Working with air compressors requires understanding its units of measurement – from sizing your compressor to calculating your energy costs and everything in between. Today on Speaking the Same Industrial Language, we’ll get back to the basics and break down some of the most common units of measurement and explain what exactly they mean.
Unit of Measurement  What It Means 

kW (kilowatt)  A measure of power equal to 1,000 watts 
kWh (kilowatt hour)  A measure of energy equal to the power consumption of 1,000 watts over 1 hour 
CFM (cubic feet per minute)  The rate at which a certain volume of air is delivered at a certain pressure valve 
SCFM (standard cubic feet per minute)  Determines weight of air based on different fixed reference conditions:
OR
OR

ACFM (actual cubic feet per minute)  Compressors are normally rated in ACFM, which is measured at the delivery point of prevailing ambient temperatures 
HP (horsepower)  A measure of the amount of power the compressor’s motor can supply 
BHP (brake horsepower)  A measure of the motor’s power without any losses from the auxiliary engine components 
PSI (pounds per square inch)  A measure of pressure resulting from a onepound force applied to one square inch 
PSIA (pounds per square inch absolute)  A measure of ambient barometric pressure as it varies with altitude and weather 
PSIG (pounds per square inch gauge)  A measure of gauge pressure relative to ambient or atmospheric pressure 
Now that we’re familiar with an air compressor’s common units of measurement, we can see how they’re used to calculate things like the cost of annual energy consumption.
Calculating the cost of annual energy consumption:
Cost ($) = (bhp) x (0.746) x (operating hours) x ($/kWh) x (% time) x (% full load bhp) Motor Efficiency
Where:
Let’s take a look at a look at an example offered by the Department of Energy:
A typical manufacturing facility has a 200 hp compressor (which requires 215 bhp) that operates for 6800 hours annually. It is fully loaded 85% of the time (motor efficiency = 95%) and unloaded the rest of the time (25% fullload bhp and motor efficiency = 90%). The aggregate electric rate is $0.05/kWh.
Cost when fully loaded =
[(215 bhp) x (0.746) x (6800 hrs) x ($0.05/kWh) x (0.85) x (1.0)] / 0.95 = $48,792
Cost when partially loaded =
[(215 bhp) x (0.746) x (6800 hrs) x ($0.05/kWh) x (0.15) x (0.25)] / 0.90 = $2,272
Annual energy cost = $48,792 + $2,272 = $51,064
Keep in mind that in order to meet optimal efficiency and conserve energy, only the minimum number and size of compressors should be used to meet the required capacity and pressure of your applications.