What is a Watt Hour?

What is a Watt Hour?

By Martin Camacho

As you may have noticed, we use Watt Hours to explain the capacity of most of our products. For many of us, a Watt Hour isn’t something all that familiar. So, here’s a brief explanation of what it is.

A Watt Hour is a unit of measurement for power over a period of time (an hour), or in our case, a way of measuring capacity. One Watt hour is equal to one Watt of average power flow over an hour. One Watt over four hours would be four Watt Hours of power. As an example, a 100 Watt light bulb on a 400 Watt Hour battery (like the Yeti 400) would last, on paper, 4 hours.

A Watt, the measure of power, is usually calculated using this equation: Watts = Volts x Amps. To explain a little further, we will use a plumbing analogy. If we have a water pipe; Volts would be a measure of the water pressure (force) in the pipe, Amps would be a measure of the current or flow through the pipe. A Watt would be the measure of of what you can do with that water, like turning a water wheel. So, how do we determine Watt hours?

Watt Hours are calculated using a similar equation when dealing with batteries. An example of this would be that the Yeti 400 contains a 33 Amp Hour battery operating at 12 Volts. 12 Volts x 33 Amp Hours = 396 Watt Hours or roughly 400 Wh. Not only are Watt Hours a good unit of measurement for capacity, but it is also pretty universal when finding out how many times one of our GZ products will recharge something with it’s own battery in it (like a phone, tablet, or laptop). The Equation to find the Watt Hours of a battery gives us a universal measurement despite batteries on the market varying greatly in operating voltage and mAh.


  1. Filippo Zonta 2 years ago

    Hello, I need to select the correct Panel for direct charging my DSLR batteries using the 12v DC solar port. Using the equation Wh = mAh * Volt I compared the battery capacity respect to power capacity of the solar ports.
    For example : battery 2900mAh * 7.2 V = 20.8 Wh

    18,2 Wh 2,3h for 2×2900 nomad20
    13Wh 3,2h for 2×2900 nomad13

  2. Jack Colton 2 years ago

    The example of a 100 watt lamp operating from a 400 Watt hour lead acid battery is flawed.
    It does not account for de-rating battery capacity. If the battery Amp hour rating is based on the 20 hour discharge rate, any faster than 20 hour discharge rate will de-rate battery capacity. Lamp will not light at full brilliance for stated time.

  3. Andrew 2 years ago

    I was just about to purchase a Yeti 400; but… Was concerned about the Amp Hours available with this device (33Ah), and realized that Goal Zero is still using AGM batteries.

    When will Goal Zero transition to LiFePo4 batteries or at the very least Lithium Ion batteries? A competitor of yours (EnerPlex) makes a similar sized generator that weighs about 40 pounds to the Yeti400’s 32 pounds and they use Lithium Ion batteries with a capacity nearly 3 times that of the Yeti400 (83Ah).


  4. DON 2 years ago

    Watt a great explanation! Thanks.

    • Renata 2 years ago

      Cute. Hahaha!

  5. Cale 2 years ago

    Wouldn’t one watt over 4 hours be 0.25 Watt/hr and one watt-hr over 4 hours be four watt-hours?

  6. Caspar 2 years ago

    Great explanation!
    Can you also explain how to calculate the capacity of the internal battery of a phone, tablet or laptop or other device, in order to calculate the number of times it can be recharged?

    • Martin Camacho 1 year ago

      Hey Caspar,

      Absolutely. First you need to know two things: the nominal battery voltage and the mAh capacity. I usually go to GSMarena for this info. Once you get that, you use the equation W= V x A (Watts = Volts x Amps or Watt Hours = Volts x Amp Hours) (There are 1,000 mAh in an Ah) to get the Wh capacity of the battery inside a phone, tablet, or laptop. This lets you compare battery sizes, apples to apples.

  7. Art grimes 1 year ago

    Curious when the Yeti 1250 will have Lipo4 type battery available

  8. ED 1 year ago


  9. Dan 4 months ago

    I would like to power a 300W heater for a few hours at night while camping.

    How long would a YETI400 power it?

    Thank you,

    • Author
      goalzero 2 months ago

      Thanks for reaching out! Please feel free to reach out to our solutions center team by phone at 1-888-794-6250 or by email at support@goalzero.com They would be happy to help you out!

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