Off-Grid Battery Runtime Calculator
Estimate usable battery energy and runtime after discharge and efficiency limits.
What this calculator shows
Voltage multiplied by amp-hours gives nominal watt-hours. The calculator applies the selected depth of discharge and efficiency, subtracts steady solar contribution from load, then estimates runtime.
How to use it
Use the complete bank voltage and amp-hour rating, not one battery’s figures when the bank changes them. Choose a safe depth of discharge for the battery chemistry and enter average continuous load rather than appliance nameplate maximums where measured data is available.
Useful examples
- Estimating whether a caravan battery can run a fridge overnight.
- Testing the effect of turning off a heater, pump or inverter load.
- Comparing runtime at different battery protection limits.
Important limits
Temperature, battery age, wiring losses, inverter standby use, variable loads and battery-management cut-offs change real runtime. Do not use this estimate to bypass manufacturer limits or electrical advice.
Nominal and usable capacity differ
A 24-volt 200-amp-hour bank is nominally 4,800 watt-hours. At 80% discharge and 90% combined efficiency, about 3,456 watt-hours are available to loads in this simplified estimate. Preserving a reserve generally improves reliability and may extend battery life.
Worked example
With a steady 400-watt load and no solar input, 3,456 usable watt-hours lasts about 8.6 hours. If daylight solar contributes a steady 200 watts, the net battery load becomes 200 watts and the theoretical runtime doubles while that input continues.