How do i measure electrical power over time ?

[DeepCut]

Hi.

I've just read this : http://en.wikipedia.org/wiki/Power_%28physics%29

but i don't understand how i would calculate say, if my 1kW heater was on for 3 minutes, how many watts would it have used ?


Thanks,

Gary.

[fritznien]

Hi.

I've just read this : http://en.wikipedia.org/wiki/Power_%28physics%29

but i don't understand how i would calculate say, if my 1kW heater was on for 3 minutes, how many watts would it have used ?


Thanks,

Gary.

watts is a unit of power, the rate energy is used. a thousand watts is a thousand for one second or 1000 hours.
energy is the power times the time it is used.
3 minutes times 1000 watts is 3000 watt minutes or one twentieth of a kilowatt hour.
fritznien

[rave154]

Deep,

power ( watts ) is a measure of joules burned (energy ) per second, much like speed is a measured of miles covered for hour...

so, your 1KW heater left on for 3 minutes would consume...

P := 1KW x 180(seconds) ..

  := 1000 x 180 = 180,000 joules of energy used.

[DeepCut]

Thanks both of you that's totally clarified it for me


Gary.

[TinselKoala]

Thanks both of you that's totally clarified it for me


Gary.

Did it? Let's see if I can muddy the waters a bit then.


Your original question wasn't answered, though, and although the answers you got are correct they don't clear up the confusion, IMHO, so here's my two dollars worth:

The original question: "If my 1kW heater was on for 3 minutes, how many watts would it have used ?"

And the answer -- which I don't see in the otherwise excellent answers -- is 1000 Watts. (not "Used" but "dissipated" is preferred)

This is because the Watt is a unit of power and is "instantaneous" --- and more importantly IS NOT A CONSERVED QUANTITY in that the number of Watts "into" a system does not necessarily equal the number of Watts "out".

The Watt is a Rate of Energy Usage. Energy is the conserved quantity: what goes in is equal to what comes out. But it doesn't have to come in and go out at the same Rate...and that's what seems to trick a lot of people into thinking they've got OU or COP>1 when all they've done is amplify power, instead of energy.

So, since the Watt is a Rate, your question is like asking: If I drive my car at 60 miles per hour for three minutes, how fast am I going?
Of course you are going 60 miles per hour.

But what's important is not how fast but how far: How Far do you go in 3 minutes at 60 mph?
If you are using 1 kiloWatt for 3 minutes, how much ENERGY have you used in that time?
So the Watt, a unit of Power, is defined as a Rate of Energy Usage: 1 Watt = 1 Joule per Second. The Joule is the accepted Energy Unit in SI.
So "how far", not "how fast" becomes 1 kW for 3 minutes = 1 kW for 180 seconds = 1000 joules/second * 180 seconds = 180,000 Joules of energy "used".
Or rather, transferred from electricity to heat.