What Size Battery Would You Need to Power Your House?

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GetOffYourGas said:
Thanks for running these numbers, Reg. I enjoyed following you on your thought process.
You're welcome! To be honest, the short-term numbers are a bit more challenging than I had imagined. (I already knew I was carrying 3 MWh across seasons.)

IMO, storage is the next big hurdle in the move to a renewable, all-electric society. But there are no simple answers, partly because we are increasing our electricity consumption (by moving to BEVs) at the same time we are attempting to clean up the electricity grid.
GetOffYourGas said:
Although I use significantly less electricity than you (I have natural gas heat), I would hate to do the calculation for my home. My panels produce very nearly zero from December-February. One of the benefits of living in snow country.
Simply put, photovoltaics get much more difficult to apply as you move away from the equator. Batteries are very likely to be a part of the long-term solution to smooth out those short-term variations. But I suspect hydrogen will eventually be the solution-of-choice for seasonal shifting of energy production. But that will take a massive reduction in cost and a significant increase in efficiency.

BTW, I do think that the Enphase AC Battery will be successful in Hawaii. Electricity is expensive there and they are very close to the equator. As such, they do not have the huge heating requirements that we have so far north.
GetOffYourGas said:
If one is truly to go off-grid, I would imagine you would want some sort of smart-home power management. For example, I come home and plug in my car to recharge, meanwhile I also turn the heat up. 30 minutes later, I'm firing up the oven to cook dinner. I know that I don't need the car charged until the morning, and would rather have dinner cooking than full heat to the house. Sounds like a challenging but fun problem to solve.
Yes, load management is something I'm a big fan of. Hopefully in the future we will be able to net-meter the energy into and out of our BEVs, including time-of-use considerations. As batteries get larger and more durable, that would provide high value to the grid, particularly if they could be charged in the daytime and discharged at nighttime.
 
RegGuheert said:
Simply put, photovoltaics get much more difficult to apply as you move away from the equator. Batteries are very likely to be a part of the long-term solution to smooth out those short-term variations. But I suspect hydrogen will eventually be the solution-of-choice for seasonal shifting of energy production. But that will take a massive reduction in cost and a significant increase in efficiency.

Don't forget other sources of renewables. I live downwind of the Great Lakes. That gives us lots of clouds/snow (which practically kills solar in the winter). It also gives us a lot of wind (no mountains upwind of us to divert/slow it down). There are more projects going in all the time. A new one was proposed for Tug Hill, just north of me:

https://www.wind-watch.org/news/2017/03/08/wind-farm-with-up-to-125-turbines-proposed-for-tug-hill/

Large-scale wind projects are, IMO, an argument against going off-grid. I hope to see the grid itself become more of a community asset, through which we all share our production and consumption of electricity.

Of course, wind doesn't eliminate the need for storage. It just reduces it. So I'm excited to see what solutions become available in the not-so-distant future. It could be hydrogen. It could also be massively distributed batteries (e.g. if V2G-equipped BEVs started to take over). It could be something I haven't heard or thought of yet.
 
RegGuheert said:
Zythryn said:
I would suggest your estimates don't show how much it would cost to go off grid. They estimate how much it would cost to go off grid with those specific batteries.
Actually, this house has been nearly off-grid in the past. For nine months of the year the 2880 Wp array provided all of our electricity and about half during the other three months. The electric water heater remained on-grid. Heat was provided using a wood-pellet stove rather than a heat pump. The clothes dryer and oven ran off propane rather than electricity. That was until the house was hit by lightning and I lost an inverter. Also, we did not have an electric car consuming over 2 MWh of electricity each year back then.

No, this calculation is intended to see what is involved in eliminating the flow to the grid with our fully-electric house. (We still have the wood-pellet stove and can operate in the case of emergencies for long periods of time, so that is not my intention.)

I applaud your energy efficiency, but once again, you take the most expensive solution and state it as a general case.


RegGuheert said:
Zythryn said:
Ironically enough, just as I was getting started in Excel, our power went out!
That is ironic! :)
Whew, back up again after a couple of hours, temp in the house only went up a couple degrees ;)

RegGuheert said:
Zythryn said:
I am planning to go back to November 1st up through today.
Unfortunately I don't have data going back prior to October 1st.
For reference, here is Virginia, my meter reversed direction around November 20, 2016. In 2015, it was around November 1. Yours likely reverses prior to that.

Our meter "reverses" direction every night.
Our first measured daily net loss was in early October, although I am sure there are days every month.
Our first measured weekly loss looks like it was the last week in November.
Monthly loss would be December.
The first monthly gain would be February, weekly looks like the first week of January and daily, well those are scattered throughout the winter.
http://www.netzeromn.com/blog/the-envelope-please

RegGuheert said:
Zythryn said:
My first thought was to plot hourly kW usage (basically kWh) to get the worst case scenario.
That's roughly what the 1-hour column provides for me.
Yep, thank you again, this is a much better way to look at it than daily. The line between hourly data and power requirements for either battery backup or going off grid is much more direct.

Zythryn said:
I could track net power use, but with such a short timeframe I'm thinking worse case is better.
This would put disproportionate weight on our car charging. One of the cars is mostly charged during sunlight hours.
So, I am tempted to subtract the car charging out of the data.
Don't you have a Tesla Model S? If so, does it have a 20 kW charger in it? That could cause you to draw power from the grid no matter when it is charged.
The standard charger in a Tesla is 10kW. The car can be set to charge at less than the maximum. I typically charge at 18A or about 4.2kW.
If I were going off grid, I would likely set them to charge at 12A/3kW. For our driving habits, that is sufficient.
I also try to charge when the sun is shining, which means the car has little impact on the net power use of the house.

