Watts vs. watt-hours

These two numbers sound alike, but they do different jobs. One tells you if a box can run a thing at all. The other tells you for how long. Watts: how fast something uses power. Watt-hours: how much power a box holds.

Think of a car

Watts are the engine. Watt-hours are the gas tank.

A big tank with a tiny engine cannot pull a trailer. A big engine with a tiny tank can pull it, but not far. A power station works the same way. You need enough of both.

The U.S. Energy Information Administration puts it like this: watts measure power at one moment, and watt-hours measure how much is used over time.

Mistake 1: enough watt-hours, too few watts

Every box has a limit on how many watts it can give at once. If a thing needs more than that, it will not run. It does not matter how much the box holds.

Our tools count 1,100 watts for a microwave. The BLUETTI Elite 30 V2 gives 600 W. So that box is fine for lights and Wi-Fi, but not for a microwave.

Some things also need a push to get going. Surge: a short burst some things need to start. Our tools count 120 watts for a running fridge and 1,200 watts for the moment it starts. That start number is from Honda's generator guide, which lists about 1,200 watts to start an ENERGY STAR fridge or freezer. By our math, a box that tops out at 600 watts cannot start a fridge.

Mistake 2: enough watts, too few watt-hours

A box can be strong and still run dry fast. Take a 1,500-watt space heater on the Anker SOLIX C1000 Gen 2. The box gives 2,000 W, so the heater runs. But look at the time.

1,024 Wh × 0.85 ÷ 1,500 W = 0.6 hours

That is about 35 minutes of heat. The same box runs a Wi-Fi router for more than two days by the same math.

Estimates. Your devices may use more or less.

Which number matters when

These are the numbers our tools count. The hours are for a 1,000 watt-hour box that runs only that one thing.

Small things that stay on all day are a watt-hours question. Big things that run for a few minutes are a watts question. Big things that stay on for hours need both, and they empty a box fast.

A fridge is a "both" case. It runs on few watts and stays on day and night, but it needs a big burst each time it starts.

Check both before you buy

• Find the thing with the most watts on your list. The box must give at least that many watts.
• Add up the watts of all the things that will be on at the same time. The box must cover the total.
• If your list has a motor, like a fridge or a pump, look at the surge number on the box too.
• Then pick the watt-hours for the hours you want. Our guide to what a watt-hour is has the math.
• A microwave's label shows cooking power, not what it pulls from the wall. Honda's generator guide lists 1,500 watts for a 1,000-watt microwave.

You do not have to do this by hand. Tell Find My Power what you want to run and for how long. It checks watts, surge and watt-hours together. You can also compare two boxes or look at our picks.

Questions people ask

Can a 1,000-watt box run a 1,500-watt heater?

No. The heater asks for more watts than the box can give. You need a box that gives at least 1,500 watts.

What does "surge" or "peak" mean on a box?

It is the short burst the box can give to start a motor. It is higher than the normal watts. Anker lists 2,000 W of output and a 3,000 W peak for the SOLIX C1000 Gen 2.

Is a kilowatt-hour the same as a watt-hour?

A kilowatt-hour is 1,000 watt-hours. A box that holds 2,000 watt-hours holds 2 kilowatt-hours.

Which number matters more for a fridge?

Both. The box needs enough watts to start the fridge and enough watt-hours to keep it cold. See how long a fridge runs on a power station.