What is specific power?
A compressor converts electricity into compressed air. Specific power tells you how much electricity it takes to make each unit of air. The formula is simple:
Specific power = total electrical input power (kW) ÷ free air delivery (m³/min or 100 cfm)
Free air delivery (FAD) is the volume of air delivered at the discharge, converted back to inlet conditions. It is not the swept volume of the airend and not the figure on the motor nameplate. One m³/min is about 35.3 cfm, so a value in kW per m³/min multiplied by about 2.83 gives kW per 100 cfm.
Specific power can be stated for a single compressor at full load, for a compressor across its load range, or for a whole compressor station including dryers and losses. Those are three different numbers, and much confusion comes from mixing them.
Why specific power matters more than motor rating
Two 37 kW compressors can deliver noticeably different amounts of air for the same electricity. Over a typical service life, electricity is usually by far the largest cost of owning a compressor, larger than purchase and maintenance together. Comparing machines on motor kW or on price alone ignores that.
- It turns an electricity bill into an engineering measure of how well the air system performs.
- It lets you compare different makes, sizes and technologies on one basis.
- It exposes losses that nameplates hide: unloaded running, poor part-load control, high pressure settings and worn airends.
- It gives a baseline, so the effect of any change can be verified rather than assumed.
How specific power is measured
A trustworthy figure needs power and flow measured together, at a defined pressure and boundary, over enough time to capture how the plant really runs.
- Define the boundary. Decide whether you are measuring one compressor package, the compressors only, or the whole station including dryers, cooling fans and drains. Write it down.
- Measure true electrical input. Use a three-phase power logger measuring kW (not just amps) on the package supply. Current alone ignores power factor, which changes a great deal between load and unload.
- Measure delivered flow. Use a flow meter on the discharge header, installed with the straight-pipe lengths it needs. Where a meter is not practical, flow can be estimated from logged load/unload time and a known FAD, but say that it is an estimate.
- Log pressure. Record discharge pressure at the same time. A specific power figure without its pressure is not comparable.
- Log for a representative period. Cover a full production cycle – weekdays, night shifts, weekends and any seasonal peak. A one-hour spot reading rarely shows the true average.
- Calculate over matching intervals. Divide energy (kWh) by air delivered (m³) over the same intervals, then express the result as kW per m³/min. Report full-load, part-load and average values separately.
Ambient and inlet temperature also matter. Hot inlet air is less dense, so a machine delivers less mass of air in a Telangana May afternoon than on a winter morning. Note the conditions with every measurement.
What does good specific power look like?
There is no single target, because the figure depends on pressure, size, technology and how well the machine is loaded. The ranges below are typical and are for orientation only. Use the manufacturer’s tested data for a specific model, and compare like with like.
| Situation (around 7 bar / 101.5 psig) | Typical kW per m³/min | Typical kW per 100 cfm |
|---|---|---|
| Efficient modern oil-injected screw, medium to large size, at full load | about 5.5–6.5 | about 16–18 |
| Smaller or older oil-injected screw at full load | about 6.5–8 | about 18–23 |
| Fixed-speed machine at partial load with load/unload control | often well above its full-load figure | often well above its full-load figure |
| Station with significant unloaded running, leaks or high pressure | commonly 8 or more | commonly 23 or more |
Pressure has a strong effect: as a rule of thumb, each extra bar of discharge pressure costs roughly 7% more power. A machine compared at 8.5 bar will look worse than the same machine at 7 bar. Always state the pressure.
Why specific power rises at part load
A compressor’s best specific power is usually close to full load. When demand falls, how the machine follows it decides how much energy is wasted.
| Control type | How it follows demand | Part-load behaviour |
|---|---|---|
| Load/unload (fixed speed) | Runs fully loaded, then unloads and runs on with no air delivered | Unloaded running still draws a substantial share of full-load power, so specific power climbs as demand falls |
| Modulating inlet | Throttles the inlet to reduce flow | Power falls only slightly as flow falls; poor at low loads |
| Variable speed drive (VSD) | Changes motor speed to match flow | Specific power stays close to its best across much of the speed range; rises at very low speed |
| Multiple machines with a sequencer | Runs the fewest machines needed, one trimming | Good if the trim machine and pressure bands are set correctly |
This is why a station average is often much worse than the brochure figure of any machine in it. The brochure states full-load performance; the plant runs at part load.
