Battery sizing guide
How to size a solar battery bank
Battery sizing starts with energy you need to deliver, not the number printed on the battery label. This guide follows the calculation from measured loads to a whole-battery layout.
1. Build a daily energy total
Use watt-hours, not watts, for the first step. A 100W appliance that runs for 3 hours uses 300Wh. Add each appliance after accounting for quantity and run time. If you have a reliable energy monitor or an electricity bill, that total is usually better than a long list of guesses.
2. Choose the period without charging
Days of autonomy multiply the daily energy requirement. One day at 4kWh needs 4kWh delivered. Two days need 8kWh. This choice is a planning scenario, not a claim that weather or charging will follow the average.
3. Account for usable capacity and losses
A battery bank cannot usually deliver every rated watt-hour under every condition. The sizing formula divides by depth of discharge, inverter efficiency, and battery efficiency. Each percentage must enter the formula as a decimal: 90% becomes 0.90.
nominal Wh = daily Wh × autonomy ÷ (DoD × inverter efficiency × battery efficiency)
Add a reserve only after the loss-adjusted nominal result. This keeps the reserve visible instead of burying it inside an unexplained efficiency factor.
Worked example
Suppose a small off-grid system must deliver 4,200Wh per day for 1.5 days. Use 90% depth of discharge, 92% inverter efficiency, 95% battery efficiency, and a 10% reserve.
- Delivered energy:
4,200 × 1.5 = 6,300Wh. - Combined usable factor:
0.90 × 0.92 × 0.95 = 0.7866. - Nominal before reserve:
6,300 ÷ 0.7866 = 8,009Wh. - Design capacity:
8,009 × 1.10 = 8,810Wh. - At 24V:
8,810 ÷ 24 = 367Ah.
If the chosen module is 12V 200Ah, two are needed in series for 24V. Two parallel strings raise the bank to 400Ah, so the layout is 2S × 2P, four batteries total, with 9.6kWh nominal.
Check the layout, not just the kWh
A kWh result can hide a voltage mismatch. The system voltage divided by one module's nominal voltage must be a positive whole number. Parallel strings then raise Ah. Recalculate installed capacity from the integer layout and compare it with the design target.
Run the numbers in the solar battery calculator, check an existing bank with the runtime calculator, or draw the topology in the series-parallel calculator.
What this calculation does not decide
It does not select protection devices, cable size, busbars, an inverter, a charge controller, ventilation, or an enclosure. Battery current limits, temperature, BMS behavior, short-circuit protection, and installation rules still need equipment-specific engineering.
Sources and further checks
- NREL PVWatts for location-based solar production checks in the United States.
- SolarMathKit methodology for the exact formula, preset status, and rounding policy.