Battery runtime is an energy balance: usable stored energy is divided by the load, with efficiency and reserve assumptions made visible.
The runtime formula
Runtime in hours = battery energy × efficiency ÷ load power. Convert watts to kilowatts before dividing. A 10 kWh battery at 90% efficiency supporting a 500 W load provides 10 × 0.90 ÷ 0.50 = 18 hours in the simplified model.
Average load matters
A refrigerator, pump, or HVAC system cycles. Use an average measured load for energy planning and check starting surge separately. A nameplate running wattage is not always the same as a full-day average.
Reserve and degradation
Usable capacity may exclude a reserve, and usable energy declines with temperature, age, and operating conditions. Run a conservative case rather than treating the nominal label capacity as guaranteed output.
Power can end the calculation early
Runtime is irrelevant if the inverter cannot support the instantaneous load. Check continuous and surge ratings, load priority, transfer time, and wiring with the system documentation.
Sources
The figures and rules in this guide are checked against:
- BU-503: How to Calculate Battery Runtime — Battery University (Cadex Electronics)
- BU-808: How to Prolong Lithium-based Batteries — Battery University (Cadex Electronics)
- UPS Selector (runtime by load) — Schneider Electric / APC
- Electricity Explained: Energy Storage for Electricity Generation — U.S. Energy Information Administration