Home Battery Outage Runtime & Solar Refill Calculator
Estimate outage survival time, model solar recharge cycles, and plan load priorities.
Last reviewed: June 2026Battery System
Solar (optional)
Loads
Outage Runtime Results
72-Hour Battery SOC Simulation
How the Home Battery Runtime Calculator works
This calculator estimates outage runtime from usable battery energy, enabled loads, inverter rating, and optional solar refill. The battery side uses usable kWh = battery capacity x number of batteries x allowed depth of discharge x round-trip efficiency. The load side converts each enabled load to daily kWh from watts and hours per day.
Runtime is then usable kWh / average load kW. Critical-only runtime repeats the same calculation using only loads marked critical. The 72-hour simulation applies hourly consumption and, if solar is enabled, adds solar production during the selected solar window.
Battery capacity vs inverter power
Capacity and power are different constraints. A 13.5 kWh battery can store energy, but the inverter determines how many watts can be served at once. A refrigerator, lights, router, and medical device may fit easily. A central air conditioner, electric oven, well pump, EV charger, or resistance heater may exceed inverter capacity or require a surge rating that this simple calculator does not model.
The inverter warning checks continuous wattage, not motor-starting surge. For pumps, compressors, and HVAC equipment, use manufacturer locked-rotor or starting-watt data and have the system designed by a qualified installer.
Solar refill assumptions
Solar refill is estimated as solar kW x peak sun hours x derate factor. Derate accounts for inverter losses, wiring losses, module temperature, soiling, shade, and other losses. Real production varies by roof orientation, season, cloud cover, wildfire smoke, snow, shading, and whether the system is configured to operate while islanded from the grid.
The U.S. Department of Energy notes that ordinary grid-tied solar generally shuts down during a grid outage for safety unless paired with a properly configured inverter and storage system. Do not assume rooftop solar alone will recharge a home battery during an outage.
Worked example
One 13.5 kWh LFP battery at 90% round-trip efficiency provides roughly 12.2 usable kWh in this model. If enabled loads average 6 kWh/day, the battery lasts about two days without solar. If a 5 kW solar array produces 5 peak sun hours at a 77% derate, the model adds about 19.25 kWh/day during the solar window. That can turn a short outage plan into a much more resilient one, but only if the inverter, transfer equipment, interconnection, and load panel are configured correctly.
Safety and planning notes
- Use load priorities: disable optional and luxury loads before increasing battery size.
- Check power limits: inverter watts and surge watts can matter more than kWh for large appliances.
- Plan for weather: storms that cause outages may also reduce solar production.
- Keep reserve: many systems should not be planned to drain to zero.
- Follow codes: storage systems need compliant installation, labeling, disconnects, and fire-safety clearances.