Estimate runtime for a portable power station or battery pack from its watt-hour rating, the watts the device draws, and a conversion loss. A 500 Wh station does not run a 500 W load for one hour, because the inverter and the voltage conversion use some of that energy.
Power station runtime calculator
The result updates from the fields above. This form does not store the numbers you enter.
What the result means
Hours is usable energy divided by the load. Usable energy is the watt-hour rating multiplied by the efficiency you enter. If the station’s continuous watt rating is lower than the device, it can shut off immediately even when this estimate shows several hours. Check that limit on the station before you trust the time.
Formula and example
Hours = (watt-hours × efficiency) ÷ watts. 500 Wh × 0.85 ÷ 100 W = 4.25 hours, or 255 minutes. A laptop charger brick labeled 65 W is not the same as the laptop drawing 65 W the whole time. Use the draw while the device is doing the work you care about.
What this number cannot tell you
Cold weather, an old battery, and a load that cycles on and off all change runtime. Phone batteries listed in mAh are not watt-hours until you multiply by the pack voltage and divide by 1000. This tool does not size a generator and does not cover medical equipment.
Questions people ask
What efficiency should I use?
Start at 85 percent if the manual does not give a figure. AC outlets lose more than a direct DC or USB-C output. If the maker publishes an AC efficiency, use that instead.
Can I add several devices?
Yes. Add their running watts and enter the total as the load. Do not add nameplate maximums if the devices are not all at that maximum together.
Why is my real runtime shorter?
The battery may not deliver its full rated watt-hours at a high load, and the display on the station is the measurement that matters once it is running.