Using specific power to compare compressors
When comparing quotations or deciding whether to replace an old machine, put every option on the same basis:
- Same pressure, stated at the package discharge.
- Package input power including fan motors and controls, not just the main motor shaft.
- FAD measured or stated to a recognised test method, such as ISO 1217, at stated inlet conditions.
- Performance at the loads your plant actually runs at, not only at 100%.
- Integrated dryer power, if one option includes a dryer and another does not.
Then multiply the power difference by your annual running hours and electricity tariff. That gives an honest running-cost comparison you can set against purchase price. Many manufacturers publish standardised data sheets showing package power and FAD, which make this comparison much easier.
Finding savings from a specific power assessment
Once you have a baseline, the causes of a poor figure usually fall into a few groups. Each can be tested and, after correction, re-measured.
- Too much unloaded running – oversized machines, wrong load/unload bands or several machines all trimming together.
- Pressure set higher than needed – often to cover a pressure drop in filters or pipework that should be fixed instead.
- Leaks – air delivered and paid for but never used; they raise demand and so total power.
- Worn airend or failing components – loss of FAD for the same power, blocked inlet filters, slipping belts or a sticking inlet valve.
- Hot inlet air – poor compressor-room ventilation recirculating hot cooling air.
- Wrong machine for the duty – a fixed-speed machine running long hours at part load where a VSD trim machine would suit.
Rank the actions by cost and benefit, carry them out, and repeat the measurement under similar conditions. Only the repeat measurement shows what was actually achieved.
How Reddy Compressors helps
Since 2002, Reddy Compressors has sold, rented and serviced compressors from our office at Tarnaka, Secunderabad. Because we are an independent multi-brand specialist, not an authorised dealer, – Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic, Gardner Denver, Sullair, Doosan, Kirloskar, Bobcat and many others – we can compare machines of different makes on equal terms. We have tie-ups with all major and reputed brands.
For a specific power assessment we agree the boundary with you, log power, pressure and flow over a representative period, and calculate full-load, part-load and station figures. We then explain where the energy goes, which corrections make sense, and the likely running-cost difference between keeping, overhauling or replacing a machine. We can carry out the service, control adjustments, leak repairs or equipment supply that follow, and re-measure afterwards. Our service teams cover all of Telangana and Andhra Pradesh, and we supply compressors and spare parts across India.
What to send us
- Make, model and nameplate photos of each compressor and dryer
- Working pressure settings and the minimum pressure your process needs
- Running hours and load hours from each controller, if available
- Recent electricity bills or meter readings for the compressor house
- Shift pattern and production days per week
- Any existing flow or power data, and the reason for the assessment
Frequently asked questions
What is a good specific power for a screw compressor?
At around 7 bar, an efficient modern oil-injected screw compressor at full load typically achieves somewhere around 5.5 to 6.5 kW per m³/min, or roughly 16 to 18 kW per 100 cfm. Smaller and older machines are usually higher. The figure depends strongly on pressure, size and load, so compare only like with like and use the manufacturer’s tested data for a specific model.
How do you calculate kW per 100 cfm?
Divide the total electrical input power in kW by the free air delivery in cfm, then multiply by 100. For example, a package drawing 45 kW while delivering 250 cfm has a specific power of 18 kW per 100 cfm. Use measured package input power, including fans, and FAD at the same pressure, not the motor nameplate rating.
Why does my compressor use so much power when demand is low?
Most fixed-speed screw compressors use load/unload control. When demand falls they unload and keep running, still drawing a substantial share of full-load power while delivering no air. Long unloaded periods raise the specific power sharply. Better pressure bands, a sequencer, a smaller machine or a VSD trim compressor are the usual remedies, depending on the demand profile.
Is a VSD compressor always more efficient?
Not always. A VSD compressor saves most where demand varies a great deal and the machine spends long periods at part load. At steady full load, a fixed-speed machine can match or slightly beat it, because the drive has its own small losses. Logging the demand profile first shows whether a VSD will pay back on your duty.
How long should power and flow be logged for an assessment?
Long enough to capture a full production cycle, including night shifts, weekends and peak days. For many plants that means at least a week of continuous logging. Plants with strong seasonal variation may need a second logging period. Short spot readings can be useful for checking a machine, but they rarely represent the true average specific power.
