What is a watt-hour?
A watt-hour is one watt used for one hour, so a box that holds 1,000 watt-hours runs a 100-watt TV for about 8.5 hours by our math.
A watt-hour is a way to count stored power. The more watt-hours a power station holds, the longer it runs your things. Watt-hours: how much power a box holds.
Think of a water bottle
A power station is like a water bottle. Watt-hours are how much water is inside. A bigger bottle lasts longer.
The other number is how fast you pour. Watts: how fast something uses power. A phone charger sips. A space heater gulps.
The U.S. Energy Information Administration says one watt-hour is the energy of one watt used for one hour. Its example: a 40-watt light bulb left on for five hours uses 200 watt-hours.
The easy math
Take the watt-hours on the box. Keep 85% of it, because some power is lost inside the box. Then divide by the watts of the thing you plug in. The answer is hours.
1,024 Wh × 0.85 ÷ 100 W = 8.7 hours
That is a 100-watt TV on the EcoFlow DELTA 3. We use this same math in every tool on the site.
Estimates. Your devices may use more or less.
How long a 1,000 watt-hour box lasts
These are the watts our tools count for each thing. By our math, a 1,000 watt-hour box runs each one, alone, for this long.
| Thing | Watts | Hours on 1,000 Wh |
|---|---|---|
| Wi-Fi router | 15 | 57 |
| Phones | 20 | 43 |
| Lights | 40 | 21 |
| CPAP machine | 40 | 21 |
| Fan | 50 | 17 |
| Laptop | 65 | 13 |
| TV | 100 | 8.5 |
| Refrigerator | 120 | 7.1 |
Your own things may differ. The label on a device shows its watts. If it only shows volts and amps, multiply them. Generac's help page shows this for a fridge.
Read the real number on the box
Makers print the watt-hours on the box and on the spec sheet. Do not go by the name alone. The Anker SOLIX C1000 Gen 2 holds 1,024 Wh. The Jackery Explorer 1000 v2 holds 1,070 Wh. Both have "1000" in the name.
We read each number on the maker's own page before we list a box. See how we check each number.
Watt-hours and kilowatt-hours
Your power company counts in kilowatt-hours. One kilowatt-hour is 1,000 watt-hours. So a 1,000 watt-hour box holds one kilowatt-hour.
The EIA says the average U.S. home bought about 899 kilowatt-hours a month in 2022. By our math that is about 30,000 watt-hours a day. One box holds a small slice of that. This is why a box runs a short list of things and not a whole home.
How many watt-hours do you need?
- Add up the watts of the things you want on at the same time.
- Multiply by the hours you want them to run.
- Divide by 0.85. That is the box size to look for.
Say you want a fridge, Wi-Fi and phones for 12 hours. Our tools count 120, 15 and 20 watts. That is 155 watts.
155 W × 12 hours ÷ 0.85 = 2,188 Wh
Our Find My Power tool does this math for you. Then you can compare two boxes side by side or see our picks.
One more thing. Watt-hours are not the whole story. A box must also give enough watts at one time. Our guide to watts vs. watt-hours explains that.
Questions people ask
Is a watt-hour the same as a watt?
No. Watts are how fast power is used right now. Watt-hours are how much power is used or stored over time.
How many watt-hours are in 1 kWh?
1,000. A kilowatt-hour is 1,000 watt-hours.
Why do you only count 85% of the watt-hours?
A box loses some power when it turns battery power into wall power. Our math keeps 85% for every box. Your box may do a little better or worse.
How many watt-hours does a fridge use in a day?
Our tools count 120 watts for a fridge. If it ran all 24 hours, that would be 2,880 watt-hours. Many fridges rest part of the time, so the real number can be lower. See how long a fridge runs on a power station.
Estimates. Your devices may use more or less. As an Amazon Associate I earn from qualifying purchases. Made with AI tools for research and drafting. Samuel Guerra reviews every guide and checks it against the sources linked on this page.