Battery & Power Station Runtime Calculator
Find out how many hours a battery will power a device. Enter the battery capacity and the device's watts. The result accounts for conversion losses that spec sheets usually leave out.
Same battery, other devices
| Device (typical) | Draw | Runtime |
|---|
How runtime is calculated
Wh = Ah × volts
The rated capacity is the energy stored in the battery cells. Getting it out costs a little. Converting to 120 V AC through an inverter usually wastes 10–15%, and the inverter draws some power just from being switched on. DC and USB ports are more efficient. That's why running a laptop or a 12 V fridge from DC lasts noticeably longer than plugging the same thing into the AC outlet.
Converting amp-hours to watt-hours
Batteries sold for RVs and boats are usually rated in amp-hours. Multiply by the nominal voltage to get watt-hours: a 100 Ah, 12.8 V LiFePO4 battery stores about 1,280 Wh. A 100 Ah, 12 V lead-acid battery stores about 1,200 Wh, but only around half of that should be used regularly. Set the reserve to 50% to reflect that.
Devices that cycle on and off
For fridges, freezers and anything with a thermostat, use the average draw, not the running watts on the label. A fridge pulling 150 W while the compressor runs might average only 40–60 W across a full day. See how long a power station runs a refrigerator for how to find that number.
Frequently asked questions
Why is my real runtime shorter than the calculator says?
Common reasons are cold temperatures (batteries deliver less when cold), an aging battery, a device drawing more than its label suggests, or the inverter's idle draw when powering a very small load. If your loads are DC-compatible, using the DC output helps.
Is the reserve setting necessary?
For portable power stations, you can usually leave it at 0%, since the unit manages its own cutoff. For lead-acid batteries, a 50% reserve protects battery life. For emergency planning, some people keep 10–20% as a buffer.