RegGuheert said:
Zythryn said:
I also need to calculate the energy needed over a longer timeframe.
This is tricky, as the length of time varies depending upon the sunshine.
This is were net metering REALLY provides its magic: The last two years I have "stored" over 3 MWh each year in Spring, Summer and Fall. This year I am hopeful to store over 4 MWh before wintertime. If I do, the thermostat may go up a couple of degrees next winter! :)
Zythryn said:
This should be fun, once the power comes back on.
Yes. We find electricity is very useful around here!

Here too!
And now that it is back...
So our highest hourly power use between November and today was 4.4kWh.
So this can be taken care of by a single PowerWall. The duration is going to be the killer.
December 5th looks like one of the higher use days. However, while the hourly high use for 4.4kWh (around dinner time), our daily use that day was 26.7kWh.
We would need two PowerWalls to supply power that whole day without any changes to our routine, if we had no solar panels.
With the solar panels, we are down to a deficit of 17kWh for that day.

Power outages, and going off grid are very different though.
Being on the grid, we can bake, cook, charge our cars, etc, whenever we like.
In the event of a day, or longer, power outage, we can easily limit the amount of cooking we do and many other activities that take power.
Going off grid though, requires a more regular focus on power use. For example, I would not have a small fridge downstairs.
We would bake during daylight hours with lots of sunshine and store the results.
I'd rely a lot more on the microwave during dark hours.

And of course, this winter wasn't the coldest we have ever had. Non-the-less, I would estimate one PowerWall would be sufficient to get us through two days of no power at reduced capacity (no car charging, no oven, etc). A day and a half of the absolute worse case scenario of zero solar power (which is unusual).

Going off grid though, with no changes, would have required about 1200 kWh of storage (peak to trough of our production graph).
This would be about 95 PowerWalls! Or about $500,000. Very expensive indeed.
Pulling the cars out of the formula, it would be about 40 PowerWalls (the garage just might be able to fit that;)).


The above is all without the cars.
 
Zythryn said:
Our meter "reverses" direction every night.
Our first measured daily net loss was in early October, although I am sure there are days every month.
Our first measured weekly loss looks like it was the last week in November.
Monthly loss would be December.
O.K. That matches us so far...
Zythryn said:
The first monthly gain would be February, weekly looks like the first week of January and daily, well those are scattered throughout the winter.
http://www.netzeromn.com/blog/the-envelope-please
O.K. That's different. February is always a significant loss for us. March is the first month with gains.
Zythryn said:
The standard charger in a Tesla is 10kW. The car can be set to charge at less than the maximum. I typically charge at 18A or about 4.2kW.
If I were going off grid, I would likely set them to charge at 12A/3kW. For our driving habits, that is sufficient.
I also try to charge when the sun is shining, which means the car has little impact on the net power use of the house.
Thanks. I had forgotten that the Tesla is programmable. I agree that charging at the lowest power level necessary makes a lot of sense.
Zythryn said:
So our highest hourly power use between November and today was 4.4kWh.
That's very good! I suppose that is the beauty of a ground-source heat pump versus a air-source heat pump like we have. Your house is WAY better insulated than ours, though your outdoor temperatures are also lower. Below about 10F, the resistive heaters come on in our air handler. I don't know their exact power rating, but I expect the entire system likely draws about 15 kW when they are on.

OTOH, it appears you have an 18.6 kW PV array on your home. Certainly in the springtime your production must approach that level. The inverter needs to be sized for the maximum power flow in either direction if you intend to keep all electricity in-house. I would think that your house may have as much as 14 kWh flowing out during some hours of the year. Peak is probably sometime in April, but you must also have fairly-high production hours in the middle of the wintertime.
Zythryn said:
So this can be taken care of by a single PowerWall. The duration is going to be the killer.
December 5th looks like one of the higher use days. However, while the hourly high use for 4.4kWh (around dinner time), our daily use that day was 26.7kWh.
We would need two PowerWalls to supply power that whole day without any changes to our routine, if we had no solar panels.
With the solar panels, we are down to a deficit of 17kWh for that day.
That's extremely good! If only more homes were built the way yours is...

I will note that I see single-day consumption numbers in the middle of December of about 3X what occurred on your graph December 5. (That would match my worst day, which was December 16, 2016.) I also see three-day drops in both the middle of December and around the 10th of January that appear to be 200 kWh total drops. The middle day in each of those drops appears to be a drop of over 80 kWh. Of course that data includes your cars, but all my data includes my LEAF, as well.

It seems to me that you will come in WAY below the 22 MWh/year that you mention on your website, even while fueling two EVs. Let's say the EVs use about 5 MWh/year total. That would mean the rest of the house consumes 17 MWh. Do you have an updated estimate where you will come in for the year?
Zythryn said:
And of course, this winter wasn't the coldest we have ever had.
No, in fact it was one of the warmest here. But it followed a warm summer in which we used much more electricity for air conditioning than normal. The result was that are total electricity usage was very close to normal.
Zythryn said:
Non-the-less, I would estimate one PowerWall would be sufficient to get us through two days of no power at reduced capacity (no car charging, no oven, etc). A day and a half of the absolute worse case scenario of zero solar power (which is unusual).
I think we'd need 25 Powerwalls to cover the same two worst-case days here. (In reality, we would just turn off the heat pump and burn wood pellets. The pellet stove only draws about 120W of electricity.)
 
