- Author
- Byron Raal, CAS Founder-Editor About the author
- Checked against
- ISO 11011
- Date last checked
- 29 August 2026
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A well-maintained modern oil-injected rotary screw compressor, measured at full load on a total package input basis, spends roughly 18 to 19 kW for every 100 cfm of free air it delivers. That is 0.18 to 0.19 kW per cfm, or 6.4 to 6.6 kW per m³/min FAD. At an indicative Australian industrial tariff of $0.30 per kWh, one cfm of continuously supplied air costs about $217 to $224 a year on 4,000 running hours, and $435 to $449 a year on 8,000 hours. Older fixed-speed machines at part load land closer to 25 to 34 kW per 100 cfm, which costs between about a third and about 90 per cent more for the same cfm. Those dollar figures are the value of the air at the stated specific power. Whether cutting demand turns into metered electricity avoided depends on how your compressor responds, which is covered below.
What “cost per cfm” actually measures
Cost per cfm is not a property of the air. It is a property of your compressor, your pressure, your controls and your tariff, and it derives from one number: specific power, electrical input divided by free air delivered.
Two plants can buy the same 500 cfm and pay bills that differ by about 44 per cent, because one spends 6.4 kW per m³/min making it and the other spends 9.2. A 132 kW compressor is neither expensive nor cheap until you know what it delivers. Two things must be true before a specific power figure means anything.
Declare the kW basis. A compressor’s total package input at full load is higher than its nominal motor rating, because the package also runs the cooling fan, the controller and any integrated dryer. Atlas Copco’s GA37, for example, pairs a 37.3 kW nominal motor with a 43.3 kW total package input, about 16 per cent higher, per its CAGI performance data sheet. That is one verified model, not a rule you can apply to yours. Pull the CAGI sheet for your own machine. Every figure on this page is total package input unless it says otherwise.
Declare the hours. A dollars-per-year figure without a stated hours basis is not a figure. Every annual number here states the hours it assumes.
The three units, and how to convert between them
Australian plants get quoted specific power in three units, often by three different people in the same week. They are the same number in different clothes, and the conversions rest on one exact constant: one foot is 0.3048 metres, so one cubic foot is 0.028316846592 m³.
- 1 cfm = 0.0283168 m³/min = 0.4719 L/s
- 1 m³/min = 35.3147 cfm = 16.667 L/s
- kW per m³/min to kW per cfm: multiply by 0.0283168.
- kW per m³/min to kW per L/s: divide by 16.667.
- kW per cfm to kW per 100 cfm: multiply by 100.
Worked through: 6.4 kW per m³/min x 0.0283168 = 0.1812 kW per cfm, which is 18.1 kW per 100 cfm, which is 6.4 / 16.667 = 0.384 kW per L/s. One machine, four numbers, no disagreement. If a supplier’s number and yours differ by a factor of about 35, one of you is in cubic metres and the other is in cubic feet.
The benchmark this page publishes, and how it was derived
CAS publishes two constants describing the same physical thing, and this page reconciles them rather than picking one.
The CAS leak-cost model uses 0.11 kWh per m³ of free air. Multiply by 60 minutes: 6.6 kW per m³/min FAD. Multiply by 0.0283168: 0.1869 kW per cfm, or 18.69 kW per 100 cfm.
The 2025 full-load benchmark for a modern VSD oil-injected rotary screw is 6.4 kW per m³/min FAD, which is 0.1812 kW per cfm, or 18.1 kW per 100 cfm.
Those two agree to within about 3 per cent, and they are not competing claims. The 6.4 figure is a best-in-class benchmark for one machine type at its design point; the 6.6 figure is the modelling constant CAS applies when converting an arbitrary air flow to energy across a mixed plant. So the honest headline is a band, not a point: 18 to 19 kW per 100 cfm, or 0.18 to 0.19 kW per cfm, at full load, total package input basis.
A single rounded figure of 0.20 kW per cfm sits about 7 per cent above the 0.11 kWh/m³ model constant and about 10 per cent above the 6.4 benchmark, and reconciles with neither. This page does not use it, and the derived figures above supersede it.
How to work out your own cost per cfm
You need two numbers read at the same moment, and a calculator. An afternoon, not a consultant.
Step 1. Get the kW. Read total package electrical input at the compressor, not the motor nameplate. A clamp meter on the incoming supply during a representative loaded period is enough for a first pass.
Step 2. Get the flow. Read free air delivered at the compressor discharge, at the same moment. Without a flow meter, the CAGI data sheet FAD at your actual discharge pressure is a starting point, but it is a rated figure on a new machine, so treat the result as indicative.
