power usage and cost advice for 15kw

[ban-all-sheds]

"Technically true" sounds like you're saying the error doesn't really matter, which it does, hugely.

And I wasn't playing with him - I wrote what I did to show just how important the difference between 'kW' and 'kWh' is by showing how using the wrong term can make what you're saying turn into nonsense.

 

[JohnW2]

Usually seconds. 15kW = 15 kJ/second

Why?

Because that makes the arithmetic simple, given that 1 W=1 J/sec. You can have any other unit of time if you want, e.g. 15 kW = 54,000 J/h - but most people wouldn't find that so convenient.

And who has determined "usually"?

It's not a matter of 'determined'. It was the person who defined a Watt as 1 Joule/sec who made it convenient, hence 'usual'.

If the load was working for 1ns, would it not still be a 15kW load?

Of course, but that doesn't alter the fact that 15kW is the rate of energy consumption (i.e. energy per unit time).

Kind Regards, John

 

[mfarrow]

Bas, any rate unit requires a time element. For example:

• A cooker consuming gas at a rate of 5 cubic metres per hour

• A car which can travel along a road at a rate of 150 miles per hour

• A fat person is America can eat at a rate of 12 hotdogs per minute

• My kettle consumes energy at a rate of 2.2 kilowatts ≡
  My kettle consumed energy at a rate of 2.2 kilojoules per second

Watts has a time element, purely because someone thought to create an SI unit to describe J/s. It would have been a lot simpler if they hadn't!

 

[ban-all-sheds]

I agree, I worded it poorly.

The point I was trying to make is that there is no specific time which is relevant to the usage of a number of watts. The definition of a watt uses 1s as the time for which an amount of work is done, but there is no time whatsoever implied when an electrical load is described as 1W.

Yes, 1W = 1J/s, but since Joules don't get used in electricity tariffs, or appear on bills, you could just as usefully say that 1W = 1kgm²/s³ for all the good it would do.


A 15kW heater is converting energy from one form to another at a rate of 15kJ/s. Or 54MJ/h. Or 15J/ms. Or about 51,182Btu/h

So while you can equate a number of watts to a rate of work, i.e. to some amount of work per unit of time, there is no time component of a number of watts.

A 15kW heater is using 15kJ/s. It is not using 15kW/s, or 15kW per hour, or 15kW per day, it is using 15kW.

Your kettle is consuming 2.2kW. Not 2.2kW/s.

If it really was doing something described as 2.2kW/s then it would not be able to do it for very long, as the fuse in the plug would blow after a few seconds - a W/s figure is an acceleration.

 

[JohnW2]

I agree, I worded it poorly. The point I was trying to make is ....

Exactly. As I wrote to you:

More generally, Power = Energy/Time. Energy = Power * Time. But you kinow all this - you just typed a bit hastily, as we all sometimes do!

Kind Regards, John

 

[ricicle]

Ok - if you want to play semantics.

The load consumes 15kw. It is on for one hour. How long was it consuming 15kw ?

For 1 hour.

And I'm not "playing semantics", but the fact that you say that shows that you just don't get it at all.

I can assure you I do get it, more than most as it appears.
I have just asked you how long it was consuming 15kw for. You replied one hour. I previously corrected my 'per hour' statement to say that the load was consuming 15kw for one hour and you implied I was wrong.

 

[ban-all-sheds]

the 15 kw, does that mean it uses 15kw of electric PER Hour ?

If the heater is on constantly (on demand) then it will consume 15kw per hour....

Maybe I should have wrote it will consume 15kw for an hour

Indeed - you did correct it.

But there is no "maybe" about it.

15kW/h = 15000W/h = 4.17W/s = 4.17J/s²

4.17J/s² means that its rate of work is increasing all the time.

Run it for 10 hours and you will end up with this:

4.17 x 36000s

s²

i.e. 4.17x36000J

s

i.e. after 10 hours of running it's cracking on at a rate of 150120J/s, i.e. it's now consuming 150kW.

I think not.

My comment was 'I'm not "playing semantics", but the fact that you say that shows that you just don't get it at all'.

If you truly do get it then why did you claim that saying 15kW/h is wrong, and should be changed to 15kW for an hour was "playing semantics"?

Perhaps if you explain what you meant by "playing semantics" that might help.

 

[ricicle]

I said you were playing semantics in response to you trying to correct me again after I had altered my original statement in which I stated it will consume 15kW for an hour. You still persisted to say no it will consume 15kW however long it is on for which, yes, is true.
But in the context of the explanation of the heater being on constantly for an hour it would consume 15kW for an hour.

 

[JohnW2]

I do rather hope that most of us have been 'playing' to some extent. The relationship between energy and power is so basic that I would sincerely hope that any of us who have participated in this thread (with the exception of the OP) would have a full and clear understanding of it, even if blindfolded and standing on our heads after drinking a good few beers - and I would be shocked if that wasn't the case.

Kind Regards, John