How long will your power station or battery last? Enter its capacity and the device you want to run, and the calculator estimates the runtime, accounting for conversion losses and appliances that cycle on and off.
Battery Runtime Calculator
Find out how long a power station or battery will run your device.
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Quick answer
- Runtime (hours) = battery Wh × efficiency ÷ device watts. Use 0.85 for AC plugs and about 0.92 for USB or 12V ports.
- A 1,000Wh power station runs a 100W device for about 8.5 hours on AC.
- Fridges and freezers cycle, so they last about three times longer than their running watts suggest.
The battery runtime formula
The basic formula is simple:
Runtime (hours) = Battery capacity (Wh) × Efficiency ÷ Average device draw (W)
Example: a 1,000Wh power station running a 60W laptop from an AC outlet: 1,000 × 0.85 ÷ 60 = about 14 hours.
If your battery is rated in amp-hours (Ah), multiply by its voltage to get watt-hours first. A 100Ah 12V LiFePO4 battery is 100 × 12.8 = 1,280Wh. Use the “I know Ah and volts” option above to do this automatically.
Why real runtime is shorter than the label suggests
Inverter losses. Plugging into an AC outlet means the power station converts its battery’s DC power to AC, losing roughly 10–15% as heat. USB-C and 12V ports skip most of that conversion, which is why a CPAP lasts noticeably longer on a DC cable.
Idle draw. An inverter that is switched on uses some power even with nothing plugged in, often 10–20W on larger units. Turn AC output off when you are not using it.
Cold and age. Lithium batteries deliver less energy in freezing weather and lose capacity slowly over hundreds of charge cycles. Expect 10–20% less runtime in winter or on an older unit.
Devices that cycle on and off
Fridges, freezers, space heaters with thermostats and AC units do not draw power constantly. Their compressor or element switches on and off to hold a temperature. That is why the calculator asks how much of the time a device actually draws power:
- Fridges and freezers: about a third of the time (more in a hot room, or if the door is opened often).
- Window AC and thermostat heaters: roughly half the time, depending on how hot or cold it is.
- CPAP, routers, laptops and lights: constantly while in use.
Example runtimes for a 1,000Wh power station
| Device | Typical draw | Estimated runtime |
|---|---|---|
| Wi-Fi router + modem | 15W | about 56 hours |
| CPAP without humidifier (DC cable) | 40W | about 23 hours |
| Laptop | 60W | about 14 hours |
| Starlink dish | 75W | about 11 hours |
| Full-size refrigerator (cycling) | 150W running | about 17 hours |
| Space heater on low (cycling) | 750W | about 2 hours |
These are estimates. Actual draw varies a lot between models, so check your device’s label or measure it with a plug-in watt meter for the most accurate result.
Most power stations show live input and output watts on their screen or app. Plug your device in for an hour and note the average. That number gives a far better estimate than any table.
Frequently asked questions
How long will a 500Wh power station run a CPAP?
A CPAP drawing about 40W without a humidifier will run for about 11 hours on a DC cable, or about 10.5 hours through the AC outlet. With a heated humidifier it can draw 90W or more, cutting that to around 5 hours.
How long will a 100Ah battery last?
A 100Ah 12V LiFePO4 battery stores about 1,280Wh. Running a 100W load through an inverter, that is about 11 hours. Lead-acid batteries should not be drained below about 50%, so a 100Ah lead-acid battery gives roughly half that.
Does a power station lose charge when not in use?
Yes, slowly. Most lose a few percent per month in storage. Top it up every 3 months and store it at 50–80% for the longest battery life.