While PowerWalls are totally cool, they are expensive. Adding batteries to your house can cost a lot less if you go with good old fashioned lead-acid. The house is not a vehicle and the batteries sit in the garage or shed and never move, so the biggest benefit of lithium, being lightweight, is wasted. A PowerWall 2 is 14kwh and 7kw continuous discharge which can be matched by 6x 100Ah 12v marine deep cycle batteries that cost about $100 for 100Ah new. 14kwh would cost $600 instead of $5,500, nearly 10 times less. Now the PowerWall probably actually can deliver a bit more of that 14kwh than the equivalent lead-acid bank, and of course it takes up far less space. I am years away from venturing into solar for my house, but you two who are clearly way deep into it; why are you speccing PowerWalls instead of cheaper lead-acid batteries? Especially when it seems like the economic factor is the biggest decider to go/no go.
 
VitaminJ said:
While PowerWalls are totally cool, they are expensive. Adding batteries to your house can cost a lot less if you go with good old fashioned lead-acid. The house is not a vehicle and the batteries sit in the garage or shed and never move, so the biggest benefit of lithium, being lightweight, is wasted. A PowerWall 2 is 14kwh and 7kw continuous discharge which can be matched by 6x 100Ah 12v marine deep cycle batteries that cost about $100 for 100Ah new. 14kwh would cost $600 instead of $5,500, nearly 10 times less.
Actually, you need twice as many lead-acid batteries to get a similar capacity. And if you want a reliable system, you likely want to purchase 12 4-V L16 size batteries, each costing about $250 each, or about $3000 total, not including the expensive wires you will need to connect them. That gets you a 48-V battery with energy efficiency about 60% of the Powerwall 2, no capacity warranty and lots of operational limitations. You can expect to get about five years out of such batteries if you take very good care of them. If you want lead-acid batteries that will outlast the Powerwall, you will pay as much or more than you pay for the Powerwall, but they will still have the low efficiency.

If you want something that you can put in place an not have to fiddle with (until they are worn out in 10 years), then the Tesla Powerwall or the Enphase AC Battery are the way to go.
 
The house is not a vehicle and the batteries sit in the garage or shed and never move, so the biggest benefit of lithium, being lightweight, is wasted.

The biggest advantage is being able to discharge to 20% or even less, recharge, and repeated for hundreds to thousands of cycles without severe capacity loss.
 
Clear, succinct answers. Thank you, guys. I can't help but think that a smashed up Leaf with a good battery might be a good cost effective donor for a household system.
 
Since I used to design off-grid systems, I'll chime in.

1. If you've got the grid and it's reasonably reliable, especially if you've got net metering and PV, there's simply no financial justification for going off-grid unless your grid rates are in the stratosphere.

2. Using electricity for space heat/ranges/drying in an off-grid home simply isn't cost-effective unless you've got year-round hydro, and probably not even then. Occasional microwave use is okay.

3. Demand reduction is the name of the game - it's far cheaper to control your max. load by load-shifting than it is to try and run everything at once. As it's an individual home, you have complete control over how much you're willing to do this. Shave your peaks!

4. Before even thinking about going off-grid, first spend your money on getting the most efficient appliances, improving insulation, making use of passive solar, closing room doors, using task rather than area lighting (and shutting lights off as soon as you leave a room), putting always-on loads on switchable load strips etc. This ties in with number 3. It's still far cheaper, even with the huge price drop in PV and wind since I was doing this, to spend thousands on efficient loads rather than tens of thousands on extra generation and storage plus inverters. As a general rule, forget air conditioning - plan on evaporative cooling if your humidity allows, plus passive cooling (a heat pump's a good step, but better to limit its use by use of insulation and passive solar design instead).

5. Designing for year round off-grid, all you need is your worst case data in whichever season's critical for you (usually winter) , the number of days you want to have backup, and decide how much you're willing to practice demand reduction/load shifting. In other words, are you willing to hold off on doing the laundry until it's sunny/windy, use a clothesline, and make sure you're not operating your shop tools, the microwave and blender at the same time, while you've got the TV on.

6. If you want, it may make more sense to only provide enough back-up storage for whichever circuits that you consider critical in emergencies, typically enough to run the refrigerator and a few lights, plus a radio (and maybe charge a cell phone or two if you don't have a land line). Beyond a few days storage, a small genset is probably cheaper for rare outages.

The typical full size off-grid home around the Bay Area might use 2-5kWh/day (no car charging), some much less, versus the 18kWh/day average for PG&E. My usage in my current small apartment usually runs well under 2kWh, as I have a small single gas wall heater and a small gas range. Largest single electrical draw is the non-efficient apartment-sized (14 cu. ft.?) refrigerator, which is a max. of about 1.4kWh/day on the hottest days, maybe 400 Wh/day in winter. Going to a Sunfrost RF-16 running 12 or 24V DC would drop that to around 700 Wh/day, max. All other electric loads are either true/on/off or on switched power strips, to eliminate vampire loads, and I normally have just a single CF task light on. No security lights are used or wanted. All this is typical of off-grid practice (even though I'm on-grid now).

In summary, unless the average person living on-grid is willing to make major changes in their lifestyle, going off-grid is rarely a good idea financially or practically. My customers had already made that decision and mostly decided what parts of their behavior they were and were not willing to alter, so there was no problem.
 
VitaminJ said:
I can't help but think that a smashed up Leaf with a good battery might be a good cost effective donor for a household system.
Member 'offpist' did just that. It looks awesome! The difference is that one is an off-the-shelf product and the other requires quite a bit of engineering and fabrication.
 
GRA said:
Since I used to design off-grid systems, I'll chime in.
This thread is not about off-grid systems. I have been off-grid in the house I live in and that is not what I am interested in. Using fossil fuels to minimize electricity consumption had its place, but 240 VAC electricity is far more convenient. A single BEV uses signficantly more electricity than what you discussed in your post.
GRA said:
1. If you've got the grid and it's reasonably reliable, especially if you've got net metering and PV, there's simply no financial justification for going off-grid unless your grid rates are in the stratosphere.
Net metering has been taken away in Hawaii due to high rates of photovoltaics. Customers who want to add solar need to prevent their production from going out onto the grid.

