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

Wed Jun 03, 2015 6:10 pm

edatoakrun wrote:I think it is common for beneficiaries of net-metering to underestimate the costs they impose on other utility ratepayers, which in fact are substantial.


I think it is common for utilities to overestimate the costs. I still pay the same daily access charge as everyone else. They either pay me less than 1/3 of what they turn around and charge for any unused kWh or allow an indefinite carryover of credits. Eventually I will use my substantial 20-30% overproduction credits on a second EV, but I wonder on average how many net-metered customers never use those credits and thus the utility has gotten that power for free.
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RegGuheert
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Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 3:15 am

The Goal
This post is a first-cut attempt to determine the capacity of Li-ion batteries and the associated costs required to convert my zero-net-usage net-metered home to a home which has zero power flow to or from the grid. The cost will be calculated to achieve zero power flow from the house for the following lengths-of-time: 1 hour, 1 day, 1 week, 1 month, 3 months, and 9 months.

The House
- Latitude: 39 Degrees North
- 3000 sq.ft.
- Typical construction, 200-A electrical service.
- All-electric loads including heating and cooling using a 19-SEER, 8.3-HSPF heat pump (exception is propane cooktop).
- 12.75-kWp PV array

The Data
My power provider has recently made available hourly energy flow through the meter for the past few years. I have analyzed this data from March 1, 2016, until March 1, 2017. Net energy flow during this entire period was 112 kWh of production. The data has been analyzed so that the PEAK consumption and production have been determined for each of the periods listed above.

Here is one interesting bit of data which is not covered in the details I discuss below: 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."

A Few Assumptions
- Power flow during each hour is constant. (This is certainly false, but it allows me to proceed. This assumption also allows me to use the one-hour consumption number to approximate peak power flow.)
- No changes are made to the home except the addition of batteries. (Perhaps this is not ideal, but it is a starting point.)
- The weather during the analysis period is typical. (Perhaps, but I doubt it. In any case, what is *really* needed is a worst-case analysis, not typical.)
- I do not account for the additional PV which would be required to provide for the energy loss in the batteries. (There is plenty of room on the roof for the approximately 10% extra production which would be required. That much additional PV with inverters would cost about as much as two Enphase AC Batteries.)
- Self-consumption of the battery and their associated electronics are ignored. (This is a rather important assumption, since it is unlikely that current-technology Li-ion battery systems could store energy for up to 12 months as would be required to meet the longer-term requirements.)

Battery Requirements
For this section, I will compute the minimum number of Enphase AC Batteries which would be necessary to meet the energy and power requirements for a given time period. Here are the pertinent specifications I will use for these batteries:
- Energy which a full AC Battery can deliver: 1.08 kWh
- Maximum continuous AC power (input or output) of the AC Battery: 270 VA
- Cost for each AC Battery: US$1000

Code: Select all

---------------------------------------------------------------------------
|  Time  |    Peak     |    Peak    | Limited |  Number of   |    Cost    |
| Period | Consumption | Production |    By   | AC Batteries |            |
|        |     kWh     |     kWh    |         |   Required   |            |
|--------|-------------|------------|---------|--------------|------------|
| 1 Hour |     18.7    |     10.7   |  Power  |       69     |    $69,000 |
|--------|-------------|------------|---------|--------------|------------|
| 1 Day  |    149.3    |     64.7   |  Energy |      138     |   $138,000 |
|--------|-------------|------------|---------|--------------|------------|
| 1 Week |    428.7    |    349.8   |  Energy |      397     |   $397,000 |
|--------|-------------|------------|---------|--------------|------------|
| 30 Days|   1310.1    |    917.5   |  Energy |     1213     | $1,213,000 |
|--------|-------------|------------|---------|--------------|------------|
| 90 Days|   2907.0    |   1716.3   |  Energy |     1588     | $1,588,000 |
|--------|-------------|------------|---------|--------------|------------|
|270 Days|   1265.0    |   2872.5   |  Energy |     2873     | $2,660,000 |
---------------------------------------------------------------------------

Conclusions
I think it is clear that it is not reasonable to use Li-ion batteries to prevent the flow of electricity to or from my home onto the electricity grid. This is true even if I am just trying to handle the worst-case one-hour or one-day period that occurs through the course of a year.

