You have a battery, or a portable power station, and you want to know how long it will keep your fridge, TV or lights running. The answer takes one short formula. This guide shows how to work it out, with a runtime chart for a common 12 V 100 Ah battery.
The formula
- Battery Wh: battery voltage x amp-hours. A 12 V 100 Ah battery holds 12 x 100 = 1,200 Wh.
- Usable share: the part of the battery you can safely use. About 0.9 for LiFePO4 and about 0.5 for lead-acid.
- Inverter efficiency: usually 0.85 to 0.95 when you power AC appliances. Use 1 for devices that run directly on DC.
- Load watts: the combined watts of everything running at the same time.
Not sure of your appliance watts? Our guide on calculating appliance watt-hours explains how to find them.
Worked example
A 12 V 100 Ah LiFePO4 battery powers a 100 W TV through an inverter that is 90% efficient.
- Battery energy: 12 x 100 = 1,200 Wh.
- Usable: 1,200 x 0.9 = 1,080 Wh.
- After inverter loss: 1,080 x 0.9 = 972 Wh.
- Runtime: 972 / 100 = about 9.7 hours.
Runtime chart
For a 12 V 100 Ah battery with a 90% inverter:
| Appliance | Average watts | LiFePO4 (972 Wh usable) | Lead-acid (540 Wh usable) |
|---|---|---|---|
| LED light | 10 W | about 97 hours | about 54 hours |
| Router and modem | 15 W | about 65 hours | about 36 hours |
| Laptop | 60 W | about 16 hours | about 9 hours |
| TV | 100 W | about 9.7 hours | about 5.4 hours |
| Fridge (150 W, runs about 35% of the time) | about 52 W average | about 18 hours | about 10 hours |
| TV + laptop + 1 light together | 170 W | about 5.7 hours | about 3.2 hours |
| Microwave | 1,200 W | about 0.8 hours | about 0.45 hours |
The lead-acid column assumes a 50% usable share. Treat these as estimates. Real results depend on battery age, temperature and the actual draw of your device.
Try the battery runtime calculator
Pick your battery type, enter its voltage and capacity, and add the appliances you want to run. The calculator shows your runtime, the battery size you would need for a target runtime, and lets you export the result as a PDF.
Battery Runtime Calculator
How long will your battery run your appliances?
Battery
Extras (optional)
Appliances running at the same time
Result
Checks
Cycle = share of time the appliance draws full power (1 for steady loads, about 0.35 for a fridge). Usable share is about 0.9 for LiFePO4 and 0.5 for lead-acid. For a power station, enter its Wh. Estimates only. Check your battery and inverter datasheets and have wiring reviewed by a qualified electrician.
A quick shortcut
For a fast estimate with an inverter, use: Hours = Volts x Ah x 0.8 / Watts. The 0.8 covers both usable capacity and inverter loss for lithium. For lead-acid, use about 0.45 instead.
Portable power stations
Power stations list capacity in Wh, so skip the voltage step. A 1,000 Wh unit running a 100 W load lasts roughly 1,000 x 0.85 / 100 = about 8.5 hours. The 0.85 allows for conversion losses, and many units reach slightly more or less than that.
Why real runtime can be shorter
- Inverter standby draw: an inverter uses power even with nothing connected, often 10 to 50 W for larger models.
- High loads: lead-acid batteries deliver less capacity at high current, so heavy loads drain them faster than the formula says.
- Cold weather: batteries give less energy at low temperatures.
- Battery age: capacity falls over the years.
- Start-up surge: motors and compressors briefly pull more power than their running watts.
- Limits on output: your battery and inverter must be able to supply the watts at once. A small battery may shut down on a heavy load even if the formula shows some runtime.
Turn the formula around
To find the battery you need for a target runtime:
To keep a 52 W average fridge running for 24 hours on LiFePO4, you need about 52 x 24 / (0.9 x 0.9) = roughly 1,540 Wh. See our guide to sizing a solar battery bank for larger systems.
Disclaimer: This article is for general information and education only. Numbers are estimates and examples, not professional advice. Check your battery and inverter datasheets, and have any wiring reviewed by a qualified electrician.
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