Eventually, similar restrictions may come to the rest of us. That is why I am exploring what my net-zero house consumption looks like. My worst-case post was just a first cut. Next I will investigate other approaches.
 
RegGuheert said:
GRA said:
Since I used to design off-grid systems, I'll chime in.
This thread is not about off-grid systems. I have been off-grid in the house I live in and that is not what I am interested in. Using fossil fuels to minimize electricity consumption had its place, but 240 VAC electricity is far more convenient. A single BEV uses significantly more electricity than what you discussed in your post.
Which was kind of the point I was making, although I didn't explicitly say so. If you've got to drive and charge a PEV every day, it's very difficult to justify being off-grid.

RegGuheert said:
GRA said:
1. If you've got the grid and it's reasonably reliable, especially if you've got net metering and PV, there's simply no financial justification for going off-grid unless your grid rates are in the stratosphere.
Net metering has been taken away in Hawaii due to high rates of photovoltaics. Customers who want to add solar need to prevent their production from going out onto the grid.

Eventually, similar restrictions may come to the rest of us. That is why I am exploring what my net-zero house consumption looks like. My worst-case post was just a first cut. Next I will investigate other approaches.
And that should be the aim, to minimize the amount you export rather than trying to provide storage. It makes no financial sense to try to go off if you have to charge a PEV nightly. If you use a PEV once a week, can leave it parked during the day and only need to charge 1-2 kWh/day, it may be reasonable to do the calcs.
 
GRA said:
Which was kind of the point I was making, although I didn't explicitly say so. If you've got to drive and charge a PEV every day, it's very difficult to justify being off-grid.
Again, this thread is not about being off-grid.
GRA said:
And that should be the aim, to minimize the amount you export rather than trying to provide storage.
Enphase has a solution that does just that already. As I have just demonstrated, If I ever did that, my PV system which is capable of producing over 18 MWh each year would produce only 8 MWh instead:
RegGuheert said:
Through the course of a year, my house draws (and replaces) about 10 MWh of its annual usage from the grid. That is compared with a total consumption of about 18 MWh. In other words, about 56% of its total consumption comes from the grid. The other 44% comes directly from the photovoltaics without being "stored."
GRA said:
It makes no financial sense to try to go off if you have to charge a PEV nightly.
That is a completely unsupported statement. Storage can already be had for about US$0.10/kWh today:
RegGuheert said:
So, let's see how they stack up in terms of US$/kWh (discharge):
- Tesla Energy: US$3500/18,000 kWh = US$0.194/kWh (Assumes Tesla meets their price and excludes the price of the inverter.)
- Enphase Energy: US$1000/(7300*1.2 kWh*0.95*0.9*0.9) = US$1000/6740 kWh = US$0.148/kWh (1.2 kWh, 95% usable capacity, 90% round-trip efficiency, 90% average capacity over life, inverter included, Envoy excluded, assumes all 6740 kWh is used within the cycles OR Enphase bases warranty on total energy discharged, both of which are unlikely)So, let's see how they stack up in terms of US$/kWh (discharge):
- Tesla Energy: US$3500/18,000 kWh = US$0.194/kWh (Assumes Tesla meets their price and excludes the price of the inverter.)
- Enphase Energy: US$1000/(7300*1.2 kWh*0.95*0.9*0.9) = US$1000/6740 kWh = US$0.148/kWh (1.2 kWh, 95% usable capacity, 90% round-trip efficiency, 90% average capacity over life, inverter included, Envoy excluded, assumes all 6740 kWh is used within the cycles OR Enphase bases warranty on total energy discharged, both of which are unlikely)
and:
RegGuheert said:
As you can see, they have doubled the usable energy and nearly quadrupled the power capability. Price is now 1/2 the price of the Enphase AC Battery on a per-kWh basis
So, with PV at around US$0.05/kWh and storage at around US$0.10/kWh, it *should* be possible to come out far ahead by adding storage if electricity from the grid costs US$0.47 like it currently does in Hawaii.

But my "first cut" data clearly show that trying to capture ALL of my excess production is not cost effective. That's what you are saying, but that doesn't answer the questions I am trying to answer, which are:
-"If I lose the ability to put all of my electricity production onto the grid, how much of it can I rescue for myself by simply adding high-efficiency Li-ion storage to my system?"
In my case, the answer to that question clearly is "Not all of it".

So the next question becomes:
-"Since it makes no sense to provide batteries for all of my storage, how much DOES make sense?"

This question is MUCH more difficult to answer because it depends on many factors:
- Price of grid electriciity (not so easy in CA)
- Incremental cost of stored electricity.
- Local weather, which controls the production profile.
- Consumption profile.

I would like to produce the following plots:
- MWh/year "rescued" versus kWh of storage provided (unlimited power capability)
- MWh/year "rescued" versus kW of storage provided (unlimited energy storage capacity)
- MWh/year "rescued" and/or "lost" versus kWp of PV added

Ultimately, I should be able to determine up-front costs, per-kWh costs, MWh/year "rescued" and system lifetime expectations for any arrangement of additional storage (with a power limitation) and additional PV. Ultimately, it may be interesting to determine the best use of the BEV battery for load management.
GRA said:
If you use a PEV once a week, can leave it parked during the day and only need to charge 1-2 kWh/day, it may be reasonable to do the calcs.
That's an off-grid mindset. It has little bearing on what is being discussed here.