Often when I refer to net metering I call it "the magic of net metering." I use this term because I know that it is absurd to try to store 3 MWh from summer until winter using existing storage technologies. What I did not fully comprehend was how difficult it would be to handle only the worst-case hour, day or week of a given year using only batteries.

So it seems that looking at worst-case numbers leads to extremely-expensive battery solutions. Perhaps a more reasonable question with the current technology would be, "How much could my grid consumption be reduced if I add X amount of storage to my system?" That is a much more difficult question to answer because it requires a form of simulation to answer. Another interesting question might be, "What can be accomplished using a combination of additional PV in addition to AC batteries?" In addition to some form of basic simulation, this will require matching up hourly data from the PV array with the net meter data to determine whether the picture can be improved significantly.

Ultimately, it seems clear that heating with electricity is a challenging task for a renewable energy source which produces the least whenever the most heat is needed. This picture gets much more challenging as the latitude increases.
RegGuheert
2011 Leaf SL Demo vehicle
2011 miles at purchase. 10K miles on Apr 14, 2013. 20K miles (55.7Ah) on Aug 7, 2014, 30K miles (52.0Ah) on Dec 30, 2015, 40K miles (49.8Ah) on Feb 8, 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?

Wed Mar 08, 2017 6:33 am

RegGuheert wrote:...

Battery Requirements
For this section, I will compute the minimum number of Enphase AC Batteries which would be necessary to meet the energy and power requirements for a given time period. Here are the pertinent specifications I will use for these batteries:
- Energy which a full AC Battery can deliver: 1.08 kWh
- Maximum continuous AC power (input or output) of the AC Battery: 270 VA
- Cost for each AC Battery: US$1000

Code: Select all

---------------------------------------------------------------------------
|  Time  |    Peak     |    Peak    | Limited |  Number of   |    Cost    |
| Period | Consumption | Production |    By   | AC Batteries |            |
|        |     kWh     |     kWh    |         |   Required   |            |
|--------|-------------|------------|---------|--------------|------------|
| 1 Hour |     18.7    |     10.7   |  Power  |       69     |    $69,000 |
|--------|-------------|------------|---------|--------------|------------|
| 1 Day  |    149.3    |     64.7   |  Energy |      138     |   $138,000 |
|--------|-------------|------------|---------|--------------|------------|
| 1 Week |    428.7    |    349.8   |  Energy |      397     |   $397,000 |
|--------|-------------|------------|---------|--------------|------------|
| 30 Days|   1310.1    |    917.5   |  Energy |     1213     | $1,213,000 |
|--------|-------------|------------|---------|--------------|------------|
| 90 Days|   2907.0    |   1716.3   |  Energy |     1588     | $1,588,000 |
|--------|-------------|------------|---------|--------------|------------|
|270 Days|   1265.0    |   2872.5   |  Energy |     2873     | $2,660,000 |
---------------------------------------------------------------------------

Conclusions
I think it is clear that it is not reasonable to use Li-ion batteries to prevent the flow of electricity to or from my home onto the electricity grid. This is true even if I am just trying to handle the worst-case one-hour or one-day period that occurs through the course of a year.

Often when I refer to net metering I call it "the magic of net metering." I use this term because I know that it is absurd to try to store 3 MWh from summer until winter using existing storage technologies. What I did not fully comprehend was how difficult it would be to handle only the worst-case hour, day or week of a given year using only batteries.

So it seems that looking at worst-case numbers leads to extremely-expensive battery solutions. Perhaps a more reasonable question with the current technology would be, "How much could my grid consumption be reduced if I add X amount of storage to my system?" That is a much more difficult question to answer because it requires a form of simulation to answer. Another interesting question might be, "What can be accomplished using a combination of additional PV in addition to AC batteries?" In addition to some form of basic simulation, this will require matching up hourly data from the PV array with the net meter data to determine whether the picture can be improved significantly.

