Battery Bank Size Calculator
Size your off-grid battery bank the right way. Get amp-hours, series/parallel configuration, battery count, and total cost based on your daily use, autonomy, and battery chemistry.
How It Works
A battery bank stores energy for use when your solar panels or generator aren't producing. To size it correctly, you account for your daily energy use, how many days of backup you want (autonomy), how deeply you can discharge the batteries (chemistry-dependent), and losses through the inverter and cold temperatures.
Why Chemistry Matters
Flooded Lead-Acid & AGM (50% DoD): You can only use half the rated capacity before damaging the battery. This means you need twice the rated capacity of your actual needs. They're cheaper upfront but heavier and shorter-lived (500–1,500 cycles).
LiFePO4 Lithium (80% DoD): You can use 80% of rated capacity. They cost more upfront but are lighter, last 3,000–6,000 cycles, and don't need maintenance. For long-term off-grid use, LiFePO4 is often cheaper per cycle.
Why Higher Voltage Systems Use Fewer Batteries
A 48V system needs 1/4 the amp-hours of a 12V system for the same watt-hours (since Wh = V × Ah). Higher voltage means lower current, thinner wires, and less energy lost as heat. Most whole-home off-grid systems use 48V.
Tips for Battery Bank Sizing
Oversize slightly: Round up on parallel strings. It's better to have a little extra capacity than to run short on a cloudy week.
Mind the charge rate: Lead-acid should charge at 10–13% of Ah capacity; LiFePO4 can handle 0.5C (50% of Ah). Make sure your charge controller and panels can deliver enough current.
Keep batteries warm: Lead-acid loses capacity fast below 32°F. Insulate or heat the battery box. LiFePO4 handles cold better but shouldn't be charged below freezing.