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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How quickly I fill a jug is the measure of KW (kilowatts).
How much the jug can hold is the KWh (kilowatthour)
I’d compare it to water usage.
You pay for water by the amount you’ve used, not the flow rate. And that in essence is the kWh vs the kW.
edit: I’ll expand this answer out…
If I want to fill a (small) swimming pool with a hose pipe, I can turn on the tap by any amount I want to up to the max. I could drip water in, or I could just fully open the tap. The end result will be the same though – I end up with a full swimming pool. It’s just if I drip water in it will take much, much longer. BUT if I know the amount of time it’s taken to fill the swimming pool and the flow rate of water, I can work out the total water usage.
So, in this case, I have the water flow rate (cubic meters per second) multiplied by time (hours). There is a mix of time units (seconds and hours) which don’t easily cancel out.
Electricity is the same. Note first that a Watt is defined as **Joules per second**. So I have a flow rate (kW). I also have the amount of time I’m using that flow rate for (hours). So if I want to know the total usage I multiply the two together: kWh.
KW says how much power is used each second. If a device uses 1kw and is turned on for 1 hour, it has used 1kwh. When you turn on the same device for a half hour it has used 0,5kwh.
Kw can be compared to the amount of water flowing out of the crane each second. But you are charged per the netto amount of water used, kWh can be compared to liters in this example.
Think of KW as the capacity of a pipe and KWH as the amount you consumed. That’s why you pay by KWH while KW defines the peak max rate at which you can consume.
The difference is between power and energy. It’s more intuitive to use a mechanical example to explain them:
Energy (KWh or Joules) is the total amount of force used to accelerate an object to a particular speed over a particular distance. E.g. The energy used to push a barrel up a hill.
Power (KW or horsepower) is the rate at which that force can be applied to get the object to a particular speed. E.g. Whether you are pushing the barrel up the hill quickly or slowly.
The energy used to push the barrel up the hill is the same whether it took you 5 mins or 5 hours. The rate of work (the power) is different.
To apply it to the electricity bill, you dont pay more if you use electricity quickly for 5 mins or slowly for 5 hours, they are interested in how much electrical energy you used in total when charging you.
Everyone’s answers are correct but they miss the point of why kW are confusing.
It’s confusing because kW is a rate, and we are used to rates being expressed in units per time. Dollars per hour. Miles per hour. The multiple the rate by the amount of time the rate applies for and you get the total quantity of dollars paid or miles traveled.
kW is a rate but the ‘per time’ is built in.
kW are “kWh per hour”.
A kilowatt is 1,000 joules per second. A joule is just a standard unit of energy. If you express kilowatt as the fraction 1,000 joules / 1 second, then multiply that by 1 hour (3,600 seconds), you find that the seconds unit cancels out. One kWh is the same thing as saying 3,600,000 joules of energy.
Electricity is measured in watts, because it is really easy to use when working with electricity. Watts = volts * amps. If you know your voltage and amperage, you know the watts. If you know the watts and how long you pulled those watts, you know how much energy you used.
For instance (making all of these numbers up). If you have an electric motor that operates at 9 volts and draws 2 amps, you know you need 9 volts * 2 amps = 18 watts to power the motor. If you need a battery to power the motor for 3 hours, then the battery must have a capacity of at least 18 watts * 3 hours = 54 watt hours.
W is watt its a measure of power E/t energy per time often expressed is J/s.
KW is a kilowatt its a 1000 watts.
KWh is a measure of energy, if power is P = E/t then P×t = E×t / t = E. So J/s × s = J. 1 KW is 1000 J/s and 1000 J/s × 3600s (1h) = 3 600 000 J of energy. If your electric device consumes 500 W of power in 2 hours it would have consumed 1 KWh of energy.
Another analogy is horsepower in cars. Horsepower is like kW, the actual strength. The work the car did, like pulling its weight over some distance, is like kWh.
Turn your heater on? It’s drawing 1kW = 1000 watts.
Turn it off after a second? Zero watts now in use.
Turn it on for an hour? You’ve used 1kWh of power.
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