How Long Can Backup Power Last?
Estimate backup runtime for batteries and generators using load, capacity, fuel and recharge assumptions.
Battery runtime
A rough estimate starts with usable watt-hours divided by average load in watts. Real runtime will be lower because of inverter losses, reserve settings, temperature and battery condition.
Generator runtime
Runtime depends on fuel consumption at the actual load and the amount of fuel available. Manufacturer runtime figures may be stated at a particular percentage of rated load.
Solar extension
Solar can extend battery runtime when production exceeds part of the load, but daily energy harvest varies with weather, season, shading and panel orientation.
Reduce the load
The cheapest way to extend runtime is often to power fewer devices, use efficient lighting, cycle appliances and avoid unnecessary resistive heating.
Plan for uncertainty
Keep margin rather than designing to the exact theoretical number. Outages and real-world loads rarely follow perfect assumptions.