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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Look at the units to determine the difference.
A kilowatt is Joules per second. So a kilowatt measures the rate of energy used. This is similar to how a car gets miles per gallon.
Then there’s the kilowatt hour. So an hour is the same kind of unit as the second, time, so it’s the same thing. So a kilowatt hour is really just Joules per second, times however many seconds – (J/s)*s. The time units cancel out, and we’re left with Joules. This is a measurement of energy used.
Your car gets say 10 miles per gallon. But when you go to the gas station, you buy gas based on your miles per gallon and how many gallons you have used.
Your house uses say 10 kw of energy (every second, 10000 Joules of energy is used). When you pay the electric company, you pay for the 10 kw times the number of hours the electricity has been on.
Maybe a better explanation is speed. I go running at 10 miles per hour. If I want to know how far I have ran, I need to know how long I have been running. I am not a good runner, so I can only run for a few seconds. So we have miles per hour times seconds – (M/h)*s. The time units cancel out so we’re left with just miles (more like feet, but feet and miles are the same thing – distance). This is the same thing as kwh.
It’s rate vs. amount. You have to maintain a rate over a period of time to achieve something.
As an example use traveling in a car. Rate would be equivalent to speed say 60 mph. But it’s only by going 60mph for a while that you get somewhere. Say you’ve travelled 60 miles, you have to maintain 60mph for an hour to get there (amount). Let’s say I hop in a cab and want to go somewhere. If I paid by rate he/she could simply accelerate to 60mph and then stop and ask for money. When I get in a cab I want to go somewhere so I am paying for he/she to maintain a rate over a period of time to actually cover the distance desired.
It’s like the difference between speed and distance travelled, where speed would be the kw and distance would be the kwh. When you pay for travel you generally pay for how far you go rather than the speed of the journey.
When you fill your car up with fuel, they don’t charge by how fast the fuel goes into the car (kw) but rather by the speed it goes into your car multiplied by the time you spend doing that, which is the amount (kwh).
I always think of an electric kettle. It consumes 3 kW when on but it only runs for like 2 minutes. So boiling water costs 3 kWh * 2 mins = 0.1 kWh or about 1 cent. Compare this to a modern light bulb: it consumes just 0.01 kW but is on for 10 hours a day, so we get the same 0.1 kWh.
This example shows that two things matter for overall energy consumption (kWhs): power (watts) and time the appliance is on.
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