Watts vs watt hours, explained
Updated
Watts measure a rate
A watt is a unit of power: how fast an appliance uses energy at a given moment. A 12 W Wi-Fi router draws about 12 W whenever it is on. A space heater on its 1,500 W setting draws 1,500 W. The U.S. Energy Information Administration describes watts the same way, as power at a specific moment.
Watts say nothing about time. A heater that runs for a minute and one that runs all night draw the same watts. That is why a power station needs a second number.
Watt hours measure an amount
A watt hour (Wh) is the energy of one watt used for one hour. Multiply watts by hours and you get watt hours, the way speed times time gives distance. A 100 W load for 10 hours uses 1,000 Wh. So does a 1,000 W load for one hour.
Larger amounts come in kilowatt hours. One kilowatt hour (kWh) is 1,000 Wh, the unit on a utility bill and on the yellow EnergyGuide label, covered in how to read an appliance label.
Which number on a power station is which
| Rating | Unit | What it limits |
|---|---|---|
| Capacity | Wh | How much energy the battery stores, so how long it runs |
| Continuous AC output | W | How much it can supply at the same moment, for as long as the battery lasts |
| Surge or peak output | W | A short burst above the continuous output, for a motor that is starting |
| Solar input | W | How fast solar panels can refill it |
| AC charging input | W | How fast a wall outlet can refill it |
Capacity is the only rating in watt hours. The others are rates in watts. When you compare stations, compare capacity with capacity and output with output.
Two checks that fail for different reasons
A station can have plenty of energy and still fail, or plenty of power and still run out early. The calculator checks both, and the two failures look different.
Power first. Take a 1,500 W space heater for 4 hours a day on a station with 1,000 Wh of capacity and 1,500 W of output. The output check passes, exactly. The energy runs out: 4 hours at 1,500 W is 6,000 Wh, and 1,000 Wh × 0.85 = 850 Wh lasts 34 min with the heater on.
Energy second. A CPAP at 40 W for 8 hours a night is the opposite case. Every station in the data set supplies 40 W; the smallest output among the 54 is 200 W. The question is energy: a night uses 320 Wh, so the same 850 Wh covers 850 ÷ 320 = 2.7 nights.
From watt hours to hours
Runtime is usable watt hours divided by watts. Start from the rated capacity, keep the share that reaches the outlets (85% by default, see why you get less than the rated capacity), then divide by the load.
- Rated capacity
- 1,000 Wh
- Usable
- 1,000 × 0.85 = 850 Wh
- Load
- 100 W
- Runtime
- 850 ÷ 100 = 8.5 h
A day of loads, worked out
Real lists mix loads that run all day with loads that run for a few hours. Add the watt hours each one uses in a day, then compare the total with the usable watt hours.
| Appliance | Running watts | Hours a day | Watt hours a day |
|---|---|---|---|
| Refrigerator | 150 W | 24 | 1,200 Wh (1.2 kWh a day) |
| Wi-Fi router | 12 W | 24 | 288 Wh |
| CPAP | 40 W | 8 | 320 Wh |
| Total | 202 W | 1,808 Wh |
The refrigerator line uses its daily energy rather than 150 W × 24 h = 3,600 Wh, because a compressor switches on and off through the day. On a 2,042 Wh station: 2,042 × 0.85 = 1,736 Wh usable, and 1,736 ÷ 1,808 × 24 = 23.0 h of this routine, just short of a full day. The 202 W of running watts is what the output has to cover when everything is on at once.
Your own list
The runtime calculator does this sum for every station in the data set and shows the working on each result.
Amp hours and volts
Some batteries list amp hours (Ah) instead of watt hours. Multiply by the battery voltage to convert: watt hours = amp hours × volts. A 100 Ah battery at 12.8 V holds 1,280 Wh. An amp hour figure says little until you know the voltage, so compare power stations in watt hours.
Sources
- Measuring electricity, U.S. Energy Information Administration, checked October 6, 2026.
- Power station and appliance figures: the Enough Watts data set, each record with its own sources and dates.