If there’s one thing worth understanding before you buy backup power, it’s this. Nearly every confusing spec sheet, and most of the buying mistakes I see, come down to mixing up watts and watt-hours.

Think of a water tank

Picture a tank with a tap on it.

  • Watt-hours (Wh) are how much water is in the tank. That’s capacity, or energy.
  • Watts (W) are how fast water comes out of the tap. That’s power.

A huge tank with a tiny tap can’t fill a bathtub quickly, and a big tap on a tiny tank empties it in seconds. You need both to be the right size.

Watts: can it run my appliance?

Every appliance needs power at a certain rate while it’s on:

  • Phone charger: about 15 W
  • Laptop: about 60 W
  • Refrigerator: about 180 W while running
  • Microwave: about 1,500 W
  • Space heater: about 1,500 W

A power station or generator has a maximum output in watts. Go over it and it shuts off. A 300 W power station can’t run a 1,500 W microwave, however full the battery is.

Watt-hours: how long will it last?

Watt-hours are just watts multiplied by time:

Wh = W × hours

  • A 60 W laptop for 5 hours is 300 Wh.
  • A 100 W TV for 3 hours is 300 Wh.
  • A 1,500 W heater for 12 minutes (0.2 h) is also 300 Wh.

Same energy, very different appliances. That’s why a 1,000 Wh power station can run a 100 W load for about 8.5 hours once you take off the roughly 15% conversion loss, but a 1,000 W load for only about 50 minutes.

Kilowatts and kilowatt-hours

  • 1 kW = 1,000 W
  • 1 kWh = 1,000 Wh

Your utility bills you per kWh. The average U.S. home uses around 29 kWh a day, which is why no portable power station runs a whole house for long, and why it pays to plan around the essentials.

Amps, volts and amp-hours

  • Watts = volts × amps. A 10 A appliance on a 120 V outlet uses 1,200 W.
  • Wh = Ah × volts. A 100 Ah, 12 V battery stores about 1,200 Wh.
  • Phone power banks list mAh at 3.7 V, so 20,000 mAh is about 74 Wh.

Putting it together

When you shop for a power station, check two things:

  1. The output in watts covers everything you’ll run at the same time, plus the startup spike from any motors.
  2. The capacity in watt-hours covers the energy you need between recharges, divided by 0.85.

The runtime calculator and the power station size calculator handle both for you.