eli5 What is the difference between kW and kWh


eli5 What is the difference between kW and kWh

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A kW (kilowatt) is a unit of power, it measures the rate at which electrical energy is flowing from one place to another (units of energy per time)

A kWh (kilowatt-hour) is a total unit of energy, equal to the amount of energy transferred with 1 kilowatt over 1 hour (units are just energy)

The main difference between kWh and kW is in what they measure. To put it simply, a kilowatt is a measure of power and a kilowatt-hour is a measure of energy; power is the rate at which something uses energy, and energy is the capacity to do work.

To put it simply, kW is how much juice something needs to work, and kWh is how much juice something has that can be used to make that thing work.

You’d use kW to measure something like a light bulb, and you’d use kWh to measure something like a battery.

kW means the energy consumed as 1kW = 1 Joules per second. kWh means the power consumed per hour. the difference is that the power means the energy rate of consumption.

A kW (kilowatt) is a unit of power, equal to about 1.3 horsepower. A kWh (kilowatt hour) is a unit of energy (AKA work), the amount generated by operating a 1 kW motor for 1 hour.

Kilo-Watts measure power, which is how fast you can apply/consume energy (energy divided by time). Kilo-Watts-hour measure energy, particularly the energy you consume in one hour by applying one kilo-watt of power during that hour (divide energy over time and multiply it by time= energy).


Electricity is energy, basically. It flows, and the SI unit of energy is a joule. It’s useful to know how fast energy is flowing, AKA power, so one joule per second is one watt. A thousand watts is a kilowatt, AKA a thousand joules of energy flowing every second. That’s enough to power a medium-sized window unit air conditioner at full blast.

Now, let’s say we’ve left that unit on for an hour. How much energy did it use? Well one kilowatt times one hour would be 3600 kilojoules (60 minutes times 60 seconds) but that’s too much math, so let’s call it one kilowatt-hour. (It’s a unit which includes time divided by time, which is dumb.)

Kilowatts tell you amount of energy flow, which tells you what you can power. KWh tells you what you used.
EDIT: Forgot the kilo, thanks for comment.

Watt is a unit of power. It’s defined as joules per second, where joules is a unit of energy. Basically, it’s the speed of energy transfer.

Watt hour basically means “if i’m transferring at *x* watts, how much energy was used in an hour”. It’s like saying MPH hour: “if I’m traveling at *x* mph, how far did I travel in an hour”. So, watt hours is just another unit of energy, just like joules.

I’ve got a 1000W microwave here – that means when it’s on, it’ll use 1000W – a kilowatt – to heat up beans. If I leave it on for an hour, it will have used 1 kilowatt-hour, and created some very burnt and possibly on fire beans. **Watts measure how much energy something uses, and watt-hours is how long it uses that energy for.** The k/ kilo part is a short way of saying 1000.

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.

W or watt (kW kilowatt 1000watt) is the performance of something usually electric devices. A heater that is like 500W can give 500J of energy to whatever per second. So P=E/t. KWh is when you multiply the performance by some time E=P×t h so one hour 3600s × 0.5kW = 1800 kWh. So its a unit of energy.

There are some weird units like that. Like mAh on batteries miliAmperhour. Its a unit of charge. Amps are a unit of current which is charge/time, I=Q/t so I×t=Q. Units would be A×s=C but battery manufacturers dont like Coulomb for some reason. So instead they use mAh. 1 C = 1 A×1s -> 1 As, the same way 1 J = 1 W × 1s -> 1 Ws.