Update: I am actually searching for really simple, intuitive ways to explain it. I have a background in engineering, but am struggling to explain why we “pay for kwh”, and not kw (on our electricity bill) to someone who doesn’t. I have tried in many ways but maybe I’m not giving the right examples or making the right comparisons. I am really searchig for a way to ELI5.
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One is a unit of energy and the other is a unit of power.
Energy is, I believe, a pretty intuitive concept. You spend energy to make things happen. It’s what battery capacity is measured in.
Power is just a measure of how fast energy is being used by something. In other words, energy per unit time. Kind of like how speed is how much distance you cover per unit time.
The SI system unit for power is the watt (W). One watt of energy isn’t actually all that much; in home electrical systems, it’s much more common to refer to watts in the thousands, also known as kilowatts (kW). In an electrical context, a measurement in kilowatts will tell you how power-hungry an appliance is. How much energy it must consume per second to do its work.
The SI unit for energy is the joule (J). One watt is one joule used up per second. As we said before, one watt isn’t actually a lot. Most electrical appliances measure their power draw in kilowatts. One kilowatt consumed over the course of one second would be one kilojoule (kJ). But that’s still a little… unhelpful. One second isn’t a very long time to measure power consumption over. People interested in knowing how much power something has consumed (like, say, your electric company) want to work in bigger units, like… whole hours. An hour has 3,600 seconds in it. So a thousand watts consumed continuously over 3,600 seconds makes 3,600,000 joules, or 3.6 megajoules (MJ). That’s still a somewhat clumsy unit, since now it’s really big and has that awkward factor of 3.6 in it. So what they instead do is say “you’ve used one kilowatt over the course of an hour”, which they will measure as one “kilowatt-hour” (kWh). It’s measuring the same thing as joules, just in a unit that’s more convenient for the electric company to measure and bill you for.
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