Battery Life Calculator
Estimate how long a battery will run a load, from its capacity in mAh, Ah or Wh and the load in milliamps, amps or watts.
- Run time
- Usable capacity
- Load current
- Load power
- In days
- Energy Used
Saved Batteries
Items you add are saved in this browser.
| Name | Capacity | Load | Run Time | Remove |
|---|
How to Estimate Battery Life
Battery capacity is usually given in milliamp-hours (mAh) or amp-hours (Ah): how many amps the battery can supply for one hour. Divide the usable capacity by the load current to get the run time in hours. If the capacity is in watt-hours or the load is in watts, convert with the battery voltage (watts = volts × amps).
Worked Example
A 2,500 mAh battery powering a 250 mA load would last 2,500 ÷ 250 = 10 hours if every bit of capacity were usable. At 85% usable, that's 10 × 0.85 = 8.5 hours. A 100 Ah, 12 V deep-cycle battery running a 60 W load draws 60 ÷ 12 = 5 A; at 50% usable that's 100 × 0.5 ÷ 5 = 10 hours.
Run Time at Common Loads
| Battery | 100 mA | 500 mA | 1 A | 5 A |
|---|
85% of rated capacity usable.
Tips
Real run time is usually shorter than the simple estimate. Batteries deliver less at high currents and in the cold, and lead-acid batteries in particular lose effective capacity at high draw (Peukert's effect). Inverters and voltage converters also waste 5% to 15%. Use the usable-capacity setting to leave a margin and protect battery life.