Key takeaways
- Choose critical-load or whole-home backup scope before selecting capacity.
- Convert each appliance into watts and watt-hours instead of relying on the utility bill total.
- Check inverter surge and battery discharge power as well as energy.
- Model recharge after an outage, not only discharge duration.
1. Set the backup boundary
Critical-load backup supports a selected circuit group, while whole-home backup must support the coincident demand and surge of a much larger load set. The two scopes should not share the same sizing assumption.
- List essential, deferrable, and prohibited backup loads.
- Record the largest motor, compressor, pump, or heating surge.
- Decide which loads can be shed automatically during an outage.
2. Build a 24-hour load table
For each load, multiply operating watts by expected hours to estimate energy, but keep continuous and surge watts in separate columns. The battery must meet both the power and energy requirement.
- Load name and backup priority
- Continuous watts and starting surge
- Expected daily hours or duty fraction
- Daily watt-hours and outage behavior
3. Convert load energy into battery energy
Required nominal energy is greater than the load's watt-hours when reserve, the permitted state-of-charge window, inverter losses, temperature, and project aging assumptions are included.
- Request nominal and usable battery energy.
- Confirm the inverter efficiency assumption at the relevant load.
- Include a reserve for critical loads and uncertain outage length.
- Check maximum discharge power at the selected battery count.
4. Test solar and grid recharge
A battery that can cover the outage may still be poorly sized if the available solar or grid charge power cannot restore it before the next operating period.
- Model seasonal photovoltaic production rather than one annual average.
- Reserve daytime energy for live loads before assigning surplus to charging.
- Check inverter charge power and the battery's allowed charge rate.
- Define behavior when solar remains low for consecutive days.
