Watt-hours vs watts: what's the difference?

Power stations are sold by two numbers that sound almost the same: watt-hours and watts. They answer two different questions. Get them straight and you'll know both what a station can run and how long it can run it.

The fuel tank picture

• Watt-hours (Wh) are the size of the tank. It's how much energy the battery holds.
• Watts (W) are how fast you use it. It's like your speed, or how wide you open the tap.

A big tank used slowly lasts a long time. The same tank used fast empties quickly.

The runtime formula

Hours ≈ Wh × 0.85 ÷ W

The 0.85 is there because some energy is lost turning the battery's DC power into 120 V AC for your plugs. We assume 85%. The calculator above lets you try 80% to 90%.

Worked example

Take the Jackery Explorer 1000 v2. Its spec page lists 1,070 Wh and 1,500 W rated output.

• Running a 100 W load: 1,070 × 0.85 ÷ 100 = about 9 hours
• Running a 500 W load: 1,070 × 0.85 ÷ 500 = about 1.8 hours
• Running a 1,500 W heater: 1,070 × 0.85 ÷ 1,500 = about 36 minutes

Same tank, very different results. That's why a power station is great for a fridge, CPAP, Wi-Fi and phones, and poor for heaters.

The watts number on a power station

A power station also lists watts. This is the most its inverter can supply at once, like a speed limit.

• Rated (continuous) watts: what it can run all the time. The Jackery above is 1,500 W.
• Surge watts: a short burst for motors starting up. The Jackery lists 3,000 W.

If your load is bigger than the rated watts, the station shuts off, no matter how full the battery is. A small station like the EcoFlow RIVER 2 Max 500 has 499 Wh but only 500 W of output. So it can run a laptop for a long time but can't run a microwave at all. See why a power station shuts off under load.

So check both:

1. Watts: is the station's rated output higher than everything you'll plug in at once?
2. Watt-hours: does the tank last as long as you need?

What's a kWh?

A kilowatt-hour (kWh) is 1,000 watt-hours. Your power bill uses kWh. A big home-backup station may hold 4 or 5 kWh. A modern full-size fridge uses about 1.0 to 1.6 kWh a day, so a 1,000 Wh station gets it through roughly half a day to most of a day. Our fridge guide works it through.

Things that cycle on and off

A fridge doesn't run its compressor all the time. It cycles. So its average watts are much lower than its running watts. Use the average for runtime, but the running and surge watts to check the station can start it. The calculator has a fridge option that does this for you.

Amp-hours vs watt-hours

Some batteries are listed in amp-hours (Ah). To get watt-hours, multiply by the battery's volts:

Wh = Ah × V

A 12 V, 100 Ah battery holds about 1,200 Wh. Watt-hours let you compare batteries of different voltages fairly, which is why power stations use them.

What about generators?

A generator's "tank" is its fuel. Its watts work the same way: running watts are what it can supply all the time. How long it runs depends on fuel and load. See how long a generator runs on 5 gallons of gas.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.