kilowatt hour

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Danny Phillip Doeden

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the power company sells us power using the unit kilowatt hour. The definition of watt is one joule per second. Isn't kilowatt hour using time twice. Since time is used to define watts, I would think time can't be used again.
 
A "kilowatt hour" is derived by multiplying the unit "kilowatts" times the unit "hours." That eliminates the parameter "time." The result of this multiplication has the units of "joules," not "joules per second per second."

Taken back to fundamental units,
  • "Kilowatt" has units of (kilogram-meter squared)/(second cubed)
  • "Kilowatt Hour" has units of (kilogram-meter squared)/(second squared)
  • "Joule" also has units of (kilogram-meter squared)/(second squared)
 
What Charlie said. One unit of time is actually canceled out.
Kilowatt = kW_________m?*kg/s?
Kilowatthour = kW*h___m?*kg/s?
Joule = J_____________m?*kg/s?
BTU, calories, foot-pounds also have the same basic SI units as kWh.
 
Reiterating what everyone else has already said, but attempting to confuse someone:

Q1. The power company sells us power using the unit kilowatt hour.
Response: Correct.

Q2. The definition of watt is one joule per second.
Response: Correct

Q3. Isn't kilowatt hour using time twice.
Response: It can be said that 'time' is being "used" twice, but because 'time' is in the denominator for WATT, and 'time' is in the numerator for HOUR', the second time it's used, it cancels-out one of the 'time's.

Q4. Since time is used to define watts, I would think time can't be used again.
Response: Sure it can. Just as the defination of WATT uses 'meters' twice, and 'time' three times.
 
rrsatx said:
Reiterating what everyone else has already said, but attempting to confuse someone:

Q1. The power company sells us power using the unit kilowatt hour.
Response: Correct.

Q2. The definition of watt is one joule per second.
Response: Correct

Q3. Isn't kilowatt hour using time twice.
Response: It can be said that 'time' is being "used" twice, but because 'time' is in the denominator for WATT, and 'time' is in the numerator for HOUR', the second time it's used, it cancels-out one of the 'time's.

Q4. Since time is used to define watts, I would think time can't be used again.
Response: Sure it can. Just as the defination of WATT uses 'meters' twice, and 'time' three times.
Click to expand...

Good response, and Welcome!
 
rrsatx said:
Q3. Isn't kilowatt hour using time twice.
Response: It can be said that 'time' is being "used" twice, but because 'time' is in the denominator for WATT, and 'time' is in the numerator for HOUR', the second time it's used, it cancels-out one of the 'time's.
Click to expand...

I was getting ready to type this exact thing. At the risk of over-explaining, I'll clarify a little. The denominator is the bottom of the fraction and the numerator is the top. In the OP, you point out that a Joule is a Watt per second. Written in equation form, that is J=W/s. The s is on the bottom of the fraction. kWh are kilowatts multiplied by hours (an hour is just 3600 seconds, so we're still really just using seconds) and in equation form that is kW*h. Now the seconds are on the top of the fraction. When you multiply two fractions together and there is a term that is the same on the top and the bottom, it cancels out. Therefore, the end results has no seconds in it at all.
 
Danny Phillip Doeden said:
Isn't kilowatt hour using time twice.
Click to expand...

What? Because a watt/second is a joule?

Watt/hour isn't a joule. But it is 3600 joules. You can make a kilowatt hour out of that by multiplying it by 1000.

You could say the same thing about miles per hour. 2580'/hour.

0.7166'/Second.

I didn't even read the whole thread but check out what happens with E=MC squared.

(energy)=(Mass) Times (the speed of light) squared.

The speed of light is 186,000 miles per second or 300,000,000 meters per second.

In the equation E=MC squared, you have to realize that velocity is distance divided by time.

This celibrated equation forces us to realize that time has at least two dimensions.

The reason is that "C" (the speed of light) already has a time component, (distance/time) now, because that is squared, time has at least two dimensions. It's an area.

You can use time twice. And far more.

By the way, you're not using time twice in this case, only changing it's unit of measure.

I know. :grin: I couldn't help it.
 
Last edited:
Now Sam:

Now Sam:

physis said:
What? Because a watt/second is a joule?

Watt/hour isn't a joule. But it is 3600 joules. You can make a kilowatt hour out of that by multiplying it by 1000.

You could say the same thing about miles per hour. 2580'/hour.

0.7166'/Second.

I didn't even read the whole thread but check out what happens with E=MC squared.

(energy)=(Mass) Times (the speed of light) squared.

The speed of light is 186,000 miles per second or 300,000,000 meters per second.

In the equation E=MC squared, you have to realize that velocity is distance divided by time.

This celibrated equation forces us to realize that time has at least two dimensions.

The reason is that "C" (the speed of light) already has a time component, (distance/time) now, because that is squared, time has at least two dimensions. It's an area.

You can use time twice. And far more.

By the way, you're not using time twice in this case, only changing it's unit of measure.

I know. :grin: I couldn't help it.
Click to expand...

Sam,

"watt/sec" doesn't make sense. "watt-sec" does.
 
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