# Thread: Power question (re: kWh)

1. ## Power question (re: kWh)

Possibly silly question - why are electricity/power bills measured in kilowatt-hours? Shouldn't they just be measured in Watts (or better still Joules?). Shouldn't we just be billed for the total number of joules of energy that we've used?

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Originally Posted by EDG_
Possibly silly question - why are electricity/power bills measured in kilowatt-hours? Shouldn't they just be measured in Watts (or better still Joules?). Shouldn't we just be billed for the total number of joules of energy that we've used?
A watt is a joule per second -- it is a rate, just like gallons per hour. A kilowatt-hour is a thousand watts (a rate), times the number of hours (a time), so it is in fact proportional to the total energy you used. You do get billed for the number of "gallons," not the rate at which the "water" flows. FYI, a kW-hour is (1000J/s)*3600s = 3.6MJ. That's a lot of energy.

3. Yes, and like you wanted..... it IS measured in watts. Kilo watts.
And there is no free lunch.
You can use less if you work at it. Make it a hobby of using what you need.... only. This works.

4. Yeah, but why is it kilowatt-hours? That's really "joules per seconds multipled by hours", right - which seems a bit redundant, given that a watt is a joule per second already.

And the water volume example is different - a gallon is just a volume, not something per unit time. "Gallons per hour" is "volume per unit time" (which makes sense). Kilowatt hours is "energy per unit time multiplied by (a different) unit time", so why not just leave it as energy?

5. Kilowatt hours are energy. It's energy divided by time multiplied by time. The time units cancel (and, as stated above, 1 kWH = 3.6 MJ).

Oh, and kilowatt hours are used because it's a fairly convenient unit.

6. The idea is that you want to know the actual amount of energy. Watts are the energy quantity divided by time. If you want the energy quantity itself (such as joules), rather than the rate of use (joules per hour) you need to multiply by time, thus cancelling out time and getting the energy quantity on its own.

i.e. E = ENERGY, T = TIME
Watts = E/T

I think the suggestion for why use that arrangement of units is because otherwise you end up getting your bill with amount of joules listed in scientific notation, which would likely make lots of folks heads spin.

To get the energy on it's own, multiply by T to get E.

E/T * T = E

ETA: well played, cjl, well played.
Last edited by Spoons; 2010-May-26 at 04:25 AM. Reason: beaten to the punch

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Originally Posted by EDG_
Yeah, but why is it kilowatt-hours? That's really "joules per seconds multipled by hours", right - which seems a bit redundant, given that a watt is a joule per second already.

And the water volume example is different - a gallon is just a volume, not something per unit time. "Gallons per hour" is "volume per unit time" (which makes sense). Kilowatt hours is "energy per unit time multiplied by (a different) unit time", so why not just leave it as energy?
No, the water example is an exact analogy. You are paying for the gallon. Wattage corresponds to gallon/minute flow. Multiply that by the total time and you get the total number of gallons.

Now the real question is why kilowatt-hours and not joules or megajoules? The reason is tradition. Power engineering, like other professions, has its own habits. Power in kilowatts is a convenient scale for industrial purposes. But why hours and not seconds? Partly because the measuring instruments (your average retro wattmeter) can only resolve power integrated over relatively long time scales (not seconds), and partly because the cost of power is so low that it takes MJ to get up to the order of dimes.

8. Originally Posted by Geo Kaplan
Now the real question is why kilowatt-hours and not joules or megajoules?
That was the question posed all along. I don't believe EDG needed to be told that time/time cancels.

Your explanation as to tradition and convenience sounds correct to me.

9. Originally Posted by Geo Kaplan
Power in kilowatts is a convenient scale for industrial purposes.
I was thinking the same thing, but more on the home front.
The average person* knows that a specific appliance or bulb or something is rated at x kWh. So; the math for running something for a period of time becomes easy when looking at your bill.

* ok; maybe the average person doesn't know (or care)... but it is within thier reach.

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