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Calculating Battery Bank Needs/Size

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TheHappyNomads

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I am slowly purchasing equipment in order to build out an off grid system for my house. We just moved to CA and missed out on NEMA 2. So far I've purchased these as the main components for my system:

(10) 48v 415W Q.PEAK DUO L-G6.3 / BGT 415 [ used panels ]
(2) LV6548 MPPTs

As I think about battery size need I'm a little stuck. Avg electricity usage = 500kWh/month with occasional bumps close to 600kWh with a main service panel @ 200A. I guess what I'm looking for is to have an effective and efficient battery bank to power our home. So would a 12kWh \ 15kWh \ 19kWh be sufficient?
 
The rates on NEM 3.0 vary by the hour. It is difficult to know the sweet spot in terms of battery capacity. You might zero in on some of the battery rack systems that can be incrementally added to as your needs are defined. If you have spreadsheet skills you might be able to download your consumption. That way you can see how much capacity you need to carry you through high rate periods. Then you can model production of those ten panels on PV Watts to see if they have enough capacity to charge the batteries. Charging from grid at opportune times might also work.
 
I pulled a quote I received from a local solar provider when I was looking to squeeze into NEMA 2.0 back in January. Here's what they calculated based on my PG&E usage:

3.60 kWp (DC) w/ 9 400w Panels
5,748 kWh Annual Usage
5,419 kWh Production Yr. 1 (estimated)
 
TheHappyNomads said:
I am slowly purchasing equipment in order to build out an off grid system for my house. We just moved to CA and missed out on NEMA 2. So far I've purchased these as the main components for my system:

(10) 48v 415W Q.PEAK DUO L-G6.3 / BGT 415 [ used panels ]
(2) LV6548 MPPTs

As I think about battery size need I'm a little stuck. Avg electricity usage = 500kWh/month with occasional bumps close to 600kWh with a main service panel @ 200A. I guess what I'm looking for is to have an effective and efficient battery bank to power our home. So would a 12kWh \ 15kWh \ 19kWh be sufficient?
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I have just gone through a similar process. You really need to look at how many watts you use per day and how much and at what time. Averages or monthly totals can be very deceiving. In my case, I did all the averages figured I needed 4 X 100ah batteries and put in 5 so I would have extra. After a week or so of actual usage, I realized that I needed at least 3 more. What was happening is that my batteries would fill up by 2pm or so but then my demand would jump in the early evening as the electric stove comes on and everyone decides to have a shower (it is a beach house so people come in and shower in the evening). That activity would use up 30-40% of the battery and then the AC would run all night and then some early risers would get up and use the stove, coffee maker, etc. before the sun got up. I was running out of battery sometime in the middle of the night. I also had a case when some folks decided to do laundry in the evening and that makes things worse. I would suggest just monitoring for a week or two and possibly look at changing some schedules (ie do laundry in the afternoon, if possible, etc.).
 
Benzimidazolone said:
I have just gone through a similar process. You really need to look at how many watts you use per day and how much and at what time. Averages or monthly totals can be very deceiving. In my case, I did all the averages figured I needed 4 X 100ah batteries and put in 5 so I would have extra. After a week or so of actual usage, I realized that I needed at least 3 more. What was happening is that my batteries would fill up by 2pm or so but then my demand would jump in the early evening as the electric stove comes on and everyone decides to have a shower (it is a beach house so people come in and shower in the evening). That activity would use up 30-40% of the battery and then the AC would run all night and then some early risers would get up and use the stove, coffee maker, etc. before the sun got up. I was running out of battery sometime in the middle of the night. I also had a case when some folks decided to do laundry in the evening and that makes things worse. I would suggest just monitoring for a week or two and possibly look at changing some schedules (ie do laundry in the afternoon, if possible, etc.).
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Super helpful! Cheers ?
 
The good thing is that battery capacity can generally be expanded if the initial system is a bit light.
For fully off grid I would be at three days minimum. About 60 kWh.
 
time2roll said:
The good thing is that battery capacity can generally be expanded if the initial system is a bit light.
For fully off grid I would be at three days minimum. About 60 kWh.
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Heard. I won't be truly and completely off-grid more like grid adjacent or grid back-up.
 
time2roll said:
For fully off grid I would be at three days minimum. About 60 kWh.
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This.

I just built a 14kWh battery pack for about $3k. Don't forget the Federal Tax Credit (if you have enough tax liability).
Easy to put in parallel with another 14kWh battery pack. After the first pack (so you know what you are doing), consider Grade B cells.
Also plan for how you will place all the batteries (heavy duty shelving - these are heavy).

Rack batteries have nice enclosures available, but cost more ($9,000 for 6 EG4 in a rack, compared to around $6,000 DIY). Harder to "fix" a rack battery.
 
DIYrich said:
This.