Some things we've already learned about two all-electric houses:
- My 3000 sq.ft. all-electric house at 39 degrees north latitude with standard 1990s construction, an air-exchange heat pump, an air-exchange heat-pump water heater, one EV, and 12.75 kWp(DC) of PV currently carries about 3 MWh from the warmer months into wintertime and stores about 10 MWh total in the grid through the course of a year. Peak 7-day consumption is around 450 kWh.
- Zythryn's all-electric house at 45 degrees north latitude with high-efficiency modern construction, a ground-sourced heat pump, two EVs and 18.6 kW of PV currently carries 1.2 MWh from the warmer months into wintertime. Peak 3-day consumption is around 200 kWh.

I suspect that Enphase is collecting a massive amount of very similar data using their Envoy-S-based systems. Tesla, OTOH, likely does not have access to nearly as much data as Enphase. Still, I have yet to see any sort of sizing guide from either company. It's not surprising since its certain the answer is extremely site-dependent.
 
Here's a table indicating how much energy would be kept off the grid in my system with various amounts of storage included:

Assumptions:
- Storage is lossless
- Energy flow in one hour is limited to capacity (or less)

Code:
--------------------------------------------------------
| Storage Capacity | Energy Produced | Energy Consumed |
|        kWh       |       kWh       |       kWh       |
|------------------------------------------------------|
|           0      |       9995      |      9883       |
|           1      |       9618      |      9505       |
|           2      |       9285      |      9172       |
|           5      |       8360      |      8245       |
|          10      |       6968      |      6852       |
|          15      |       5833      |      5714       |
|          20      |       5079      |      4957       |
|          25      |       4586      |      4462       |
|          30      |       4243      |      4117       |
|          35      |       4014      |      3885       |
|          40      |       3863      |      3732       |
|          45      |       3736      |      3602       |
|          50      |       3643      |      3506       |
|         100      |       3162      |      3023       |
|         200      |       2917      |      2749       |
|         500      |       2690      |      2423       |
|        1000      |       2190      |      1922       |
|        2000      |       1190      |       922       |
|        3000      |        190      |         0       |
|        4000      |          0      |         0       |
--------------------------------------------------------
As expected, the first storage added has the most impact on the power flow onto the grid. Adding more storage has less and less impact on the result.

Now to try to add in some losses and a realistic power limitation.
 
RegGuheert said:
...
O.K. That's different. February is always a significant loss for us. March is the first month with gains.
It was a VERY unusual February, and our first in this house. I don't expect this to repeat, or much less be the norm.
That said, we have installed our panels at a steeper angle. It gives us better efficiency in the winter, and helps snow to sluff off.
RegGuheert said:
That's very good! I suppose that is the beauty of a ground-source heat pump versus a air-source heat pump like we have. Your house is WAY better insulated than ours, though your outdoor temperatures are also lower. Below about 10F, the resistive heaters come on in our air handler. I don't know their exact power rating, but I expect the entire system likely draws about 15 kW when they are on.

Yeh, the groundsource is definitely the way to go, if you can. For some renovations or builds it just doesn't make sense.
Resistance heat will definitely chew up the energy. We have one two as our backup to the backup. So far, it hasn't kicked off at all.
We had a string of sub zero days (one of the days the high temp was -2(f)). Our system wasn't working properly, and we didn't notice for 3 days :)

RegGuheert said:
OTOH, it appears you have an 18.6 kW PV array on your home. Certainly in the springtime your production must approach that level. The inverter needs to be sized for the maximum power flow in either direction if you intend to keep all electricity in-house. I would think that your house may have as much as 14 kWh flowing out during some hours of the year. Peak is probably sometime in April, but you must also have fairly-high production hours in the middle of the wintertime.

In February we are peaking just under 18kW at any one moment. I expect that to peak this month due to the steeper angle. In the summer we don't have use for all the power, so the less than optimal angle works just fine.
If I had unlimited storage, I'd shoot for the more optimal summertime angle as it would give us more power for the entire year. But this way we get more power when we need it and less when we don't.

RegGuheert said:
That's extremely good! If only more homes were built the way yours is...
Thank you! They are out there, and many more are being built. However it is still a small percentage of all houses and needs to expand. I have been to a couple of Net Zero renovations which amazed me. I would have never thought you could take a 100+ year old house and turn it net zero (with only a small solar array).

RegGuheert said:
I will note that I see single-day consumption numbers in the middle of December of about 3X what occurred on your graph December 5. (That would match my worst day, which was December 16, 2016.) I also see three-day drops in both the middle of December and around the 10th of January that appear to be 200 kWh total drops. The middle day in each of those drops appears to be a drop of over 80 kWh. Of course that data includes your cars, but all my data includes my LEAF, as well.

Clouds are a wonderful thing, or not ;) Those three days in the middle of December were three very cloudy, and cold days.
So our HVAC went up, and on the 17th alone we used 62kWh to charge the vehicles. I don't recall if I was planning for a trip or just had both needing to charge on the same day.
In January there was a day when we used 71kWh to charge our cars, sending the total power use just over 101kWh.
However, I don't need to plan for that as it would never happen if I didn't have the grid.
If I were off grid, we would charge on a much more even schedule than no charging for two or three days and a full charge the next.
With the battery backup, I would never charge the cars while the power is out.
With an off grid setup, I would only charge at home on sunny days, or at public chargers.

RegGuheert said:
It seems to me that you will come in WAY below the 22 MWh/year that you mention on your website, even while fueling two EVs. Let's say the EVs use about 5 MWh/year total. That would mean the rest of the house consumes 17 MWh. Do you have an updated estimate where you will come in for the year?

