kW Question

[SLaubach]

I was asked a qeustion by one of the maintenance techs that i could not answer. I have a machine that has a nameplate kW rating of 2.7, what unit of time is that? is it generally per hour, minute, etc...

thanks in advance

 

[Steve Bailey]

A kilowatt is a measure of power. Power is the rate of energy transfer. However, power itself is independent of the timescale.

1 kilowatt is equal to 3413 BTU/hour or 0.95 BTU/second
1 kilowatt is equal to 44.24 Ft-Lbf/minute or 0.74 Ft-Lbf/second.

 

[NorthControl]

At any time!

That is the machines "usage" of power or power rating

ex 2,7 kWh is how much it uses in one hour

pr second would be kWs if used.....

 

[NorthControl]

Sorry

Sorry S B, you're too fast for me....

 

[SLaubach]

That is the machines "usage" of power or power rating

ex 2,7 kWh is how much it uses in one hour

pr second would be kWs if used.....

OK reread my post i have misled you. I understand KW is a measure of power, i just dont know over what time scale the power is measured. if i assume it is kw/hr so the cost to run this machine for (1) - 20hr day would be ...

2.7kW x 20hr x $.075 = $4.05 per day?

 

[Rick Densing]

I'm a little confused, the namplate is kW not kWh. so the cost to run this machine for (1) - 20hr day would be ...

2.7kW x 20hr x $.075 = $4.05 per day?

I am assuming you mean the the price per kilowatthour is $0.075 and you are running it at full load, yes.

KW is an instantaneous number.

 

[Steve Bailey]

Assuming $.075 per KwH electrical cost and 100% efficiency of the machine (motor?) then operating it at full rated power for 20 hours would cost $4.05.

If the device is 95% efficient, then you will have to put energy into it at a rate of 2.84 Kw to get 2.7 Kw out.

 

[SLaubach]

great ! thanks.

 

[Ken M]

The actual instantaneous amount of energy being consumed by the machine is measured in Joules. And that's what the electric company charges you for: energy, not power.

Now, 1 Watt is equal to an energy consumption (or production) of 1 Joule per second. So if your machine consumes 1 Joule of energy every second, and you run it for 1 minute, the electric company will bill you for 60 Joules. Of course as with some other measurements (like Farads) the basic unit is often hopelessly impractical for everyday use. Billing you for Joules would require very wide paper for the big numbers you would need. So the convention was adopted many years ago of expressing energy as power x time. After all, the definition of power is energy / time. So rather than talk about so many Joules, we use kiloWattHours to scale the measurement up to something useful. In other words 1 kWh is 3,600,000 Joules. So your machine, rated at a power of 2.7kW consumes 2700 Joules every second (alright assuming 100% efficiency!) After 1 hour you've consumed 3600 x 2700 Joules and will be billed for them, or as you would recognise it, 2.7kWh. If your machine was only on for 30 minutes you get billed for 1800 x 2700 Joules or 1.35kWh. It doesn't mean your machine has lost power (the 2700 factor is the same), it means it was on for a shorter period of time.

Regards

Ken

 

[SLaubach]

Again, thank you all for helping me.

What a great board with intelligent people! i recommend it to many people when the subject come up.

 

[TConnolly]

2.7KW means that the machine uses energy at the rate of 2,700 Joules per second. For converting that to KWH see this post in another thread:

http://www.plctalk.net/qanda/showpost.php?p=147540&postcount=6

 

[10baseT]

Here we go , let's upset someone else .

STEVE you terrible man !

1 kilowatt is equal to 3413 BTU/hour or 0.95 BTU/second
1 kilowatt is equal to 44.24 Ft-Lbf/minute or 0.74 Ft-Lbf/second.

How about 1 Watt is 1 Joule/second
and 1 Joule is 1 Newton Metre
or (on earth) 10 Joules are required to lift 1 Kg through 1 Metre
We can also state that a Joule is one coulomb through a PD of 1 volt .

But please don't confuse people by stating conversions factors , a Kilowatt is just that and a horsepower is a horsepower , I wouldn't expect to see them used simultaniously .

 

[SimonGoldsworthy]

I thought I'd add some fuel to this fire - what about KVA ? I thought large commercial electricity customers were charged on KVAh (with penalties being invoked on KVArh limits being exceeded). Anyone have any direct experience of this ?

 

[10baseT]

Quite right Simon , power factor comes in to it , so if a consumer has a power factor other than unity , then with KVA metering , then they ( the consumer) get to pay for it .

 

[Steve Bailey]

Tell you what 10baseT,

You answer questions your way, and I'll answer questions my way. I won't take issue with your methods except to point out when you make a mistake. Whether or not you show a corresponding amount of respect depends on whether or not you are a 'terrible man'.

But please don't confuse people by stating conversions factors , a Kilowatt is just that and a horsepower is a horsepower , I wouldn't expect to see them used simultaniously .

At the time I posted my initial response, the question on the table was

what unit of time is that?

I gave few conversion factors in an attempt to show that energy transfer at the rate of 1 kilowatt is independent of the timescale used to express it. Notice that for the two examples I cited, I gave two different units of time.

Since a kilowatt is a measure of the rate of energy transfer, I thought about using a more familiar rate as an example. It doesn't matter if you say a car is traveling at 60 miles per hour or 88 feet per second. Either way, you're describing the same phenomenon. I was concerned that the analogy might add to the confusion rather than clarify things.

I didn't see where anyone other than you used KW and HP simultaneously.