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.

In: 24

One is a rate of power expenditure, and one is a quantity of energy spent (or stored.. batteries also have a Wh value associated with them, or more commonly mAh which can be converted to Wh using the voltage of the battery (milliAmp hours)). Power (Standard unit of Watts) is defined as energy per time (Joules/Second)

kW is the *rate* of power consumed when you use a device.

If a device requires 1 kW and you use it for 1 hour, you have used a *quantity* of 1 kWh.

This is how energy companies measure your power usage, by how many kWh you use per month. Different amounts put you in different tiers, which give you different rates.

Edit: added the words “spent (or stored)” for clarity, changed a few things for accuracy

Think of it as gas mileage on your car. The trip computer will tell you at any one time how much fuel you’re currently using but to know how much you’ve used in total you need to multiply that by the distance you’ve travelled.

A kW rating tells you how much energy a device uses at any one moment, but to know how much energy you’ve used in total you need to multiply that by how long you’ve used it for.

The standard unit of energy is the Joule. Joules could be used to describe the energy stored in a battery, in a gallon of gasoline, in food (Calories are also a unit of energy), and so on. Energy being transferred over time is such a commonly used concept, that we gave the unit its own name: the Watt. 1 Watt = 1 Joule per second.

1 Watt is actually a small amount, so often we use kilowatts (kW) instead. 1 kW = 1000 W = 1000 J/s.

A kilowatt-hour is the amount of energy needed to transfer energy at 1000 Watts for an hour. There are 3600 seconds/hour, so 1 kWh =1 kJ/s * 1 hour = 1 kJ/s * 3600 seconds = 3600 kJ = 3.6 MJ.

We often use kWh instead of megajoules because we want to answer questions like “how many hours will my battery last?” and not “how many seconds will my battery last?”. They’re technically equivalent, but kWh is more convenient.

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.

kW is instantaneous power. How much work you are doing right now.

kWh is energy, how much energy you burned over a time period. Could kind of be considered the average power over an hour. Using 1kW for an hour constantly uses the same energy as using 2kW for only 30 mins.

You can think of riding a bike. How hard you pedal is the power, it’s kW (though in reality good luck pedalling at 1kW for any extended period!). By the end of the cycle the energy you burned can be measured in kWh. Pedalling at 1kW for an hour burns more energy than pedalling at 0.5kW for an hour.

Also worth mentioning that energy is power X time. You literally multiply the kW by how many hours to get the energy (though usually you’d measure in Joules, which uses seconds not hours).

I often use the analogy with a water tap and a bucket. Power is best associated with how much you open the tap, while energy is how much the bucket is filled with water.

This way it’s easy to figure out how you can have accumulated (or spent) a lot of energy even with very little power, because it also depends on time.

May not be 100% accurate but you get the idea. HTH.

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