Ultimately, it seems clear that heating with electricity is a challenging task for a renewable energy source which produces the least whenever the most heat is needed. This picture gets much more challenging as the latitude increases.


I do agree going off grid with Li batteries is expensive.
I don't understand some of your numbers though...

In the "one hour" line, you show a shortfall of 8kWh, and then 69 needed batteries?
Shouldn't that be ~8 of the 1.08kWh batteries?

Next, I would suggest you battery cost is very high.
If buying a sizable system, $0.60 would be closer to actual costs, less if you can do the install yourself.

Interesting methodology, I am going to try that myself. We are further north (45 lat) and do heat 100% with electricity.
I don't plan to take us off grid, but we do want battery backup available good for a couple of days in the winter.
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RegGuheert
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Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 7:06 am

Zythryn wrote:In the "one hour" line, you show a shortfall of 8kWh, and then 69 needed batteries?
Shouldn't that be ~8 of the 1.08kWh batteries?
Actually, that's not a shortfall of 8 kWh. What those two columns indicate are that during one particular hour over the course of the past year, net 18.7 kWh was consumed and during a DIFFERENT particular hour, likely in a different month, net 10.7 kWh was produced. (Note that I do not have two separate meters for production and consumption.) So I take the larger of these two numbers in my calculations. In the "Limited By" column, I note whether the number of batteries required is limited by their "Energy" capacity or their "Power" capability. In the case of the 1-hour result, the batteries need to be able to produce AT LEAST 18.7 kW (likely more) to be able to keep power from flowing through the meter. Since the AC Battery can only put out 270 W, I would need 18.7 kW/0.27 kW or 69 batteries to meet that need. (The actual need would likely be a bit higher because the peak power draw during that hour is likely above 18.7 kW.)
Zythryn wrote:Next, I would suggest you battery cost is very high. If buying a sizable system, $0.60 would be closer to actual costs, less if you can do the install yourself.
I didn't make up the price. That is the retail price for the Enphase AC Battery converted to US$. Each battery costs $1000. I didn't add anything for installation since I figure you can likely purchase the units for less than the retail price. You are correct that I did not include any volume discounts in my calculations. I don't think that would change the conclusions very much.
Zythryn wrote:Interesting methodology, I am going to try that myself. We are further north (45 lat) and do heat 100% with electricity. I don't plan to take us off grid, but we do want battery backup available good for a couple of days in the winter.
I'm interested to see what numbers you get, as well. Yeah, I don't have any intention to purchase batteries because net metering is such a good deal. These only make sense in areas where electricity is very expensive and produced electricity is not valued highly, such as in Hawaii. But I want to understand how well these things would fit into my application.
RegGuheert
2011 Leaf SL Demo vehicle
2011 miles at purchase. 10K miles on Apr 14, 2013. 20K miles (55.7Ah) on Aug 7, 2014, 30K miles (52.0Ah) on Dec 30, 2015, 40K miles (49.8Ah) on Feb 8, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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

Wed Mar 08, 2017 7:26 am

RegGuheert wrote:The Goal
This post is a first-cut attempt to determine the capacity of Li-ion batteries and the associated costs required to convert my zero-net-usage net-metered home to a home which has zero power flow to or from the grid. The cost will be calculated to achieve zero power flow from the house for the following lengths-of-time: 1 hour, 1 day, 1 week, 1 month, 3 months, and 9 months.


To improve this, several points:

1) The "Enphase AC Batteries" product is an integrated inverter, charger and battery. If you separate these functions, the cost of longer term storage should fall dramatically. For the battery only, $150/kWh today, $100/kWh is expected by 2020, and current trends project $40/kWh by 2030.

2) You have a very close match between production and consumption. Excess production (ie more solar cells) is probably cheaper, as will require far less storage, and that storage is more expensive than the needed solar cells.