Our original model was for the cars and house to each use about 11 MWh/year.
We are WAY ahead of that on the cars so far. However, I expect we will be putting more miles on as the summer arrives and we start having more EV and Net Zero events to travel to.
I still expect to come in well below the 11MWh estimate, probably closer to 7. If I am lucky though, perhaps I will get closer to your 5MWh number :D

RegGuheert said:
Zythryn said:
And of course, this winter wasn't the coldest we have ever had.
No, in fact it was one of the warmest here. But it followed a warm summer in which we used much more electricity for air conditioning than normal. The result was that are total electricity usage was very close to normal.
Interesting, does your AC take as much energy as your heating?
We moved in September, so we haven't yet seen how the house performs in the Summer. We have had a couple of cold snaps this winter, but I suspect it is one of the top five warmest winters for us as well.

As for those that don't have to deal with winter, net zero gets much easier!
https://www.youtube.com/watch?v=fkQBVoS9lAo
 
I have modified the spreadsheet to include losses and to limit the flow of power. So I am going to build a table to determine the number of kWh "rescued" each year with various numbers of Enphase AC Batteries.

Enphase AC Battery Specifications used (1 unit):
- Usable energy capacity: 0.95*1.2 kWh
- Maximum AC power: 270 VA
- One way energy efficiency: 95%

Assumptions:
- Power flow is constant over each hour
- Control algorithm for battery operation is optimal
- Batteries last 10 years
Code:
----------------------------------------------------------------
|  No.  | Storage Capacity | Energy Produced | Energy Consumed |
| Units |        kWh       |       kWh       |       kWh       |
|--------------------------------------------------------------|
|    0  |           0      |       9995      |      9883       |
|    1  |        1.14      |       9549      |      9480       |
|    2  |        2.28      |       9155      |      9124       |
|    3  |        3.42      |       8780      |      8785       |
|    4  |        4.56      |       8417      |      8457       |
|    5  |        5.70      |       8063      |      8137       |
|    6  |        6.84      |       7722      |      7828       |
|    7  |        7.98      |       7386      |      7525       |
|    8  |        9.12      |       7060      |      7230       |
|    9  |       10.26      |       6744      |      6945       |
|   10  |       11.40      |       6441      |      6671       |
|   11  |       12.54      |       6154      |      6411       |
|   12  |       13.68      |       5892      |      6173       |
|   13  |       14.82      |       5651      |      5956       |
|   14  |       15.96      |       5431      |      5756       |
|   15  |       17.10      |       5229      |      5573       |
|   16  |       18.24      |       5044      |      5406       |
|   17  |       19.38      |       4877      |      5255       |
|   18  |       20.52      |       4729      |      5121       |
|   19  |       21.66      |       4593      |      4998       |
|   20  |       22.80      |       4470      |      4885       |
|   25  |       28.50      |       3983      |      4444       |
|   30  |       34.20      |       3658      |      4148       |
|   35  |       39.90      |       3450      |      3958       |
|   40  |       45.60      |       3306      |      3825       |
|   45  |       51.30      |       3187      |      3715       |
|   50  |       57.00      |       3093      |      3628       |
----------------------------------------------------------------
What we see here is that adding storage brings down net the amount of energy delivered to the grid faster than it brings down the amount of energy consumed. In other words, even though consumption from the grid comes down, net consumption increases.

If I assume the batteries all last 10 years in this type of service, I get a price of US$0.24/kWh for one unit, US$0.31/kWh for 10 units, US$0.40/kWh for 20 units and US$0.80/kWh for 50 units. In other words, in my application, I cannot get down to the theoretical cost for these units. I assume this is because they do not receive enough cycles as I used in that calculation. Only if they lasted significantly longer than 10 years would they become an affordable solution in my application (even if the price of electricity were the same as it is in Hawaii).
 
Zythryn said:
In February we are peaking just under 18kW at any one moment. I expect that to peak this month due to the steeper angle. In the summer we don't have use for all the power, so the less than optimal angle works just fine.
If you wanted to capture all this production in batteries, you would need a powerful inverter.
Zythryn said:
If I had unlimited storage, I'd shoot for the more optimal summertime angle as it would give us more power for the entire year. But this way we get more power when we need it and less when we don't.
At my latitiude, I use about 50 degrees elevation to optimize for wintertime while still providing decent summertime production. At your latitude, I imagine an even steeper angle may be in order. What pitch did you use?
Zythryn said:
Clouds are a wonderful thing, or not ;) Those three days in the middle of December were three very cloudy, and cold days.
So our HVAC went up, and on the 17th alone we used 62kWh to charge the vehicles. I don't recall if I was planning for a trip or just had both needing to charge on the same day.
In January there was a day when we used 71kWh to charge our cars, sending the total power use just over 101kWh.
However, I don't need to plan for that as it would never happen if I didn't have the grid.
If I were off grid, we would charge on a much more even schedule than no charging for two or three days and a full charge the next.
With the battery backup, I would never charge the cars while the power is out.
With an off grid setup, I would only charge at home on sunny days, or at public chargers.
Makes sense. With the limited capacity of the LEAF, we need to charge at night when the car is needed first thing in the morning.
Zythryn said:
Interesting, does your AC take as much energy as your heating?
Not at all. The temperature differential for cooling is much lower than for heating, so the power consumption is much lower. (Also, there is no resistive heating element!) Most heat pumps are optimized for cooling, but we bought one that is optimized for heating. Still, it uses much less for cooling. In any case, we rarely run the air conditioner. 2016 was an exception in that we ran it for about two months.
Zythryn said:
We moved in September, so we haven't yet seen how the house performs in the Summer. We have had a couple of cold snaps this winter, but I suspect it is one of the top five warmest winters for us as well.
Yeah, I was concerned that a cold winter would follow our warm summer, which would be the worst case for our net meterings, but it didn't happen. Rather than a strong La Nina to follow the strong El Nino, it was just a weak one (with projections of *another* El Nino to follow).
 
