Cost of Grid Power Versus Running a Generator

      Generators are a poor deal when fuel prices rise, but commercial power bills can get complicated. Here's a look at how the numbers crunch. March 3, 2006

Question
Currently we run our sawmill operation off a generator. With the high fuel prices, I am looking at hooking up to the line. I am trying to figure out how many kilowatt hours of electric I would be using. I am using 440v 3 phase (measured across 2 legs). An average of 100 amps on each phase while running.

I know that volts x amps = watts/1000 = kw. My question is, do I use 220 or 440 as my volts, seeing as 1 leg is 220 (if measured leg to ground)? And also do I multiply this by 3, seeing that it pulls 100 amps per phase?

Forum Responses
(Sawing and Drying Forum)
From contributor E:
Power = (3sqrt) x V x I or 1.732 x 440 x 100 = 76,210 W = 76.210 kW. The power company will also be interested in your peak demand and your power factor. You would multiply the above equation by your PF to get actual watts, which is how your consumption is billed, but the power company must deliver VA or kVA, so if you have a low PF, you get charged more because the power company must size the line and transformers to account for your reactive current.



From contributor D:
Generators are a pretty poor deal these days. You will average, at best, about 12kWh per gallon of fuel burned. So if diesel costs $1.80 per gallon, that is 15 cents per kWh for electricity. There are few places where electric rates are that high.


From contributor W:
You need to keep in mind that utilities bills for commercial accounts usually include 3 parts. First is the basic charge which is billed every month, second is the demand charge which is based on the connected load (kW), third is the energy charge based on the time the load is on (kWH). You will also get a bill every month and most utilities base this minimum on the installed transformer capacity. This minimum will be billed every month even if you don't use it. Therefore, before switching to the grid, you need to consider the amount of time (hours) that you operate. If this is just a weekend business and can live with the extra down time for maintenance, then stay with the generator. If this is a full time business, then electricity is probably the way to go. Also, a three phase service will cost several thousand dollars to have installed.


From contributor R:
The mill down the road cuts about 400,000 feet a month and their electric bill is around $5000.


From the original questioner:
Thanks for all your responses. It looks like hooking up to the line is what we have to do. When I started sawing a few years ago, I was buying off road diesel for less than $1.00 a gallon. I doubt we'll ever see that again!

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