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Battery Life Calculator: How Long Will a Battery Last
Enter the battery capacity and voltage, the load that it powers, how much of the capacity you will actually use and the efficiency of any inverter or converter. The calculator estimates the runtime.
The runtime calculation
Battery capacity is energy stored. A rating in amp-hours becomes watt-hours when multiplied by the nominal voltage: Wh = Ah × V. A rating in milliamp-hours is divided by 1,000 first. The load is also converted to watts, with W = A × V if you know the current. Runtime is then the usable energy divided by the load power:
hours = capacity (Wh) × depth of discharge × efficiency ÷ load (W)
A 100 Ah, 12 V battery stores 1,200 Wh. If only 50 percent may be used, 600 Wh is available, and a 100 W load would run for six hours when no other losses apply.
Depth of discharge and efficiency
Depth of discharge is the share of the rated capacity you are prepared to use. Some battery chemistries tolerate deep discharges better than others, and cycling a battery very deeply can shorten its life. The maker's datasheet gives the recommended limit, so use that rather than a generic figure. The 80 percent default here is only a starting point.
Efficiency covers the loss in an inverter, regulator or converter between the battery and the load. If a 12 V battery feeds a mains inverter, a figure of 85 to 95 percent is common, but check the inverter specification. Direct 12 V loads need no inverter, so enter 100.
Why real runtime is often shorter
- High discharge rates. Many batteries deliver less than their rated capacity when drained quickly. This is often described by Peukert's effect for lead-acid types.
- Cold temperatures. Capacity falls in the cold, sometimes sharply.
- Ageing. Capacity declines with cycles and years.
- Varying loads. Motors and compressors draw surges, and devices cycle on and off. Use an average figure for the load.
- Voltage cut-off. Devices may shut down before the battery is empty.
For a safer estimate, lower the depth of discharge, reduce the efficiency a few points and enter the highest average load you expect. Then compare the answer with a short real test: run the load for an hour and note how far the state of charge falls. Measured data beats any formula, and a simple log of voltage over time also shows how the battery ages from season to season.
Estimating and safety
This tool is for estimating and learning. Batteries store a lot of energy and can overheat, vent gas or ignite when overloaded, short-circuited, overcharged or damaged. Wiring, fusing, charging and ventilation for battery banks must follow the maker's instructions and the local electrical codes, and a qualified electrician or engineer should design and inspect anything beyond small consumer devices. Never use a runtime estimate as the only basis for a life-safety or medical backup system.
Worked example
A 12 V, 100 Ah battery running a 100 W load through an inverter that is 90% efficient, using at most half of the capacity.
Inputs
| Battery capacity | 100 |
|---|---|
| Capacity unit | Amp-hours (Ah) |
| Battery voltage | 12 V |
| Load | 100 |
| Load unit | Watts (W) |
| Usable capacity (depth of discharge) | 50 % |
| Efficiency of inverter or converter | 90 % |
Result
About 5.40 hours from 540.0 Wh of usable energy.
| Stored energy | 1,200.0 Wh |
|---|---|
| Usable energy after depth of discharge and efficiency | 540.0 Wh |
| Load power | 100.000 W |
| Current at battery voltage | 8.333 A |
| Runtime | 5.40 hours |
- Real batteries deliver less capacity at high currents, in the cold and as they age. Treat the result as a best-case planning figure.
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
How do I calculate battery life?
Convert capacity to watt-hours, multiply by the usable share and efficiency, and divide by the load in watts. The result is the runtime in hours.
How do I convert amp-hours to watt-hours?
Multiply the amp-hours by the battery voltage. A 12 volt battery of 100 amp-hours stores 1,200 watt-hours of nominal energy.
How long will a 100Ah battery run a 100W load?
A 12 V, 100 Ah battery holds 1,200 Wh, so a 100 W load would run for 12 hours at full discharge. With 50 percent usable capacity it runs for about six hours.
Why should I not use 100 percent of the battery?
Deep discharge can shorten the life of many battery types and can cause low-voltage shutdowns. The datasheet gives the depth of discharge the maker recommends.
Does this work for phone and laptop batteries?
Yes. Enter the capacity in mAh or Wh from the label, the nominal voltage, and the average power use. Screen brightness and workload change the real value a lot.
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References and sources
- Battery makers publish rated capacity, discharge curves and recommended depth of discharge on datasheets; those figures take priority over generic defaults.