watt vs kilowatt.hour


by kuin
Tags: kilowatthour, watt
kuin
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#1
Nov27-12, 01:25 AM
P: 5
Hello,

getting confused about watt vs watt.hour

in the Watt's wiki page, an example is given on how to calculate wattage (power):

A person having a mass of 100 kilograms who climbs a 3 meter high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due to gravity times height divided by the time it takes to lift the object to the given height gives the rate of doing work or power
So, (100kg x (9.81m/s2) x 3m) / 5s = 588 kg.m2/s3

this system needs aroud 600 watts of power to accomplish its task. ok.

Then, lower in the page, an example is given on how to calculate watt.hour(energy):

For example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watt-hours (Wh), 0.1 kilowatt-hour

Now, if I go back to my first example, it's being done in 5 seconds for 588,6 watts

if it was done in 1 second, it would be 2943 watts

if it was done in 3600 seconds, it would be 0,8175 watts

So, if I do the work on a 1 hour (3600 sec) time span, can say I use 0,8175 watt.hour?

And if I lift the thing in 1 second, but constantly, for 1 hour, do I use 2943 watt.hour?

This is confusing me a little, so I guess my question also is a little confused. I looked many places where it was promised that this confusion would evaporate after reading their explanations. Now I'm here because I'm still a little lost.

thanks
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sophiecentaur
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#2
Nov27-12, 02:47 AM
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It may help to ignore the actual figures for a start. The Watt is a unit of Power - the rate of doing work or using energy. The kW hour is a unit of Energy - just like the more basic Joule. If you read each problem / example carefully you will always see whether Power or Energy is what is needed.
Power is Energy / Time
Energy is Power X Time
Hold that in your head and the numbers will look after themselves.
russ_watters
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#3
Nov27-12, 05:35 AM
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Quote Quote by kuin View Post
This is confusing me a little, so I guess my question also is a little confused. I looked many places where it was promised that this confusion would evaporate after reading their explanations. Now I'm here because I'm still a little lost.

thanks
Welcome to PF!

Your confusion isn't affecting your ability to do the math, so I'm not sure what there is we can do to help other than to tell you to trust your understanding, because you understand correctly.

...Or ask us another question...

ParamTv
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#4
Nov27-12, 06:37 AM
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watt vs kilowatt.hour


NO need to confuse over it. Just Take it easy. I try to make it clear.
Actually the rate of work done is called power. I mean for calculating the power you must know the work done. ... Work done per sec is called power and measured in Watt. Work done per minute is also power. Work done per hour is also power but measured in Watt.hour.
Actually Watthour And killowatthour is represent the power produce by a machine in 1 hour.

Suppose a machine produce 1500watt power in one sec. then it will produce 1500X60 watt in a minute and 1500x60x60 in a hour or i can say 1500x60x60/100 Kwatthour (KWH)
hope you understand
thanks
sophiecentaur
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#5
Nov27-12, 06:51 AM
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Yep. Two issues here. 1. Believe the Physics. 2. Believe the arithmetic.
russ_watters
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#6
Nov27-12, 08:19 AM
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Quote Quote by sophiecentaur View Post
Yep. Two issues here. 1. Believe the Physics. 2. Believe the arithmetic.
3. Believe the [mathematical] definition of the words.
nasu
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#7
Nov27-12, 10:12 AM
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Quote Quote by ParamTv View Post
NO need to confuse over it. Just Take it easy. I try to make it clear.

Actually Watthour And killowatthour is represent the power produce by a machine in 1 hour.
No, they represent energy and not power.
A watt-hour represents a specific amount of energy (3600 J) no matter in what time is this energy used or transformed.
Even saying "power produced in 1 hour" is meaningless.
The machine "produces" energy. Power shows you how fast is done, not necessarily how much of it.
1 watt -hour of energy may be "produced" in 1 second or 1 billion years. It's still 1watt-hour.
sophiecentaur
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#8
Nov27-12, 12:11 PM
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Quote Quote by nasu View Post
No, they represent energy and not power.
A watt-hour represents a specific amount of energy (3600 J) no matter in what time is this energy used or transformed.
Even saying "power produced in 1 hour" is meaningless.
The machine "produces" energy. Power shows you how fast is done, not necessarily how much of it.
1 watt -hour of energy may be "produced" in 1 second or 1 billion years. It's still 1watt-hour.
I agree.
I find people's determination to use the wrong terms, in this way, very annoying. It's as if they really don't get it. People who get aerated about the 'offside rule', as if it's actually important. But they will use scientific terms interchangeably like some poet who can't bring himself to use the same word twice in a sentence and scrabbles around to find a shoddy equivalent. The fact is that there is, with very few exceptions, only one word for each scientific concept. It just does not help anyone to have the alternative (wrong) terms used instead. It doesn't make the subject any more approachable - just the reverse.

