Calculating a Kilowatt hour

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Red Wiggler

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How do you calculate for a Kilowatt hour?

I have a customer that is designing a new facility and was wondering what his monthly power consumtion would be if the following estimate of usage is calculated...

480 volt 3 phase service

He estimates that during the day he will have an estimated 800 amp load for 12 hours. He thinks that the remaining 12 hours would be estimated for a 250 amp load.

What would be his monthly Kilowatt hour consumption?
 
480*800*1.732=665,088/1000


665 *12 hours =7980

7980*15 days = 119700


480*250*1.732= 207840/1000

207.8 *12 hours = 2494

2494*15 days = 37411




119700 + 37411 = 157111 kwh
 
mikeames, shouldn't you be using 30 days in your formulas?

The op said 12 hours at 800 amps and the other 12 hours at 250 amps...
but that's just one day. Not 800 amps one day & 250 amps the next day etc...
 
Yea but I figured 12 hours is half a 24 hour day, so 15 solid days of a 12 hour load and 15 solid days of another load = 30 solid days of both loads.
 
Your right I messed myself up thinking too much in the head. I did 15 because the reason I explained above but then realized I used 12 hours instead of 24. Sorry bone head move on my part. My numbers are wrong.
 
mikeames said:
480*800*1.732=665,088/1000


665 *12 hours =7980

7980*30 days = 239400


480*250*1.732= 207840/1000

207.8 *12 hours = 2494

2494*30 days = 74820




239400 + 74820 = 314220 kwh
Click to expand...
here ya go.

Add: There will be probably be a power factor with the load, so you would really take the result times something like 80% or 90%. But, this is such a guess anyway that it is not critical. Plus, you would probably get charged a pf penalty if you were down at 80% anyway. Just worth mentioning for future reference.

pf = kW/kVA
and
kVA^2 = kW^2 + kvar^2
 
Last edited:
Why have all of the calculation examples assumed that the loading on the weekends would be the same as the laoding on the weekdays?:-?

Is the intent to provide a 'high' estimate?
 
jim dungar said:
Why have all of the calculation examples assumed that the loading on the weekends would be the same as the laoding on the weekdays?:-?

Is the intent to provide a 'high' estimate?
Click to expand...

Because the OP did not provide that info and we were giving the most straight fwd response based on the facts provided.
 
mikeames said:
jim dungar said:
Why have all of the calculation examples assumed that the loading on the weekends would be the same as the laoding on the weekdays?

Is the intent to provide a 'high' estimate?
Click to expand...
Because the OP did not provide that info and we were giving the most straight fwd response based on the facts provided.
Click to expand...
He said day, not weekday.
 
Red Wiggler said:
How do you calculate for a Kilowatt hour?

I have a customer that is designing a new facility and was wondering what his monthly power consumtion would be if the following estimate of usage is calculated...

480 volt 3 phase service

He estimates that during the day he will have an estimated 800 amp load for 12 hours. He thinks that the remaining 12 hours would be estimated for a 250 amp load.

What would be his monthly Kilowatt hour consumption?
Click to expand...

=(365*460*SQRT(3)*0.8*((800*12)+(250*12))/(1000))/12

244281.5kWHr

(365Days/year*460Volts*1.732 square root of 3*0.8 power factor*((800A*12hours)+(250A*12hours))/(1000 Watt/KiloWatt)/12months/year
 
weressl said:
=(365*460*SQRT(3)*0.8*((800*12)+(250*12))/(1000))/12

244281.5kWHr

(365Days/year*460Volts*1.732 square root of 3*0.8 power factor*((800A*12hours)+(250A*12hours))/(1000 Watt/KiloWatt)/12months/year
Click to expand...
I looked at that for a bit. Seemed entirely correct.
But having done what amounted to the same calculation, I got a slightly different answer - about 255,000 kWh. Made me look a bit more closely.

The disparity was in the voltage.
You used 460V. I used 480V.
No big deal.
 
Jim pointed out they were working the weekends. Now Laszlo has them working all the holidays. At least they only have to work 12 hours a day.:D
 
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