Does pressure setting affect specific power?
Yes, strongly. As a rule of thumb, each extra bar of discharge pressure costs roughly 7% more compressor power. Pressure higher than necessary also increases leakage and unregulated demand. Reducing pressure to the true minimum your process needs, after fixing avoidable pressure drops, is one of the simplest ways to improve specific power.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Specific power comparison
Compare two compressors on what matters over life: the electrical power needed per unit of air delivered, and what that costs a year.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Machine A: input power (kW): 75
- Machine A: delivered flow (m³/min): 11.5
- Machine B: input power (kW): 80
- Machine B: delivered flow (m³/min): 12.6
- Running hours per year: 6000
- Air actually needed (m³/min): 11
Result
- Machine A specific power: 6.52 kW per m³/min
- Machine B specific power: 6.35 kW per m³/min
- Energy to deliver the air needed: A: 4,30,435 kWh per year
- Energy to deliver the air needed: B: 4,19,048 kWh per year
- Energy difference: 11,387 kWh per year in favour of B
- Cost difference: Enter your electricity tariff (cost per kWh) above
Method. Specific power = input kW ÷ delivered flow (m³/min) at the same pressure and reference conditions. Annual cost = kW × hours × tariff. Use the full-load input power on the same test basis for both machines.
What to take from it. A small gap in specific power is multiplied by thousands of running hours. Compare on the same pressure and reference conditions.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Load/unload cycle energy
Turn measured load and unload times (or the logged percentage of time in each state) and measured package kW into average power, annual energy, the energy spent running unloaded, and specific power based on the air actually delivered. An optional second set of times compares a change.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Times entered as: Measured cycle times (seconds)
- Loaded package power (kW): 82
- Unloaded package power (kW): 28
- Loaded time (s per cycle, or percent): 40
- Unloaded time (s per cycle, or percent): 60
- Stopped time, motor off, 0 if none (s per cycle, or percent): 0
- Power while stopped, controls only (kW): 0
- Running hours per year with this cycle pattern: 6000
- Free air delivered when loaded, FAD (m³/min): 12.5
- After change, optional: loaded time, all three 0 = not used (s or percent): 0
- After change, optional: unloaded time (s or percent): 0
- After change, optional: stopped time (s or percent): 0
Result
- Cycle time: 100 s (36 cycles per hour)
- Loaded fraction: 40 %
- Unloaded fraction: 60 %
- Unloaded power as a share of loaded power: 34.1 %
- Average package power: 49.6 kW
- Annual energy: 2,97,600 kWh per year
- Unload energy (buys no air): 1,00,800 kWh per year = 33.9% of the annual energy
- Loaded specific power: 6.56 kW per m³/min
- Average air delivered: 5 m³/min
- Effective specific power over the cycle: 9.92 kW per m³/min
- Energy cost per year: Enter your electricity tariff (cost per kWh) above
Method. Two input modes. Cycle timing: enter measured seconds per cycle. Fraction: enter the share of time loaded, unloaded and stopped (for example 40, 60, 0, from controller loaded/running hours); no cycle time is reported in this mode. Average kW = (P_load × t_load + P_unload × t_unload + P_stopped × t_stopped) ÷ (t_load + t_unload + t_stopped); the states must not overlap. Annual kWh = average kW × the hours this pattern runs. Unload energy = P_unload × unloaded fraction × hours. Average air delivered = loaded fraction × loaded FAD; effective specific power = average kW ÷ average air delivered. Unloaded power is never assumed to be zero: enter measured package kW (unloaded read after blowdown has settled). Scenario comparison: enter the predicted times after a change (bigger receiver, wider band, auto-stop) in the “after change” fields; the same powers, FAD and hours are used and the kWh difference is shown. An auto-stop timer is modelled by moving unload time into the stopped state (check the motor’s permitted starts per hour). For different day and night patterns, run the tool once per pattern and add the results. The rapid-cycling warning below 30 s per cycle is a screening value used by this tool. Money is shown only when you enter your own tariff.
What to take from it. A large unload-energy share, or an effective specific power well above the loaded figure, means a fixed-speed machine is running lightly loaded: look at storage, pressure band, sequencing, auto-stop or a smaller/VSD trim machine.