"How long will it actually last?" is the question everyone asks and almost no product page answers honestly. The marketing math assumes a perfect world with zero losses. Real batteries do not work that way.
The calculator below gives you a realistic estimate. Pick your capacity and an appliance, and it accounts for inverter losses and the fact that things like fridges cycle on and off rather than run flat out.
Runtime calculator
Cycling appliance: it only draws power about 33 % of the time (the motor stops once it reaches temperature).
Estimated runtime
≈ 1d 2h
Estimate for 1000 Wh usable at ~85 % (inverter losses + real depth of discharge). Real numbers vary with temperature, battery age, and appliance efficiency.
How the runtime is calculated
The honest formula is simpler than it looks:
Runtime (hours) = Usable capacity (Wh) / Appliance draw (W)
But two corrections turn the fantasy number into a realistic one:
- Usable capacity, not rated capacity. You lose roughly 10 to 15% to inverter and conversion losses. A 1000 Wh station gives you closer to 850 to 900 Wh at the outlet. The calculator uses the usable figure, which is why its answers are lower and more honest than a simple division.
- Duty cycle for anything that cycles. A fridge does not draw its rated wattage constantly. It runs, reaches temperature, and shuts off, repeating all day. So a fridge "rated" at 150W might average 50 to 70W over 24 hours. The calculator factors this in for appliances that cycle.
If you ignore both, you overestimate runtime by a wide margin. That is the single most common reason people feel let down by a unit that was actually working fine.
Rough runtime by capacity
Ballpark figures, losses included. Treat them as a starting point and confirm with your own appliance wattage.
| Appliance (typical draw) | 500 Wh | 1000 Wh | 2000 Wh |
|---|---|---|---|
| Phone charge (~15W) | ~25+ charges | ~50+ charges | ~100+ charges |
| Laptop (~50W) | ~8 to 9 h | ~17 h | ~34 h |
| CPAP machine (~40W, no heat) | ~10 h | ~21 h | ~42 h |
| Mini fridge (cycling, ~40W avg) | ~10 h | ~21 h | ~42 h |
| Full-size fridge (cycling, ~60W avg) | ~7 h | ~14 h | ~28 h |
| Coffee maker (~1000W, brief) | a few brews | several brews | many brews |
| Space heater (1500W) | ~20 min | ~35 min | ~70 min |
Notice the bottom rows. Anything that heats drains a battery fast. That is physics, not a weak unit. If you want to run heating appliances for real time, you need a much bigger capacity or a different power source.
Which capacity should you choose
Three simple tiers based on what the runtime math usually tells people:
- 500 Wh: electronics, lights, a fan, a CPAP for one night. Great for light camping. Not for a fridge overnight.
- 1000 Wh: the popular middle. A fridge for most of a day, a home office backup, a solid camping unit.
- 2000 Wh and up: longer outages, several appliances, or pairing with solar so you can recharge daily and effectively run indefinitely in good weather.
Once the calculator shows you a target capacity, the buying guide covers the other two specs that matter (chemistry and inverter output), and the power station vs generator comparison helps if you are still weighing fuel-based backup.