3) A simulation is probably required. A year's worth of data isn't a large enough sample of weather to accurately reflect climate. "Worse case" is probably too tough, you need to ask how many "9"s you need. 99%? 99.9% 99.99% 99.999% and so on. As part of the simulation, a plan for load shedding as the storage empties is important, as it would let different loads have different service qualities. You might accept 90% for a swimming pool heater or something like that, 99.9% for the house heat and want 99.999% for the freezer (1 hour of outage per decade). Adjust "9"s to your expectations.

4) Probably cheaper to improve insulation first.

5) A grid is still useful. Think of Florida in winter having excess power, and Boston, with a winter need for power. Summer, reverse.
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Zythryn
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Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 9:08 am

RegGuheert]Actually, that's not a shortfall of 8 kWh. What those two columns indicate are that during one particular hour over the course of the past year, net 18.7 kWh was consumed and during a DIFFERENT particular hour, likely in a different month, net 10.7 kWh was produced. (Note that I do not have two separate meters for production and consumption.) So I take the larger of these two numbers in my calculations. In the "Limited By" column, I note whether the number of batteries required is limited by their "Energy" capacity or their "Power" capability. In the case of the 1-hour result, the batteries need to be able to produce AT LEAST 18.7 kW (likely more) to be able to keep power from flowing through the meter. Since the AC Battery can only put out 270 W, I would need 18.7 kW/0.27 kW or 69 batteries to meet that need. (The actual need would likely be a bit higher because the peak power draw during that hour is likely above 18.7 kW.)[/quote]

Ah, I understand better, thank you for clearing up my misunderstanding.

[quote="RegGuheert wrote:
Zythryn wrote:Next, I would suggest you battery cost is very high. If buying a sizable system, $0.60 would be closer to actual costs, less if you can do the install yourself.
I didn't make up the price. That is the retail price for the Enphase AC Battery converted to US$. Each battery costs $1000. I didn't add anything for installation since I figure you can likely purchase the units for less than the retail price. You are correct that I did not include any volume discounts in my calculations. I don't think that would change the conclusions very much.


Choosing the correct batteries for your project is critical.
The cost you quote would be vastly different with a more capable battery.
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.

I apologize if you inferred from my statement that I thought you were making up the price. With your mention of the specific battery I did not. My point was you are picking one ill suited to the job, making it unnecessarily expensive.

For example, if you use a Tesla PowerWall (AC version) a battery pack would cost about $7500 installed (guess, base price is $5500). For that you get 14.5kWh capacity, delivered at up to 4kW (5kW peak surge).
This change alone cuts your cost estimates in half.

Zythryn wrote:Interesting methodology, I am going to try that myself. We are further north (45 lat) and do heat 100% with electricity. I don't plan to take us off grid, but we do want battery backup available good for a couple of days in the winter.
I'm interested to see what numbers you get, as well. Yeah, I don't have any intention to purchase batteries because net metering is such a good deal. These only make sense in areas where electricity is very expensive and produced electricity is not valued highly, such as in Hawaii. But I want to understand how well these things would fit into my application.[/quote]

I agree with the conclusion, but not the underlying calculations.
First, if you were to go off grid, you probably wouldn't have any single hour where your draw was 18.7kWh. That is huge, unless you are including an electric car or two.
Even with a fully electric house, we have never used more than 8kW, maybe 10kW. And if we wanted to be careful to avoid spikes, we wouldn't ever cross 6kW.
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Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 10:02 am

Ironically enough, just as I was getting started in Excel, our power went out!

I'd like to ask for some help in how to get a really good estimate.
I love the idea of doing this hourly.

I am planning to go back to November 1st up through today.
Unfortunately I don't have data going back prior to October 1st.

My first thought was to plot hourly kW usage (basically kWh) to get the worst case scenario.
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.

I am not too concerned about just having data over the winter months as here in MN we use far more energy in the winter than we do in the summer.

