# Why are Watt Hours not Watts per Hour? Are they the same thing?

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Why are Watt Hours not Watts per Hour? Are they the same thing?

In: 1289 Because its Watts applied for an hour not for every hour.

5 wH means 5 Watts applied for a hour. 5 w/h is 0.08 w applied every minute for every hour. Watt hours is multiplying power and time to give an energy: it’s energy divided by time (power) times time, to give energy.

When we say “<something> per <something else>”, it means the first thing divided by the second. So watts per hour would have units of energy divided by time *squared* and would be something like how fast some power is changing. It’s basically going the opposite direction of watt-hours.

A nice analogy that might help is thinking about energy as being like distance. In that analogy, power is like distance per time, or speed. To get total distance from some speed, you need to multiply the speed (power) by how long you have been going that speed (time) to give total distance. Watt-hours as a unit would be kind of like mph-hours or something. It sounds weird, but it’s just a distance. Speed per time is an *acceleration* in this analogy and tells us how fast our speed is changing. Acceleration has units of distance per time squared, just like watts per hour would have units of energy per time squared and would tell us how fast some power is changing. Watt Hours are units of energy, power multiplied by time.

Watts are a unit of power, energy divided by time.

Watts per hour doesn’t really make all that much sense, unless you mean the change in power output over time. Watts per hour is a bit redundant. Watts is a measurement of energy used per second with the units of joules and seconds. Watts = (Joules/seconds). Watt hours is a total amount of energy used. If we take the formula for Watts and multiply by hours(3600 seconds) the result is joules or energy. Watt Hours are a unit of watts multiplied by hours. Watts *per* hour would be a unit of watts *divided* by hours, which would obviously produce a very different result.

We are more familiar with the phrase “per hour” because we commonly use it to measure speed in miles or kilometers per hour. But you might have heard a comparable unit to Watt Hours used in a business setting when people talk about man hours. The idea being if ten people worked on a task for a 40 hour businesses week, then the task was completed in 40 hours but took 400 total man hours to complete (i.e. it would theoretically take 400 hours of work for one person).