To estimate power costs, you will need to know the following:
- What is the cost per KWH?
- How many hours per year does the compressor run?
- At what capacity will the compressor run or how many hours will the compressor run at various load levels?
- What are the brake horsepower requirements of the compressor at the required load levels?
- What is the motor efficiency?
It is important to use actual CFM requirements to figure the load level of the compressor. Do not base power cost calculations on comments like, “About half the time we run at full load and about half the time we run at 70% of full load.” Full load for one machine may not be the same as full load for another machine. Always determine the exact air requirement in order to provide the customer with a power cost calculation that approximates his situation.
Motor efficiencies vary from horsepower to horsepower and from manufacturer to manufacturer within horsepower ranges. The only way to accurately figure power costs will be to use the motor efficiency number on the nameplate of the actual motor being used.
With the above information in hand, annual power costs can be estimated by using the following formulas:
- kW=BHP x .746/motor efficiency
Example – Find the kW of a 100 HP, normal efficiency motor running at a 95 HP load.
- kW = 95 x .746/.93 = 76.2
- Cost per hour = KWH x power cost
Example – Find the cost per hour to operate the compressor in the above example assuming a cost per KWH of 7 cents.
- Cost per hour = 76.2 x 0.07 = $5.334
To find the annual power costs, calculate the cost per hour of operating at the various anticipated load levels and multiply by the anticipated number of hours that the machine will operate at those load levels.