Enter Values
Formula
Runtime (hours) = V × Ah × efficiency ÷ load watts
This is an estimate. Actual runtime varies with battery chemistry, age, temperature, and discharge rate.
Example
A 12 V, 100 Ah battery supplying 200 W at 85% efficiency has an estimated runtime of 5.10 hours.
What the Estimate Includes
Multiplying nominal voltage by amp-hour capacity gives nominal watt-hours. The efficiency input reduces that energy to account for inverter, wiring, and conversion losses before dividing by the load watts.
For an AC appliance powered through an inverter, use the appliance's actual input watts and include inverter losses in the efficiency value.
Why Actual Runtime Can Differ
- Lead-acid capacity decreases at high discharge rates due to the Peukert effect.
- Cold temperature, battery age, cell imbalance, and cable losses reduce available energy.
- Many batteries should not be discharged through their full nameplate capacity.
- Loads may cycle or draw startup power that is much higher than normal operation.
Use the battery manufacturer's usable-energy, discharge-rate, and minimum-state-of-charge guidance for a design estimate.