Speaking the same industrial language: understanding a compressor’s common units of measurement

October 10, 2017

5 minutes

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: 14.7 Pounds per Square Inch (psi) 60 Degrees Fahrenheit (^°F) 0% Relative Humidity (RH) OR 14.7 Pounds per Square Inch (psi) 68 Degrees Fahrenheit (^°F) 0% Relative Humidity (RH) OR 14.7 Pounds per Square Inch (psi) 68 Degrees Fahrenheit (^°F) 36% Relative Humidity (RH)
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 one-pound 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

Understanding compressor power: HP, BHP, and kW

Of all the units in the table above, the ones most directly tied to compressor power are HP (horsepower), BHP (brake horsepower), and kW (kilowatt). Together, these three measurements paint a complete picture of how much power a compressor consumes, how efficiently it converts that power into compressed air, and what it will ultimately cost to run.

Understanding compressor power: HP, BHP, and kW

Compressor power and horsepower (HP)

Horsepower is one of the most commonly referenced indicators of compressor power. In simple terms, HP tells you how much power the compressor's motor can supply to drive the compression process. A higher HP rating generally means the compressor is capable of delivering greater pressure and flow, but HP alone does not tell the full story of a compressor's real-world power consumption or efficiency.

It's important to note that HP is a nameplate rating, it reflects the motor's maximum power output under ideal conditions. In practice, the actual power required to drive the compressor will vary depending on operating pressure, load conditions, and the efficiency of the motor and drive system.

Compressor power and brake horsepower (BHP)

BHP, or brake horsepower, is a more precise measure of compressor power because it accounts for the actual power delivered at the compressor shaft, excluding losses from auxiliary components such as cooling fans, pumps, and other supporting systems. Because BHP reflects the true mechanical power being used to drive compression, it is the value used in energy consumption calculations rather than the nameplate HP rating.

As you can see in the energy cost formula below, BHP is the starting point for calculating what a compressor actually costs to run. A compressor with a lower BHP relative to its output is a more power-efficient machine, it delivers more compressed air per unit of energy consumed.

Compressor power and kilowatts (kW)

While HP and BHP are the traditional units for expressing compressor power in the United States, kilowatts (kW) are the standard unit of power in the international system and are increasingly used in industrial energy management. Since electricity is billed in kilowatt-hours (kWh), converting compressor power from BHP to kW is an essential step in calculating energy costs. The conversion is straightforward: 1 HP = 0.746 kW, which is why you'll see this factor appear in the energy cost formula below.

Understanding your compressor's power rating in kW is especially useful when benchmarking energy efficiency, comparing compressors from different manufacturers, or working with energy auditors to identify opportunities for cost savings in your compressed air system.

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:

bhp = compressor shaft horsepower
Operating hours = number of hours your compressor operates
1 hp = 0.746 kW
$/kWh = electricity rate
percent time = percentage of time running at this operating level
motor efficiency = motor efficiency at this operating level

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% full-load 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.

Frequently asked questions

What does compressor power mean?

Compressor power refers to the amount of energy a compressor's motor uses to drive the compression process. It is most commonly expressed in horsepower (HP), brake horsepower (BHP), or kilowatts (kW). HP reflects the motor's rated output capacity, BHP measures the actual power delivered at the compressor shaft after accounting for mechanical losses, and kW is the standard unit used for energy billing and international comparisons. Together, these measurements help you understand how powerful a compressor is, how efficiently it operates, and what it will cost to run.

How does compressor power affect energy costs?

Compressor power is directly tied to energy costs, the more power a compressor consumes, the higher your electricity bill. This is why BHP is used as the basis for energy cost calculations rather than the nameplate HP rating, as it more accurately reflects real-world power consumption. Choosing a compressor with the right power rating for your application, not too large and not too small, is one of the most effective ways to manage energy costs. Oversized compressors consume more power than necessary, while undersized compressors are forced to run continuously at full load, which also drives up energy usage and accelerates wear.

What is the difference between HP and BHP in a compressor?

HP (horsepower) is the nameplate power rating of a compressor's motor, it represents the maximum power the motor can deliver under ideal conditions. BHP (brake horsepower) is the actual power measured at the compressor shaft, taking into account real-world mechanical losses from auxiliary components such as cooling fans and oil pumps. Because BHP reflects true power consumption more accurately than HP, it is the value used when calculating annual energy costs and comparing the operational efficiency of different compressors.

How do I convert compressor power from HP to kW?

Converting compressor power from horsepower to kilowatts is straightforward: multiply the HP value by 0.746. For example, a 200 HP compressor has a power equivalent of approximately 149.2 kW. This conversion is important because electricity is billed in kilowatt-hours (kWh), so expressing compressor power in kW allows you to directly calculate energy consumption and cost. When comparing compressors or conducting energy audits, working in kW ensures consistency, especially when evaluating equipment from international manufacturers who may rate their compressors in kW rather than HP.

How do I choose the right compressor power for my application?

Choosing the right compressor power starts with understanding your application's pressure (PSI) and flow (CFM) requirements. From there, you can determine the BHP needed to meet those demands efficiently. A good rule of thumb is to select a compressor that can comfortably meet your peak demand without running at maximum load continuously, this preserves motor efficiency, reduces energy consumption, and extends the compressor's service life. It's also worth considering future capacity needs; a compressor that is slightly larger than your current demand may be more cost-effective in the long run than one that is immediately maxed out. When in doubt, consult an Atlas Copco compressed air expert who can help you match compressor power to your specific operational requirements.