Battery Runtime Calculator
Find how long a battery or battery bank runs a load, from its volts, amp-hours, usable depth of discharge and efficiency.
Battery runtime
4:48:00
- Hours
- 4.8
- Battery energy
- 1,200 Wh
- Usable energy
- 960 Wh
The battery runs this load for about 4.8 hours.
Calculation breakdown
| Step | Formula | Result |
|---|---|---|
| Battery energy | 12 V × 100 Ah | 1,200.0 Wh |
| Usable energy | 1,200 Wh × 80% × 100% | 960.0 Wh |
| Runtime | 960 Wh ÷ 200 W | 4:48:00 |
How it works
Enter the battery and the load. Open the advanced options for series and parallel banks.
Worked example
A 12 V, 100 Ah battery running a 200 W load, 80% depth of discharge, 100% efficiency:
- Energy: 12 × 100 = 1,200 Wh
- Usable: 1,200 × 0.80 = 960 Wh
- Runtime: 960 ÷ 200 = 4.8 hours (4:48:00)
Sources
Frequently asked questions
How do you calculate how long a battery will last?
Multiply the battery's volts by its amp-hours to get watt-hours, apply the depth of discharge and efficiency, then divide by the load in watts. A 12 V 100 Ah battery holds 1,200 Wh, so at 80% depth into a 200 W load it runs 4.8 hours.
What is depth of discharge?
It is how much of the battery's capacity you let the load use. Battery University shows lithium-ion lasts many more cycles at shallow depths, for example about 400 cycles at 80% for NMC against about 6,000 at 10%. Enter your own limit.
How do batteries in series and parallel add up?
Batteries in series add their voltages and keep the same amp-hours. Strings in parallel add amp-hours at the same voltage. Either way the total watt-hours are volts times amp-hours times the number of batteries.
How do I find the load in watts or amps?
Use the label on the device, or watts = volts times amps. A load in amps is converted at your battery bank's voltage. For an AC load running from an inverter, enter the inverter's efficiency, since it uses some of the energy.
Does this match the watt-hour calculator?
It uses the same watt-hour maths. This page adds series and parallel banks, a load in amps, depth of discharge and efficiency to give a more realistic runtime. The watt-hour calculator is the quick version.
An estimate. Real runtime also depends on temperature, battery age, discharge rate and load changes, and Battery University gives no single depth of discharge to use, so check your battery's data sheet.