Power consumption question

[Syrch]

Im curious on this. If you have something that is rated to pull 240w (.24kilowatt) while in use how is that 240w calculated? Is it for an hour of use? Is it for a minute of use? Anyone know?

 

[dullard]

1 Watt = 1 Joule / second

It already has the time built in.

[*]A 240 W device will use 240 J every second.
[*]A 240 W device will use 240*60 = 14.4kJ every minute.
[*]A 240 W device will use 240*60*60 = 864kJ every hour.

 

[So]

Watts are an instantaneous rating. It is pulling 240w at any given moment when it's on. In 1 hour iw till use .240kilowatt-hours, which is the unit you buy electricity in.

 

[Syrch]

well no i figured that. But my electricity costs me 10 cents a kilowatt. So if my computer for instance is pulling .24kilowatts while in use is that for a minute or for a second or for an hour etc. And I now a PC's power consumption can fluctuate and am using that as an example only.

 

[dullard]

Originally posted by: Syrch
well no i figured that. But my electricity costs me 10 cents a kilowatt. So if my computer for instance is pulling .24kilowatts while in use is that for a minute or for a second or for an hour etc. And I now a PC's power consumption can fluctuate and am using that as an example only.

No, your electricity does NOT cost you 10 cents per kilowatt. You have that wrong.

You are probably charged 10 cents per kW*hour. Thus, using the way you look at it, it is defined per hour.

 

[Syrch]

Originally posted by: So
Watts are an instantaneous rating. It is pulling 240w at any given moment when it's on. In 1 hour iw till use .240kilowatt-hours, which is the unit you buy electricity in.

So if im reading this right in eight hours it will have used 1.92 kilowatts?

 

[So]

Originally posted by: Syrch
well no i figured that. But my electricity costs me 10 cents a kilowatt. So if my computer for instance is pulling .24kilowatts while in use is that for a minute or for a second or for an hour etc. And I now a PC's power consumption can fluctuate and am using that as an example only.

It uses 240watts at any given instant, assuming constant current draw. Watts has time built in.

 

[So]

Originally posted by: Syrch

Originally posted by: So
Watts are an instantaneous rating. It is pulling 240w at any given moment when it's on. In 1 hour iw till use .240kilowatt-hours, which is the unit you buy electricity in.

So if im reading this right in eight hours it will have used 1.92 kilowatts?

no. If you run it for eight hours you will have used 1.92kilowatt-hours, which is a different unit from kilowatts.

Watts are power, kilowatt hours, like joules are energy.

Edit: You pay 10 cents per kilowatt hour, not per kilowatt. I guarantee it.

<--power engineer

 

[dullard]

Originally posted by: Syrch
So if im reading this right in eight hours it will have used 1.92 kilowatts?

No, it will have used 1.92 kilowatt*hours.

Units don't disappear. (.240 kW) * (8 hr) = 1.92 kW*hr.

If you are charged $0.10 / (kW*hr), then you do this math:

(1.92 kW*hr) * ($0.10 / (kW*hr)) = $0.192, or just over 19 cents.

 

[Syrch]

Hmm i probably sound like a noob on this and admit that i am. I used 2169 kWh (kw*hour?) and it cost me $230. So im assuming (ass out of you and me) its 9.43 cents of kWh. I just have no clue how to try to calculate what devices are using what amount of power with the given factors.

 

[Syrch]

Originally posted by: dullard

Originally posted by: Syrch
So if im reading this right in eight hours it will have used 1.92 kilowatts?

No, it will have used 1.92 kilowatt*hours.

Units don't disappear. (.240 kW) * (8 hr) = 1.92 kW*hr.

If you are charged $0.10 / (kW*hr), then you do this math:

(1.92 kW*hr) * ($0.10 / (kW*hr)) = $0.192, or just over 19 cents.

okay i was posting my other post as you wrote this one up. That is the equation i did not know how to come up with

 

[So]

Originally posted by: Syrch
Hmm i probably sound like a noob on this and admit that i am. I used 2169 kWh (kw*hour?) and it cost me $230. So im assuming (ass out of you and me) its 9.43 cents of kWh. I just have no clue how to try to calculate what devices are using what amount of power with the given factors.

9.43 cents per kilowatt hour.

9.43 cents/kwh seems really cheap to me though...

 

[spidey07]

Originally posted by: Syrch
Hmm i probably sound like a noob on this and admit that i am. I used 2169 kWh (kw*hour?) and it cost me $230. So im assuming (ass out of you and me) its 9.43 cents of kWh. I just have no clue how to try to calculate what devices are using what amount of power with the given factors.

killowats of power used per hour = kW hour.

If you computer is on 24x7 that's .24 killowatts*24 hours of use. that's your kW hours per day. multiply this times the number of days in your billing cycle to get total kWh for the month. multiply that times your kWh cost.

 

[Syrch]

Originally posted by: So

Originally posted by: Syrch
Hmm i probably sound like a noob on this and admit that i am. I used 2169 kWh (kw*hour?) and it cost me $230. So im assuming (ass out of you and me) its 9.43 cents of kWh. I just have no clue how to try to calculate what devices are using what amount of power with the given factors.

9.43 cents per kilowatt hour.

9.43 cents/kwh seems really cheap to me though...

I've never really looked at it this closely so i couldn't tell you but thats the numbers on my bill for the month. I had 70 kWh at an average cost of 7.43 per day.

 

[So]

Originally posted by: Syrch

Originally posted by: dullard

Originally posted by: Syrch
So if im reading this right in eight hours it will have used 1.92 kilowatts?

No, it will have used 1.92 kilowatt*hours.

Units don't disappear. (.240 kW) * (8 hr) = 1.92 kW*hr.

If you are charged $0.10 / (kW*hr), then you do this math:

(1.92 kW*hr) * ($0.10 / (kW*hr)) = $0.192, or just over 19 cents.

okay i was posting my other post as you wrote this one up. That is the equation i did not know how to come up with

Hope we've cleared it up for you a little bit. Dullard is 100% right. One of the most important things they teach you in introductory science / engineering is units, units, units...

You include units with everything and cancel as you go. Only then can you be sure your answer is in the right units. Yeah, we all get lazy about it when we're plugging into a calculation we've done a million times before. But anyone who is competent does it when they're doing something new. It's certainly something I've found useful in both work and everyday life.

 

[Syrch]

I think i understand it at this point. I hope this information helps anyone else out that might want to try to drill into their electricity bill

 

[lokiju]

My brain hurts now.