Step 3. Divide, then convert. Specific power is kW divided by flow. Convert to whichever unit your benchmark uses before comparing. Do not hold a kW per cfm figure against a kW per m³/min benchmark.
Step 4. Price one cfm. Annual cost of one cfm = kW per cfm x annual running hours x tariff in dollars per kWh. Use your own contract rate, including network and market charges, not the $0.30 used illustratively here.
Step 5. Measure the gap, not the number. Divide your specific power by the benchmark for your machine class and subtract one. That percentage is your excess energy at the compressor, before distribution losses.
Worked example: two plants, same air, different bill
Plant A, ageing fixed-speed rotary screw. 132 kW total package input against 240 L/s FAD, read at the same instant. 240 L/s is 14.40 m³/min, or 508.5 cfm. Specific power is 132 / 14.40 = 9.17 kW per m³/min, which is 132 / 508.5 = 0.2596 kW per cfm, or 26.0 kW per 100 cfm. On 6,000 hours at $0.30 per kWh, one cfm costs 0.2596 x 6,000 x 0.30 = $467 a year.
Plant B, modern VSD rotary screw. 82 kW against 215 L/s FAD. That is 12.90 m³/min, or 455.6 cfm. Specific power is 6.36 kW per m³/min, 0.1800 kW per cfm, 18.0 kW per 100 cfm. On the same 6,000 hours at the same $0.30, one cfm costs $324 a year.
Plant A pays about 44 per cent more for each cfm. Against the 6.4 benchmark it sits 43 per cent high; at benchmark it would draw about 92 kW rather than 132, a gap worth roughly $72,000 a year on 6,000 hours at $0.30 per kWh.
That $72,000 is the value of the energy gap at the stated specific power and hours. It sizes the opportunity; it is not a quotation. What lands on the meter depends on how the machine responds to reduced demand: load/unload, start/stop, inlet modulation or variable speed. On a load/unload machine the compressor still draws power while unloaded, so demand reductions arrive at the meter only partially until the control strategy or the machine changes.
Benchmarks by machine class
These are 2025 industry-typical full-load figures on a total package input basis. They are benchmarks, not specifications, and your machine’s CAGI data sheet is the authority for yours.
| Machine class | kW per m³/min FAD | kW per 100 cfm | kW per cfm | kW per L/s |
|---|---|---|---|---|
| Centrifugal at design point | approx 5.65 | approx 16.0 | 0.160 | 0.34 |
| Modern VSD oil-injected rotary screw | approx 6.4 | approx 18.1 | 0.181 | 0.38 |
| CAS leak-cost modelling constant | 6.6 | 18.7 | 0.187 | 0.40 |
| Small reciprocating | 7 to 8 | 19.8 to 22.7 | 0.198 to 0.227 | 0.42 to 0.48 |
| Older fixed-speed at part load | 9 to 12 | 25.5 to 34.0 | 0.255 to 0.340 | 0.54 to 0.72 |
A centrifugal at its design point makes the cheapest air in the table and is least forgiving off it. A small reciprocating machine is not badly maintained because it sits at 7.5; that is what the class does.
What moves your number
Discharge pressure. The US DOE Improving Compressed Air System Performance sourcebook gives a rule of thumb for systems in the 100 psig range: every 2 psi increase in discharge pressure raises energy consumption by about 1 per cent at full output flow. Where 30 to 50 per cent of usage is unregulated, elevated pressure also feeds artificial demand, and the combined effect is about 1.6 to 2 per cent per 2 psi. Two psi is about 0.14 bar. Hold the scope: DOE states that rule for the 100 psig range with that unregulated share, so it describes a change near a normal plant setpoint rather than a coefficient you can stretch across any pressure you like. Scaled across the 14.5 psi in a bar it gives about 7 per cent at full output flow, which is where the 6 to 7 per cent per bar CAS publishes on its audit page comes from.
Leaks. Leaks do not change your specific power. They change how much of the air you paid for reaches something useful, which is the same thing to your budget. The DOE Sourcebook says leaks can sometimes waste 20 to 30 per cent of a compressor’s output, and that a well-maintained system should sit under 10 per cent. At 0.187 kW per cfm, 6,000 hours and $0.30 per kWh, a continuous 25 cfm leak is about $8,400 a year of air value, subject to the same control-response condition.
Part load. Compressors running at part load are generally less efficient than at full load, per the DOE compressed air system control strategies guidance. This is the biggest single reason a plant’s measured specific power sits above the full-load benchmark for its own machine.
Inlet conditions and pressure drop. Hotter, less dense inlet air means less mass per revolution, so specific power drifts up in an Australian summer plant room. Note the ambient when you benchmark. Dryers, filters and separators all cost pressure that the compressor makes back at the discharge, so filter differential shows up on the electricity bill.
