Portable power station runtime calculator
Enter your battery capacity (Wh) and what you want to run (watts). We apply realistic ~88% inverter efficiency and 92% usable capacity — not the marketing math.
Examples: phone 5–10 W · LED bulb 8 W · laptop 50 W · CPAP 30–60 W · mini-fridge 80 W · full fridge 120 W · microwave 1,100 W
Formula: hours = (capacity × efficiency × 0.92) / device_watts
Why not the marketing numbers?
If you divide 1,024 Wh by a 60 W device, you'd get 17 hours. The truth is closer to 13.8 hours, because:
- Inverter losses: converting DC battery → AC outlet loses 8–15% as heat.
- Battery reserve: BMS holds back 5–10% to protect cells.
- Temperature & age: cold weather and older cells reduce usable energy further.
Want to skip the math?
Try the scenario finder — tell us what you'll run and we'll suggest the minimum capacity. Or browse our model database with built-in scenario runtime tables.
Frequently asked questions
Why is the calculated runtime less than capacity ÷ device watts?
Because the inverter loses 8–15% of energy as heat converting battery DC to outlet AC, and the BMS holds back another 5–10% to protect the cells. The math is (capacity × inverter_eff × usable_pct) / watts — not the back-of-the-box version. See our Wh-to-runtime guide for the full breakdown.
Are the efficiency numbers accurate?
They're realistic for modern LiFePO4 units (88%) and lithium NMC units (85%), based on independent load-test data from Hobotech, Will Prowse, and OutdoorGearLab's lab. Older or budget units may run lower (82–86%). High-end units sometimes exceed 90%. We use 88% as a conservative middle ground.
Why does cold weather matter?
LiFePO4 cells lose 5–15% of usable capacity below 40°F (4°C). Most modern power stations heat the cells internally before discharge, which helps but burns some energy. If you'll use it in winter camping or unheated garages, expect 10–15% less runtime than the calculator shows.
How do I know my device's wattage?
Check the device's spec label (usually on the back or bottom — "100–240V, 50/60Hz, 1.5A" gives you 1.5 × 120 = 180 W). Or use a Kill A Watt meter ($25 on Amazon) to measure actual draw at the outlet, which is often much lower than the rated max.