Backup power
Power station runtime calculator
Runtime in hours = battery watt-hours × efficiency ÷ load in watts. For AC loads, count about 15% lost in the inverter, so a 1,000 Wh station running a 100 W load lasts about 8.5 hours, not 10.
5.8 hof runtime at 150 W
- Energy you can actually use
- 870 Wh of 1,024 Wh rated
- Average draw
- 150 W
- Losses counted
- 15% inverter
- Runtime at half the load
- 12 h
Power station runtime formula
usable Wh = rated Wh × (1 − reserve) × efficiency average W = load W × duty cycle runtime h = usable Wh ÷ average W
A power station's rated capacity (Wh) is the energy in its cells. You never get all of it out of the AC outlets: the inverter that turns 12–51 V DC into 120 V AC wastes 10–20% as heat, more at very light loads where its own idle draw (often 10–30 W) matters. USB and 12 V outputs skip the inverter and get closer to 95%.
Refrigerators, freezers and heaters with thermostats don't run continuously. The duty cycle is the share of time the compressor or element is actually on. A full refrigerator in a 70 °F kitchen typically runs 30–50% of the time; open the door often or put it in a hot garage and it climbs.
Runtime table by station size
Computed at 85% efficiency with no reserve. Columns are common capacities (Wh).
| Load | 256 Wh | 512 Wh | 1,024 Wh | 2,048 Wh | 3,600 Wh |
|---|---|---|---|---|---|
| Router + modem 25 W | 8.7 h | 17 h | 35 h | 70 h | 122 h |
| CPAP 60 W | 3.6 h | 7.3 h | 15 h | 29 h | 51 h |
| Fridge (150 W, 40% duty) | 3.6 h | 7.3 h | 15 h | 29 h | 51 h |
| TV 100 W | 2.2 h | 4.4 h | 8.7 h | 17 h | 31 h |
| Space heater 1,500 W | 9 min | 17 min | 35 min | 1.2 h | 2.0 h |
Notice the space heater: even a 3,600 Wh station runs it about two hours. Heating with batteries is rarely practical; keeping a fridge, a CPAP and the internet up is.
Why your station may die sooner
- Inverter idle draw. With a 20 W load, a 15 W idle draw cuts runtime almost in half. Use DC outputs for small loads.
- Cold. Lithium cells deliver less below freezing and most stations stop charging below 32 °F.
- Inverter limit. If a load (or its start-up surge) exceeds the inverter rating, the station shuts off no matter how much charge is left.
- Ageing. LiFePO4 stations keep about 80% capacity after 3,000+ cycles; older NMC models after 500–800.
Safety. Keep a power station dry and ventilated while charging, and never use a damaged or swollen battery. Do not connect a power station to house wiring except through a transfer switch or inlet listed for that use.
Common questions
How long will a 1,000 Wh power station run a refrigerator?
About 11–16 hours for a modern fridge: it draws around 150 W while running but runs only 35–50% of the time, so the average draw is 50–75 W. At 85% inverter efficiency, 1,000 Wh gives about 850 usable Wh. Full breakdown.
Why does my power station show less than the rated Wh?
Rated Wh is cell capacity. AC output loses 10–20% in the inverter, and many stations hold back a few percent at the top and bottom of charge to protect the cells.
Can a power station run a space heater?
Only briefly. A 1,500 W heater empties a 1,000 Wh station in about 35 minutes and a 2,000 Wh station in about 70.
Is a power station the same as a UPS?
Many now switch over in under 30 ms, which is enough for most electronics, but check the spec: a true UPS switches in under 10 ms or runs online continuously.
Guides that use this calculator
- How Long Will a 1,000 Wh Power Station Actually Last?
- Can a Portable Power Station Run a CPAP? Runtime by Size
- Can a 2,000 W Inverter Generator Run a Refrigerator?