Tell us your backup load and how many days of autonomy you want. We'll size the battery bank in kWh and modules.
Lead-acid needs a bigger rated bank than lithium for the same usable energy, because of its lower safe depth of discharge.
Further reading: Solar Battery Storage 101: Sizing, Cost, and Whether It's Worth It
The gap between "usable" and "rated" capacity is what trips people up when comparing battery quotes. A 10 kWh lead-acid bank might only deliver 4.5 kWh of real, safe backup — while a 10 kWh LiFePO4 bank delivers close to 9 kWh. Always compare usable capacity, not the sticker number.
Multiply your target daily backup load (kWh) by how many days of autonomy you want, then divide by depth-of-discharge and round-trip efficiency — for most grid-tied homes backing up essentials only, that lands around 8-15 kWh usable; for whole-home or multi-day off-grid autonomy, plan on 20 kWh or more. Enter your own load and autonomy above for an exact figure instead of a rule of thumb.
Whole-house backup multiplies cost fast. Most homeowners back up a dedicated "essentials" circuit — fridge, wifi, some lights and outlets — which is what the default 10 kWh/day estimate assumes.
1 day covers typical grid outages. If you're in a hurricane, wildfire or extreme-weather zone with multi-day outages, 2-3 days is a safer target — the calculator scales linearly.
Yes — "battery size calculator," "battery bank size calculator," "solar battery calculator," and "solar battery size calculator" all describe the same usable-capacity-vs-rated-capacity math used here, whether the battery is a single lithium unit or a bank of several lead-acid batteries wired together.
Storage solar battery sizing (whichever word order you search it as) always comes back to the same two numbers: how much energy you need to store per day, and how many days you want to cover without solar input — this calculator turns both into a usable-kWh and rated-kWh figure you can shop against real battery specs.