What one cfm costs per year
All figures below use an indicative $0.30 per kWh, a modelling tariff rather than a quoted rate, so substitute your own. Hours are annual compressor running hours: about 2,000 for a single shift, 4,000 for two shifts, 6,000 for two 12-hour shifts, 8,000 for near-continuous operation. Each cell is one cfm of continuously supplied free air, per year.
| Machine class | kW per cfm | 2,000 h | 4,000 h | 6,000 h | 8,000 h |
|---|---|---|---|---|---|
| Centrifugal at design point | 0.160 | $96 | $192 | $288 | $384 |
| Modern VSD rotary screw | 0.181 | $109 | $217 | $326 | $435 |
| CAS modelling constant | 0.187 | $112 | $224 | $336 | $449 |
| Small reciprocating, 7 kW/m³/min | 0.198 | $119 | $238 | $357 | $476 |
| Small reciprocating, 8 kW/m³/min | 0.227 | $136 | $272 | $408 | $544 |
| Older fixed-speed, 9 kW/m³/min | 0.255 | $153 | $306 | $459 | $612 |
| Older fixed-speed, 12 kW/m³/min | 0.340 | $204 | $408 | $612 | $816 |
Read across a row for what hours do to one machine, and down a column for what machine class does at fixed hours. Hours move the figure four-fold across this table; machine class moves it about two-fold. Neither is a free choice, but it does mean the run schedule and the control strategy deserve as much attention as the machine itself.
Every dollar figure here is the value of that air at the stated specific power, hours and tariff. It converts to money on your invoice only to the extent that your compressor’s control response actually reduces electrical draw when demand falls.
Once you can price one cfm, the whole-machine number follows from the same arithmetic. For annual running cost by compressor size, see what it costs to run an air compressor in Australia. For the measurement done properly, with logged data over a representative cycle rather than a spot reading, that is the scope of an ISO 11011 compressed air energy audit.
Frequently asked questions
How many kW per cfm does a compressor use?
A well-maintained modern oil-injected rotary screw compressor at full load uses about 0.18 to 0.19 kW per cfm, which is 18 to 19 kW per 100 cfm, or 6.4 to 6.6 kW per cubic metre per minute of free air delivered. A centrifugal machine at its design point is lower, around 0.16. An older fixed-speed machine at part load is commonly 0.26 to 0.34. These are total package electrical input at full load, not motor nameplate ratings, and the CAGI performance data sheet is the authority for your own machine.
How do I calculate the cost of compressed air per cfm?
Measure total package electrical input in kW and free air delivered at the compressor discharge at the same moment. Divide kW by flow to get specific power, then convert to kW per cfm. Multiply by your annual running hours and by your tariff in dollars per kWh. For example, 0.187 kW per cfm at 6,000 hours and 30 cents per kWh is about 336 dollars per cfm per year. Whether reduced air demand shows up as a lower bill depends on how your compressor responds to it, because a load or unload machine still draws power while unloaded.
How do I convert kW per cubic metre per minute to kW per cfm?
Multiply by 0.0283168, because one cubic foot is 0.028316846592 cubic metres. So 6.4 kW per cubic metre per minute is 0.181 kW per cfm, or 18.1 kW per 100 cfm. Going the other way, multiply kW per cfm by 35.3147. For kW per litre per second, divide the kW per cubic metre per minute figure by 16.667.
Why is my measured specific power so much worse than the benchmark?
The most common reason is part load. Compressors at part load are generally less efficient than at full load, so a machine sized for a peak it rarely sees reads poorly against a full-load benchmark. Other common causes are discharge pressure set higher than the plant needs, high pressure drop across dryers and filters, hot plant room inlet air, and a measurement that held package input kW against a rated flow rather than a measured one. Check the basis before concluding the machine is at fault.
Is compressed air really the most expensive utility on site?
Per unit of energy delivered to the end use, compressed air is expensive, because most of the electrical energy going into a compressor leaves as heat rather than as useful work at the tool. The US Department of Energy Sourcebook states that as much as 80 to 93 per cent of the electrical energy used by an industrial air compressor is converted into heat, and that a well designed heat recovery unit can put 50 to 90 per cent of that thermal energy to use. Whether compressed air is the largest line on your energy bill depends on what else your site runs, so compare it against your own metered loads.
Work out what your air is really costing
Price one cfm on your own plant, then decide. If the arithmetic here puts your specific power well above the benchmark for your machine class, the next step is a measurement you can defend: logged package kW and logged flow over a representative cycle, not a single spot reading.
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