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RegGuheert
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Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 7:53 pm

GRA wrote: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 wrote: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
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

GRA
Posts: 7575
Joined: Mon Sep 19, 2011 1:49 pm
Location: East side of San Francisco Bay

Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 8:11 pm

RegGuheert wrote:
GRA wrote: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 wrote:
GRA wrote: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.
Guy [I have lots of experience designing/selling off-grid AE systems, some using EVs but don't own one. Local trips are by foot, bike and/or rapid transit].

The 'best' is the enemy of 'good enough'. Copper shot, not Silver bullets.

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RegGuheert
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Re: What Size Battery Would You Need to Power Your House?

Thu Mar 09, 2017 5:01 am

GRA wrote: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 wrote: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 wrote: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 wrote: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$0.10/kWh today:
RegGuheert wrote:So, let's see how they stack up in terms of US$/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 wrote: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 wrote: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.
RegGuheert
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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RegGuheert
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Re: What Size Battery Would You Need to Power Your House?

Thu Mar 09, 2017 9:10 am

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: Select all

--------------------------------------------------------
| 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
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

Zythryn
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Re: What Size Battery Would You Need to Power Your House?

Thu Mar 09, 2017 11:36 am

RegGuheert wrote:...
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 wrote: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 wrote: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 wrote: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 wrote: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 wrote: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 wrote:
Zythryn wrote: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
Previous owner of Prius, Volt & Leaf
Current owner of Model S
http://www.netzeromn.com

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RegGuheert
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Re: What Size Battery Would You Need to Power Your House?

Thu Mar 09, 2017 2:31 pm

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: Select all

----------------------------------------------------------------
|  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).
RegGuheert
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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RegGuheert
Posts: 5621
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Re: What Size Battery Would You Need to Power Your House?

Thu Mar 09, 2017 4:21 pm

Zythryn wrote: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 wrote: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 wrote: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 wrote: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 wrote: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
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

GRA
Posts: 7575
Joined: Mon Sep 19, 2011 1:49 pm
Location: East side of San Francisco Bay

Re: What Size Battery Would You Need to Power Your House?

Thu Mar 09, 2017 5:26 pm

RegGuheert wrote:
GRA wrote: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 wrote:
GRA wrote: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 wrote: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 wrote:
GRA wrote: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$0.10/kWh today:
RegGuheert wrote:So, let's see how they stack up in terms of US$/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 wrote: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 wrote: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 wrote: 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 wrote: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 wrote:
GRA wrote: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 wrote: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 wrote: - 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 wrote: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.
Guy [I have lots of experience designing/selling off-grid AE systems, some using EVs but don't own one. Local trips are by foot, bike and/or rapid transit].

The 'best' is the enemy of 'good enough'. Copper shot, not Silver bullets.

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RegGuheert
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Re: What Size Battery Would You Need to Power Your House?

Thu Mar 09, 2017 6:30 pm

GRA wrote: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 wrote: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.
RegGuheert
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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RegGuheert
Posts: 5621
Joined: Mon Mar 19, 2012 4:12 am
Delivery Date: 16 Mar 2012
Leaf Number: 5926
Location: Northern VA

Re: What Size Battery Would You Need to Power Your House?

Fri Mar 10, 2017 8:45 am

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: Select all

-----------------------------------------------------------------------------------------
|  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.
Last edited by RegGuheert on Sat Mar 11, 2017 9:28 am, edited 1 time in total.
RegGuheert
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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