RegGuheert said:
GRA said:
Which was kind of the point I was making, although I didn't explicitly say so. If you've got to drive and charge a PEV every day, it's very difficult to justify being off-grid.
Again, this thread is not about being off-grid.
But the calcs are much the same, and as Zythryn has also calculated, having to charge the car regularly drives the costs through the roof.

RegGuheert said:
GRA said:
And that should be the aim, to minimize the amount you export rather than trying to provide storage.
Enphase has a solution that does just that already. As I have just demonstrated, If I ever did that, my PV system which is capable of producing over 18 MWh each year would produce only 8 MWh instead:
RegGuheert said:
Through the course of a year, my house draws (and replaces) about 10 MWh of its annual usage from the grid. That is compared with a total consumption of about 18 MWh. In other words, about 56% of its total consumption comes from the grid. The other 44% comes directly from the photovoltaics without being "stored."
Which assumes that Enphase isn't talking though their hat, but leaving that aside, now you not only have to provide that extra 56%, but at the time(s) you want to use it, i.e. you're comparing dispatchable versus non-dispatchable power, the difference having to be provided by storage, which jacks the cost.

RegGuheert said:
GRA said:
It makes no financial sense to try to go off if you have to charge a PEV nightly.
That is a completely unsupported statement. Storage can already be had for about US$0.10/kWh today:
RegGuheert said:
So, let's see how they stack up in terms of US$/kWh (discharge):
- Tesla Energy: US$3500/18,000 kWh = US$0.194/kWh (Assumes Tesla meets their price and excludes the price of the inverter.)
Now there's an caveat you can drive a BEV bus through! ;) Snip calcs.

RegGuheert said:
So, with PV at around US$0.05/kWh and storage at around US$0.10/kWh, it *should* be possible to come out far ahead by adding storage if electricity from the grid costs US$0.47 like it currently does in Hawaii.
Reg, you must have missed this in my previous post:
1. If you've got the grid and it's reasonably reliable, especially if you've got net metering and PV, there's simply no financial justification for going off-grid unless your grid rates are in the stratosphere.
Situations like Hawaii are definitely up above the tropopause, but you're not in Hawaii.

RegGuheert said:
But my "first cut" data clearly show that trying to capture ALL of my excess production is not cost effective. That's what you are saying,
I knew you'd get there. :D I was trying to save you some calculation time, but there's undeniably a benefit to going through all the intermediate steps, and I used to encourage all my customers to do them for themselves.

RegGuheert said:
but that doesn't answer the questions I am trying to answer, which are:
-"If I lose the ability to put all of my electricity production onto the grid, how much of it can I rescue for myself by simply adding high-efficiency Li-ion storage to my system?"
In my case, the answer to that question clearly is "Not all of it".

RegGuheert said:
So the next question becomes:
-"Since it makes no sense to provide batteries for all of my storage, how much DOES make sense?"

This question is MUCH more difficult to answer because it depends on many factors:
- Price of grid electriciity (not so easy in CA)
- Incremental cost of stored electricity.
- Local weather, which controls the production profile.
- Consumption profile.

I would like to produce the following plots:
- MWh/year "rescued" versus kWh of storage provided (unlimited power capability)
- MWh/year "rescued" versus kW of storage provided (unlimited energy storage capacity)
- MWh/year "rescued" and/or "lost" versus kWp of PV added

Ultimately, I should be able to determine up-front costs, per-kWh costs, MWh/year "rescued" and system lifetime expectations for any arrangement of additional storage (with a power limitation) and additional PV. Ultimately, it may be interesting to determine the best use of the BEV battery for load management.
More power to you for going through it all.

RegGuheert said:
GRA said:
If you use a PEV once a week, can leave it parked during the day and only need to charge 1-2 kWh/day, it may be reasonable to do the calcs.
That's an off-grid mindset. It has little bearing on what is being discussed here.
See above.

RegGuheert said:
Some things we've already learned about two all-electric houses:
- My 3000 sq.ft. all-electric house at 39 degrees north latitude with standard 1990s construction, an air-exchange heat pump, an air-exchange heat-pump water heater, one EV, and 12.75 kWp(DC) of PV currently carries about 3 MWh from the warmer months into wintertime and stores about 10 MWh total in the grid through the course of a year. Peak 7-day consumption is around 450 kWh.
I've highlighted your main issues, Reg. It's a case of "if I were you, I wouldn't try to get there from here." ;)

RegGuheert said:
- Zythryn's all-electric house at 45 degrees north latitude with high-efficiency modern construction, a ground-sourced heat pump, two EVs and 18.6 kW of PV currently carries 1.2 MWh from the warmer months into wintertime. Peak 3-day consumption is around 200 kWh.
Likewise.

RegGuheert said:
I suspect that Enphase is collecting a massive amount of very similar data using their Envoy-S-based systems. Tesla, OTOH, likely does not have access to nearly as much data as Enphase. Still, I have yet to see any sort of sizing guide from either company. It's not surprising since its certain the answer is extremely site-dependent.
All variable RE calcs are site and situation dependent, Reg, and quite frankly, the calcs for going completely off-grid are simpler than what you're trying to do, but I look forward to seeing yours. I remain convinced that for most people it's simply not cost-effective (outside of the conditions I've mentioned), but I can be convinced otherwise. BTW, is there any particular reason that you're concentrating on Li-ion for storage? Deep cycle L-A batteries are a commodity and priced accordingly, and work just fine for stationary storage. OTOH, unless you opt for more expensive gel cells there is a fair amount of maintenance involved, which takes us to the whole cost vs. convenience discussion. On that note, I was amused to see you write this a few posts back:
Using fossil fuels to minimize electricity consumption had its place, but 240 VAC electricity is far more convenient.
Absolutely, and it's also more convenient than using limited production, more expensive but more efficient DC appliances. I'm glad to see you acknowledge the main point I've been making here for the past 5+ years, that most people will opt for convenience over efficiency as long as the cost difference is affordable, and that's even true for someone like you who places a much higher priority on energy efficiency than most people do.
 
