A power station runtime calculator answers the one question that matters in an outage or at a campsite: how long will it actually run? The number on the box is not the answer. Inverter losses, the battery reserve and cycling loads like a fridge all cut real runtime by 20-30%. Use the calculator below, then check the formula and the wattage table to size the right unit.
The power station runtime formula
Runtime (hours) = Capacity (Wh) x 0.9 x Efficiency / Load (W)
- 0.9 is the usable share of the battery. The BMS shuts down before 0%, and cold weather lowers capacity further.
- Efficiency is about 0.85 through the AC outlet (inverter) and about 0.9 through DC or USB-C ports.
Example: a 1,024 Wh power station running a 40 W CPAP from the AC outlet lasts 1,024 x 0.9 x 0.85 / 40 = about 19.5 hours, or two to three nights.
Wh vs mAh: converting power bank capacity
Power banks are rated in mAh at the 3.7 V cell voltage, not at the 5-20 V output. Convert with: Wh = mAh x 3.7 / 1000.
| Power bank | Energy | Phone charges (about 15 Wh each) |
|---|---|---|
| 10,000 mAh | 37 Wh | about 2 |
| 20,000 mAh | 74 Wh | about 4 |
| 26,800 mAh | 99 Wh | about 5-6 (airline carry-on limit is 100 Wh) |
| 50,000 mAh | 185 Wh | about 10 |
How many watts do common devices use?
| Device | Average power |
|---|---|
| Wi-Fi router | 6-12 W |
| Starlink Mini | 20-40 W |
| Starlink Standard (Gen 3) | 75-100 W |
| Laptop | 30-65 W |
| CPAP without humidifier | 30-60 W |
| Full-size fridge (averaged over a day) | 60-100 W |
| 55-inch LED TV | 60-100 W |
| Gas furnace blower | 300-600 W |
| Space heater | 1,500 W |
A plug-in watt meter or the display on the power station itself gives the real number for your devices. Measure them instead of trusting the label on the power brick.
How long will a power station run a fridge?
A modern full-size fridge averages 60-100 W over 24 hours, even though the compressor draws 100-200 W while running. A 1,000 Wh station gives roughly 8-12 hours, a 2,000 Wh unit 16-24 hours. The compressor needs a startup surge of 600-1,200 W, so the inverter must be rated for it. Keeping the door closed matters more than any setting.
How to get more runtime
- Use DC and USB-C ports instead of the AC outlet whenever a device allows it. The inverter alone burns 10-30 W even with a tiny load.
- Switch the AC output off when nothing is plugged into it.
- Keep the battery warm. LiFePO4 delivers less energy in the cold and must not be charged below 32 F (0 C).
- Skip resistive heat. Space heaters, kettles and hair dryers empty a 1 kWh station in about 35 minutes.
Powering satellite internet? See Starlink Mini power bank guide. Building your own battery monitor? Read ESP32 battery voltage monitoring.
Running Starlink off-grid?
Which power banks actually start a Starlink Mini, what cable you need and how many hours you get.
Read the Starlink guideFAQ
How long will a 1000 Wh power station last?
Divide about 765 usable watt-hours by your load: around 76 hours for a 10 W router, 19 hours for a 40 W CPAP, 9-10 hours for an 80 W fridge and 30 minutes for a 1,500 W heater.
Why does my power station die sooner than capacity divided by watts?
Because of inverter losses (10-15%), the unit’s own idle draw, the BMS reserve and temperature. Real runtime is typically 20-30% below the simple division.
Is a 300 Wh power station enough for a CPAP?
Usually for one night: about 230 usable Wh covers 5-7 hours at 30-40 W without the heated humidifier. Use the DC adapter for your machine if one exists.
Sources: Starlink Mini power supply (Starlink support) and DishyTech: How Much Power Does Starlink Use?.