I heard some clown of an energy minister on BBC Radio, recently, talking about "storing Power"!!! What hope do we have?
nasu
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#9
Nov27-12, 12:50 PM
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Quote Quote by sophiecentaur View Post

I heard some clown of an energy minister on BBC Radio, recently, talking about "storing Power"!!! What hope do we have?
Well, for a minister I think it's OK. He's not trying to learn or teach (thanks God) physics.
In common language, power is used so often for "energy" that maybe you should close you eyes. It's a "power station" or "power plant" after all. At least in North America.
A car factory produces cars which are stored in parking lots. So what does a power plant produce (and store)?
sophiecentaur
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Nov27-12, 01:07 PM
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I clearly made the mistake of hoping that he should know what he's talking about.
If he's as ignorant as that, how can he distinguish between sense and nonsense when he's being sold some idea by a flash salesman? The distinction could be highly relevant to making a good decision.
For instance, what use is a 5MW wind turbine if it only operates for 30 days per year?
(You wouldn't be confused, by common language, yourself would you??? )
K^2
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Nov27-12, 01:33 PM
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Quote Quote by sophiecentaur View Post
For instance, what use is a 5MW wind turbine if it only operates for 30 days per year?
Sell excess back to electric company in these 30 days, use power from the grid for the remaining 335 days. At 5MW, you are still going to be making a net profit.

:p
nasu
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#12
Nov27-12, 03:02 PM
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Quote Quote by sophiecentaur View Post
For instance, what use is a 5MW wind turbine if it only operates for 30 days per year?
(You wouldn't be confused, by common language, yourself would you??? )
Oh, I did not pay attention that you said energy minister.
This makes some difference.
nasu
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Nov27-12, 03:05 PM
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Quote Quote by K^2 View Post
Sell excess back to electric company in these 30 days, use power from the grid for the remaining 335 days. At 5MW, you are still going to be making a net profit.

:p
And how is the electric company going to store all the power it buys from people?
sophiecentaur
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Nov27-12, 03:23 PM
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Quote Quote by nasu View Post
Oh, I did not pay attention that you said energy minister.
This makes some difference.
Well, that's what he calls himself. It can't be 'Minister of Power'; the PM wouldn't like that.
sophiecentaur
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Nov27-12, 03:25 PM
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Quote Quote by K^2 View Post
Sell excess back to electric company in these 30 days, use power from the grid for the remaining 335 days. At 5MW, you are still going to be making a net profit.

:p
Hmmm
Not sure you could actually fit a 5MW turbine on your roof. I was thinking more of those big devils out at sea. They are owned and run by the Elec company.

Did you consider the capital cost of one that big?
the_emi_guy
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#16
Nov27-12, 03:32 PM
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Kuin,

Here is an analogy for you.

The bottled water factory produces 8,000 gallons per day.

I can store 10 gallons in my pantry.

See the difference between "gallons per day" and "gallons"?
sophiecentaur
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Nov27-12, 03:34 PM
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Quote Quote by the_emi_guy View Post
Nasu,

Here is an analogy for you.

The bottled water factory produces 8,000 gallons per day.

I can store 10 gallons in my pantry.

See the difference between "gallons per day" and "gallons"?
If that's beer then we can have a party.
CWatters
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#18
Nov27-12, 04:42 PM
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A lot of the confusion comes because the energy companies use kWh as a measure of energy which it is (because its power * time) but it's not a basic unit like the joule. To make matters worse you sometimes find people talking about "kWh per hour" which is the same as saying kW.


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