Peak Demand

[marasmit]

What is the correct formula to use if you need to find the peak demand for electrical usage over a given period of time. i know the ave. min. max KW and KVAR.

 

[charlie b]

There is no formula. Indeed, what you are asking cannot be done.

Think of it this way: Suppose I told you that I drove a total of 100 miles, and that the trip took me 2 hours. You could calculate my average speed to be 50 MPH. But tell me, from the information I have given you, what my fastest speed was over that two hour period. Did I drive 70 mph for part of the time, and did I speed up to 80 mph to pass someone along the way? "Peak demand" is like "highest speed." Neither the total nor the average value can be used to tell the peak value.

So if you need to know the peak value, you need to look at the historical record of measurements, and look for the highest value that was recorded.

 

[QES]

Easy, get the number from your POCO.

 

[charlie]

. . . get the number from your POCO.

I assume that a project manager is in the process of putting a new service together. You are, of course, correct if is a large existing service where they have demand meters installed.

 

[physis]

If anybody would know about this it would be Charlie E. Isn't there a peak meter, hell, I could build one if nobody else does, but then there's probably not a very large demand for something like that.

 

[charlie b]

Easy, get the number from your POCO.

That might be a useful source of information. But it does not answer the question.

 

[bob]

What is the correct formula to use if you need to find the peak demand for electrical usage over a given period of time. i know the ave. min. max KW and KVAR.

When you say you know the "max KW and KVAR", what do you mean? That could be the peak demand. Generally the max demand is the greatest demand over a certain period, say 15 minutes. Peak demand may be the max instantaneous demand. What are you looking for?

 

[charlie b]

When you say you know the "max KW and KVAR", what do you mean? That could be the peak demand.

Good catch. I had stopped reading at the word "ave." Mea culpa.

If you really do know the highest readings of KW and KVAR, you can obtain a conservative value for peak KVA demand by applying the Pythagorean Theorem: KVA is the square root of the sum of KW squared plus KVAR squared.

However, unless you have a time line history of both readings, you won't know if the peak value of KW occurred at the same time as the peak value of KVAR. If they didn't, and if you use the equation I describe above, then you will get an answer that is larger than it has to be. That is what I meant when I called it a "conservative value."