Large DIY Battery Bank for a House

[nick4673]

You really need no more than 60kWh battery if your peak use is 40kWh only on worst days. This would account for 20% calendar aging and 20% reserve (10/90) for longevity. With your power needs <10kW you can still stay in 48V ( actually 51V with 16S LFP cells ) range.
So, you'd need no more than 1200AH at 51V, which can be done with either 200AH cells in 16S6P ( 96 cells ) or 300AH cells in 16S4P ( 64 cells ).
It is a massive battery and I would consider reducing the need for AC in some other ways if possible ( insulation, whole house fan, etc ) since AC is the killer app. However, this is doable if money is not a problem. I have seen such systems done a few times.
You will need enough solar panels to charge the battery, I'm guessing at least 10kW-15kW array, depending on your geo location.

40Kwh is an average. My peak was 70kwh. I’m in South Carolina in a brand new house. My house is on 75 in the summer with all of the fans going. If I go any warmer the Mrs. will have a fit. With being in SC solar is better than average so that’s a plus. The house is really well insulated. I was in a condo half the size of the house 5 mins down the road and my summer electric bills were a lot higher at least 50%. Same thermostat settings.

 

[electric]

Balance between battery size and solar size is a critical thing to figure out. At peak AC load the solar production is also at peak, so battery is not used to power loads during the day, but must be charged during the day to provide power at night. I don't have a formula for this, it's been the most difficult balance to calculate for off grid systems. You need enough solar to power loads and charge the battery during day hours and enough battery to power loads during night hours, and also consider seasonal changes and rainy/cloudy days. So many variables my head spins.
However, solar is usually limited by available south/west facing roof, so you can maximize that up front. The battery can always be increased later, that is the easiest part, just add more cells. Usual rule of thumb is battery kWh should be 5x of max solar kW output. Tweak this up/down based on your specific needs.

 

[SCClockDr]

Thank you all for responding to my post, but I haven’t received replies to my original question. If you were asked to design an 100KwH battery bank using LiFePO4 cells, which size cells would you use? 100ah seems to be the most readily available, but I have also seen plenty of higher ah rated cells.

I did address your question, get the largest capacity cells readily available to your market. Add to your cell count 4 additional cells supporting an autonomous lighting system. Those would be donor cells should the battery need a cell down the road.

Added:
My earlier post, #3.