Twelve, 24 and 72 hours of autonomy are different operating scenarios. Start with critical functions and an energy budget, not a battery label.
Priorities
Separate water, communications, ventilation, refrigeration and essential lighting from deferrable loads. The list depends on occupants and installed systems.
Three time horizons
A 12-hour interruption may allow some tasks to be deferred. A 24-hour scenario needs a daily balance; a 72-hour scenario also needs recharge, supplies and operating limits.
kW and kWh
Power determines simultaneous operation; energy determines duration. An assumed constant 0.20 kW load needs 2.4, 4.8 and 14.4 kWh for 12, 24 and 72 hours respectively, before losses.
Starting power and usable capacity
Pumps and compressors may have starting demands that need documentation or measurement. Assess capacity using allowed depth of discharge, losses and BMS limits.
Solar and generator support
Annual PV yield does not guarantee recharge during a particular three-day winter outage. Generator support requires safe connection, fuel and outdoor siting with exhaust kept away from building openings.
Controls and calculator limits
Define start sequencing and shedding of non-critical loads. The calculator estimates an energy budget; it does not verify protection, selectivity, compatibility or safe grid transfer.
| Item | Parameter | Check / result |
|---|---|---|
| 12 h | 0.20 kW × 12 = 2.4 kWh | Before losses |
| 24 h | 0.20 kW × 24 = 4.8 kWh | Before losses |
| 72 h | 0.20 kW × 72 = 14.4 kWh | Before losses |
What this means for your project
List loads with power, operating hours and starting data. Then coordinate the battery, inverter, protection scheme and backup testing.
Sources and scope of review
- US DOE — Solar and resilience basics
- US DOE — Solar integration: energy and storage basics
- CPSC — Generator and carbon monoxide hazards, 2026