Solar battery calculator
Pick essentials or whole-home backup and see the battery capacity you would need, worked out from your own bill.
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Start with what you want to keep running
Battery size is a decision about outages, not about panels. Decide first whether you want a few essentials kept alive for a day or the whole house, because the two answers differ by a large factor. The tool offers both with a switch under your results.
The formulas, with the constants
- essentials battery = 10 kWh a day x 1 day / 0.9 depth of discharge
- whole-home battery = average daily kWh / 0.9
- average daily kWh = annual kWh / 365
The 10 kWh a day for essentials is a planning figure, not a measurement of your house. If you only need a fridge, internet and lights, your real number may be lower; adding well pumps, medical gear or air conditioning raises it fast. The 0.9 depth of discharge leaves a margin so the battery is not run flat every time.
A worked example
The average Florida home uses 1,104 kWh a month (EIA, 2024 residential average), or 36 kWh a day. Essentials-only backup comes to 11.1 kWh of battery. Whole-home backup for one day comes to 40.3 kWh, which is 3.6 times larger. That gap is why most households back up essentials only.
What the tool does not model
Real batteries have a maximum power output, which limits how many appliances you can run at once. A large air conditioner or well pump needs a surge many times its running load. Inverter ratings, wiring and the battery's round-trip losses also matter. This calculator sizes energy storage in kilowatt-hours only, so take the number to an installer and ask them to check power output against your actual loads.
Backup is not the same as savings
Batteries rarely pay for themselves through bill savings in places with flat rates, and since the homeowner federal credit has ended, the case rests on resilience and on your local rate structure. We list parts with affiliate links so you can compare prices; see the disclosure on this site. Prices change, so we show none here.
Planning estimate. Permits, interconnection approval and a licensed electrician are required for grid-tied systems.
Questions
How big a battery do I need for backup?
For essentials such as the fridge, lights, internet and a few outlets, a planning figure is about 10 kWh a day, which the tool turns into roughly 11 kWh of battery. A whole home needs your full daily average, which is often several times larger.
What is depth of discharge?
It is how much of a battery's rated capacity you can use without shortening its life. The tool assumes 90 percent, so it divides the energy you need by 0.9 to get the rated size.
Will a battery lower my electric bill?
Only in some cases. A battery mainly gives backup. It saves money when your utility charges much more at some hours than others or pays little for exported power. Check your rate plan before paying for storage you do not need.
Does the battery change the number of panels?
Not in grid-tied backup mode, where the grid still covers the gaps. In off-grid mode the array and battery are sized together, which the off-grid calculator on this site handles.
Can I install a battery myself?
Small portable stations plug in and need no wiring. A battery connected to your home's electrical system is electrical work that needs a licensed electrician, a permit and often utility approval.