I just built a 14kWh battery pack for about $3k. Don't forget the Federal Tax Credit (if you have enough tax liability).
Easy to put in parallel with another 14kWh battery pack. After the first pack (so you know what you are doing), consider Grade B cells.
Also plan for how you will place all the batteries (heavy duty shelving - these are heavy).

Rack batteries have nice enclosures available, but cost more ($9,000 for 6 EG4 in a rack, compared to around $6,000 DIY). Harder to "fix" a rack battery.
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Thanks for this great info!
 
Its a really difficult thing to nail down.

Your largest loads (should) be during the day which can hopefully be powered direct from solar, minimizing the overnight battery storage requirement.
But on the other hand, you cannot rely on getting your average daily requirements of solar, reliably every single day of the month.

It could be horribly gloomy for several days in a row, then several sunny days with more solar than you are able to store in your battery.

Even if you keep meticulous records over an entire year of careful monitoring and data logging before you buy anything, its still a judgement call.
It may be more economic to just accept that owning the worlds largest and most expensive battery is just not going to happen, and something a bit less, coupled to a gasoline powered battery charger may be more sensible.

That may only need to be run on a very few odd occasions during the very worst of mid winter, and the nuisance value may be far outweighed by the eventual cost saving.

In the end, all you can do is make a best guess, and from your own experience improve or upgrade as you get to know your system better.
 
rodrick said:
With the loads you expect the 19kwh would barely cover your daily usage, to cover you for a couple of days of low or no pv production I would be looking at 60kwh or find a way to cut my daily usage
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SOK 48v 100ah Just watched Will's most recent video on these and from what I understand you saying is that I would need 2 full racks of these (10x batteries) in order to hit that 50kwh (~60kwh). Seems like a lot of freaking battery storage if what you're saying is correct -- not doubting it is. Just new to the battery discussion.
 
It depends on how much PV you have. You listed 10x 400W, so 4kW. Expect to produce 24kWh or so in a day.
It also depends on how much power you draw during peak hours (4 to 9 PM?) and how much at night.

The goal is to use power harvested middle of the day to avoid paying high rates of peak times. Maybe also to backfeed during highest export rate times. If you produce enough (and don't use during the day), then use at night.

I think something between zero and 24kWh could be optimum. Assuming you're not trying to survive multi-day grid failures with poor sun.
 
TheHappyNomads said:
SOK 48v 100ah Just watched Will's most recent video on these and from what I understand you saying is that I would need 2 full racks of these (10x batteries) in order to hit that 50kwh (~60kwh). Seems like a lot of freaking battery storage if what you're saying is correct -- not doubting it is. Just new to the battery discussion.
Click to expand...
The rule of thumb is for an off-grid battery to provide 3 days of power. That accounts for a few days of rain or gloomy days (or Canadian wild fires). You said max 600kWh/mo, which is about 20kWh/day, or 60kWh for 3 days.

The added benefit is that with 60 kWh of batteries, you can have peak power of 250 amps at 240v, assuming your inverter can deliver.

The battery also qualifies for the 30% federal tax credit.

One option is to use a few rack batteries to get permitted, and DIY the rest.

Otherwise, a cheap generator and chargeverter can recharge your battery. Probably want a propane tank for fuel.
 
I suppose it depends on the size of your thumb !
Batteries will be by far the most expensive part of your system, and for most of us a HUGE obstacle to overcome.

Probably practical to get started initially with something affordable, and include a backup power source, grid, gas/propane/diesel generator, or whatever.
Then just live with it for a while and see how it all goes.

After a year or two, you can then decide which suits you best, spending a lot more on a much larger battery, or just accepting that on a few occasions the generator needs to be started up. Nobody can really advise you.
It depends on your own particular needs, preferences and resources.
 
Thank you all for your helpful responses. I should be/should have been more clear. Our house is located in the heart of the SF Bay Area and so I'm not really able to/trying to be 100% off grid and sufficiently detached from the grid. My main goal is to use as much solar as possible to minimize my electric bills as much as possible. Our water heater and small living room fireplace insert are natural gas. I would love to replace the water heater tank with an on-demand system in the future since we only have one bathroom. After doing some research, it seems to me that it doesn't make sense to connect to the grid with the NEMA 3.0 contract in place for newcomers to the solar game. Plus, it affords the option to use non-UL listed inverters like the LV6548 and other components that I don't need/can't permit. Currently, we pay more for electric delivery services to our house from PG&E than the actual amount of electricity used. Hope this makes more sense and I apologize for the lack of clarity that led to generator/backup comments even though I greatly appreciate them.
 
Permit is separate from utility permission. Permit is by the municipality for safety. Permission to operate is by the utility to make sure you don't risk disturbing your neighbors and the grid.

UL listing is normally a Permit issue. If you touch the grid, then it might be a utility issue. If there is no way to backfeed the grid, then you may be able to avoid PTO.

A grid connected battery charger only, with the inverter only connected to the battery, is one way to do it.
 
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