eli5 What is the difference between kW and kWh
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Say I had a tank of water. If I wanted to tell you how much water my tank holds, I would give you some number in volumetric units, like “10 liters”.
Say my tank has a tap on it, where I can open the tap and let water stream out. How long does it take to empty my tank? Let’s say it takes ten seconds. So the tap on my tank allows 10 liters of water to pass through it in 10 seconds. 10 liters per 10 seconds. Or, if you simplify the fraction, 1 liter per second. That’s the “flow rate”.
We have similar concepts with energy. If I have a battery (an “energy tank”, so to speak), I can tell you how much energy it can hold. I would probably do this using a unit called the “joule”.
If I hooked up my battery to a light bulb, and lit up that light bulb until my battery went dead, then measured how long it took for the battery to be fully “drained out” (it wouldn’t actually behave like this in real life, but let’s just pretend conditions were ideal), you could calculate the energy’s “flow rate”, just like we did with the water. You can measure that in “joules per second”. This unit actually has its own special name, a “watt”.
So, a “kilowatt”, abbreviated as “kW”, is 1000 watts, or 1000 joules per second. It’s a “flow rate” for energy. This is the most common unit for energy flow rate in electrical systems, particularly the ones that send power to households over power lines. This is what the power company measures to monitor how much power you’re using. It’s a very convenient unit for them.
Back in the era where power companies literally had no better way to measure your power usage than sending a guy out to your house once a month to read the power meter, they would see how much power your house consumed over the course of an entire month. If you measured this in joules, you’d get some stupendously huge number. SI units have the prefixes to handle this, but once you get past the “mega” range, unless you’re talking about data or distances in astronomy, it gets annoying. Power companies found it much more useful to instead think about that convenient flow rate unit, the kilowatt, and considered how much power got delivered at that flow rate over the course of one hour. Essentially, imagining how much energy a battery that could output a flow rate of 1 kW for a whole hour would hold. This is the “kilowatt-hour”, abbreviated “kWh”. It measures the exact same thing that the joule does (energy), but it’s a unit that is simply more convenient for power companies to use when billing customers.
There are many examples of “convenience units” like this used in lots of different industries. The customary units still used in the United States and some parts of Canada and the UK are based on some of them.
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