GRA said:
Which assumes that Enphase isn't talking though their hat,...
Enphase does not make the batteries. Those batteries retain 80% of their capacity after 12,000 FULL cycles. And those cycles achieve 96% round-trip energy efficiency. I know of no other storage technology at this capacity level which even comes close.
GRA said:
All variable RE calcs are site and situation dependent, Reg, and quite frankly, the calcs for going completely off-grid are simpler than what you're trying to do, but I look forward to seeing yours.
Yes, they are much simpler. And, for the umpteenth time, they are totally irrelevant to the on-grid case in which you can choose to use batteries for whichever portion makes sense. In other words, adding grid-tied batteries is very similar to adding grid-tied solar. You can add them and just see what comes of it. You don't need to switch your large loads to fossil fuels to make things work.

But I'd rather understand what will happen *before* adding batteries. The calculations I am doing *cannot* be done without having the kind of data which I now have.

Please move along if you have nothing to add.
 
I found a mistake in my equation which affected the computation of the effects of a power limit. The error impacted all results with results with fewer than 40 Enphase AC Batteries. This post contains the corrected results along with some added columns of data.

Enphase AC Battery Specifications used (1 unit):
- Usable energy capacity: 0.95*1.2 kWh
- Maximum AC power: 270 VA
- One way energy efficiency: 95%

Assumptions:
- Power flow is constant over each hour
- Control algorithm for battery operation is Optimal
- Batteries last 10 years
Code:
-----------------------------------------------------------------------------------------
|  No.  | Storage  | Storage |   Energy  |  Energy  |   Energy  | Net Energy | Cost per |
| Units | Capacity |  Power  |  Produced | Consumed | "Rescued" |  Consumed  |    kWh   |
|       |    kWh   |    kW   |     kWh   |    kWh   |     kWh   |     kWh    |  US$/kWh |
|-------|----------|---------|-----------|----------|-----------|------------|----------|
|    0  |       0  |    N/A  |    9995   |   9883   |       0   |    -112    |    0.00  |
|    1  |    1.14  |   0.27  |    9597   |   9883   |     360   |     -73    |    0.28  |
|    2  |    2.28  |   0.54  |    9211   |   9175   |     708   |     -36    |    0.28  |
|    3  |    3.42  |   0.81  |    8840   |   8839   |    1145   |      -1    |    0.29  |
|    4  |    4.56  |   1.08  |    8477   |   8511   |    1372   |      34    |    0.29  |
|    5  |    5.70  |   1.35  |    8127   |   8194   |    1868   |      68    |    0.30  |
|    6  |    6.84  |   1.62  |    7787   |   7888   |    1996   |     100    |    0.30  |
|    7  |    7.98  |   1.89  |    7454   |   7586   |    2297   |     132    |    0.30  |
|    8  |    9.12  |   2.16  |    7132   |   7295   |    2589   |     163    |    0.31  |
|    9  |   10.26  |   2.43  |    6822   |   7015   |    2869   |     193    |    0.31  |
|   10  |   11.40  |   2.70  |    6527   |   6748   |    3136   |     221    |    0.32  |
|   11  |   12.54  |   2.97  |    6247   |   6495   |    3388   |     247    |    0.32  |
|   12  |   13.68  |   3.24  |    5985   |   6258   |    3626   |     273    |    0.33  |
|   13  |   14.82  |   3.51  |    5741   |   6037   |    3847   |     296    |    0.34  |
|   14  |   15.96  |   3.78  |    5512   |   5830   |    4054   |     318    |    0.35  |
|   15  |   17.10  |   4.05  |    5303   |   5642   |    4242   |     338    |    0.35  |
|   16  |   18.24  |   4.32  |    5116   |   5472   |    4412   |     355    |    0.36  |
|   17  |   19.38  |   4.59  |    4945   |   5317   |    4567   |     372    |    0.37  |
|   18  |   20.52  |   4.86  |    4789   |   5175   |    4708   |     386    |    0.38  |
|   19  |   21.66  |   5.13  |    4645   |   5045   |    4839   |     400    |    0.39  |
|   20  |   22.80  |   5.40  |    4515   |   4927   |    4956   |     412    |    0.40  |
|   25  |   28.50  |   6.75  |    4002   |   4461   |    5422   |     459    |    0.46  |
|   30  |   34.20  |   8.10  |    3667   |   4156   |    5727   |     489    |    0.52  |
|   35  |   39.90  |   9.45  |    3453   |   3960   |    5923   |     507    |    0.59  |
|   40  |   45.60  |  10.80  |    3306   |   3825   |    6058   |     519    |    0.66  |
|   45  |   51.30  |  12.15  |    3187   |   3715   |    6168   |     528    |    0.73  |
|   50  |   57.00  |  13.50  |    3093   |   3628   |    6255   |     534    |    0.80  |
-----------------------------------------------------------------------------------------
What we see here is that adding storage reduces the amount of energy consumed but it also increases the net amount of energy consumed. Of course when excess production is not allowed on the grid, only the consumption matters.

I did a quick test to determine the value of a second 270-W inverter in the AC battery, thus doubling the power capability of the unit. Interestingly, if that unit is included for free, it only reduces the per-kWh price by US$0.02/kWh for a single unit and offers no benefits after about seven units. If I assume that option costs an additional $100 per unit, there is no savings for the single-unit case and only extra costs after about seven units. That gives me an idea that Enphase's idea of how much storage to include with each inverter is approximately correct.
 
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