Solar Battery Backup Sizing: Hours, Load, and Cycle-Life Worksheet
Battery sizing starts with the loads you actually need during an outage. Backing up the entire home can multiply cost and inverter size without improving resilience. Separate essential circuits first.
Core calculation
Required usable AC energy (kWh) = Σ(load watts × backup hours) ÷ 1000. Then adjust for inverter losses and the battery’s permitted usable depth of discharge. Add margin for ageing and surge, but stay within the inverter and battery manufacturer’s supported design.
| Load | Watts | Hours | Wh needed |
|---|---|---|---|
| Lights | Measured total | Desired backup | W × h |
| Fans | Measured total | Desired backup | W × h |
| Router / electronics | Measured | Desired backup | W × h |
| Refrigerator or motor load | Running + surge | Duty cycle estimate | Use realistic cycling |
Size the backup from loads to battery
Do the energy math before choosing a battery brand or chemistry.
List essential loads. Record running watts, starting surge and how many hours each must operate.
Calculate usable AC energy. Sum watt-hours and convert to kWh.
Account for system losses. Divide by realistic inverter efficiency and allowed depth of discharge.
Check power, not just energy. Confirm inverter continuous and surge ratings can start the largest simultaneous loads.
Add ageing and service margin. Size within manufacturer limits and plan for capacity fade, temperature and future replacement cost.
Cycle life is a usage assumption
A cycle-life claim only makes sense with depth of discharge, temperature and warranty conditions. Ask how the warranty defines retained capacity, eligible usage and replacement remedy instead of comparing one headline cycle number.
Backup-energy worksheet
Required usable AC energy (kWh) = sum of essential load watts × backup hours ÷ 1000. Divide by inverter efficiency and permitted battery depth of discharge, then add surge and ageing margin. Separate essential circuits; backing up every household load can make storage and inverter requirements unnecessarily large.
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