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.
As an example, the month of December tends to be our cloudiest month, which means the batteries may need to handle the power needs for 3 weeks.
The February though, it was incredibly sunny. We produced more solar energy than any other month (Oct-Jan). This was way out of the norm though.

This should be fun, once the power comes back on.
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RegGuheert
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Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 10:40 am

WetEV wrote:To improve this, several points:

1) The "Enphase AC Batteries" product is an integrated inverter, charger and battery. If you separate these functions, the cost of longer term storage should fall dramatically. For the battery only, $150/kWh today, $100/kWh is expected by 2020, and current trends project $40/kWh by 2030.
Agreed. I picked this as a starting point since they are simple to price. One advantage that Enphase has is extremely high cycle life, like 12,000 FULL cycles. They warrant for 7600 cycles. That made them cheaper per kWh than Powerwall 1, but they are about twice the price of Powerwall 2 on that basis. Powerwall 2 also has a much higher power/energy ratio. Prices will certainly come down over time.
WetEV wrote:2) You have a very close match between production and consumption. Excess production (ie more solar cells) is probably cheaper, as will require far less storage, and that storage is more expensive than the needed solar cells.
Certainly, but this only helps with the longer time frames. The 1-hour, 1-day and 1-week worst-case situations will not change much with additional PV due to the impacts of nighttime and snow on the PV modules. A Nor'easter is about the worst-case scenario that we get here and that didn't happen this winter. I need to pull in my hourly PV production data in order to estimate how increases in PV affect the results.
WetEV wrote:3) A simulation is probably required. A year's worth of data isn't a large enough sample of weather to accurately reflect climate. "Worse case" is probably too tough, you need to ask how many "9"s you need. 99%? 99.9% 99.99% 99.999% and so on. As part of the simulation, a plan for load shedding as the storage empties is important, as it would let different loads have different service qualities. You might accept 90% for a swimming pool heater or something like that, 99.9% for the house heat and want 99.999% for the freezer (1 hour of outage per decade). Adjust "9"s to your expectations.
I will attempt a poor-man's simulation with Excel after I get some PV data in place. Then I can estimate performance based on various amounts of storage.
WetEV wrote:4) Probably cheaper to improve insulation first.
Perhaps, but I'm convinced that would have been more expensive than the additional PV with net metering since it would require expensive window replacements. Clearly not more expensive than batteries, though.
WetEV wrote:5) A grid is still useful. Think of Florida in winter having excess power, and Boston, with a winter need for power. Summer, reverse.
Agreed, but I doubt the existing grid can handle that type of power flow.
RegGuheert
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2011 miles at purchase. 10K miles on Apr 14, 2013. 20K miles (55.7Ah) on Aug 7, 2014, 30K miles (52.0Ah) on Dec 30, 2015, 40K miles (49.8Ah) on Feb 8, 2017.
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Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 11:02 am

Zythryn wrote: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.)
Zythryn wrote:Ironically enough, just as I was getting started in Excel, our power went out!
That is ironic! :)
Zythryn wrote: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 changes in the other direction around this time each year.
Zythryn wrote: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.
Zythryn wrote: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.

Our 2011 LEAF is limited 3.6 kW draw.
Zythryn wrote:I am not too concerned about just having data over the winter months as here in MN we use far more energy in the winter than we do in the summer.
Agreed. We are only net consumers of electricity from November through Februrary, roughly.
Zythryn wrote: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 wrote:This should be fun, once the power comes back on.
Yes. We find electricity is very useful around here!
RegGuheert
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2011 miles at purchase. 10K miles on Apr 14, 2013. 20K miles (55.7Ah) on Aug 7, 2014, 30K miles (52.0Ah) on Dec 30, 2015, 40K miles (49.8Ah) on Feb 8, 2017.
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GetOffYourGas
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Re: What Size Battery Would You Need to Power Your House?

Wed Mar 08, 2017 11:49 am

Thanks for running these numbers, Reg. I enjoyed following you on your thought process.

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.

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.
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