How to size a bank
There’s a clean formula for sizing battery storage. It just requires honesty about how much energy you actually use and how much of a battery’s nameplate capacity is real.
Add up your daily watt-hours. Multiply by days of autonomy. Divide by system voltage to get amp-hours of raw energy. Then divide again by efficiency and usable depth-of-discharge to get the nameplate capacity you actually need.
Ah_needed = total_Wh ÷ (V × efficiency × usable_DoD)
Frequently asked
Why divide by efficiency?
Because the energy that leaves your battery isn’t the energy that reaches your load. An inverter eats 5–15% as heat. DC-DC conversions, wiring losses, and BMS overhead all add up. Sizing for 100% efficiency gives a system that comes up short.
What’s a good safety margin?
Round up to a stock battery size - most pros round to the next 50 or 100 Ah. Hard-to-replace systems (boats, remote cabins) often spec 1.5–2x the calculated minimum to handle aging and unexpected loads.
What about temperature?
Capacity drops in cold weather. LiFePO₄ loses 10–15% at freezing, 30% below 0°F. Lead-acid is worse. If your system sees winter, account for it explicitly or oversize.