Start with the loads that must run during an outage. A 5 kW inverter does not mean you need 5 kWh of battery, and a 5 kWh battery does not necessarily deliver 5 kWh of usable AC energy. Usable capacity is reduced by permitted depth of discharge, conversion losses, temperature and battery ageing.
Build the load-hours worksheet
| Load | Watts | Quantity | Hours | Wh |
|---|---|---|---|---|
| Fans | W each | n | h | W × n × h |
| Lights | W each | n | h | W × n × h |
| Refrigerator | Use measured/estimated duty cycle | 1 | h | Average W × h |
| Router/laptop | W | n | h | Total |
Add the watt-hours, then divide by expected inverter/system efficiency and by the usable fraction of nominal battery capacity. Finally add an ageing/reserve margin. For example, if essential loads need 3,000 Wh and you assume 90% conversion efficiency and 80% usable depth of discharge, the theoretical nominal requirement is roughly 3,000 ÷ 0.9 ÷ 0.8 = 4,167 Wh before ageing reserve.
Power and energy are different constraints
The battery must supply enough energy for the required hours, while the inverter and battery must also support the instantaneous power and surge of loads such as refrigerators, pumps or motors. A battery can have enough kWh but still fail if the system cannot deliver the startup surge.
Do not size from one perfect-day solar assumption
If the battery is meant for outage resilience, decide what happens during cloudy weather and multi-hour grid failures. Separate “daily solar self-consumption” from “emergency reserve”. A system optimised to discharge deeply every evening may have little backup left when the grid fails at night.
Check the safety and warranty envelope
Use equipment and installation practices consistent with manufacturer specifications and applicable Indian electrical/safety requirements. Consult current guidance from MNRE, the Central Electricity Authority and relevant BIS standards. Battery chemistry, ventilation, protection, earthing and installation location are safety decisions, not only efficiency choices.
Decision rule: size from essential Wh, surge W, usable depth of discharge, efficiency and ageing reserve. If a vendor cannot show all five assumptions, the quoted battery size is not yet auditable.