What specific power means
Specific power is the simplest single measure of how efficiently a compressor turns electricity into useful air:
Specific power = total package input power (kW) / free air delivery (m³/min)
It is also stated as kW per 100 cfm. Because 1 m³/min is about 35.3 cfm, multiply kW per m³/min by about 2.83 to get kW per 100 cfm. Keep the units from each original data sheet on record, then compare every machine in one chosen unit.
A typical oil-injected screw compressor at around 7 bar (101.5 psig) often sits in the region of 6–7 kW per m³/min at full load, with smaller machines higher. Treat any such range only as a sense check; the figure for a specific machine must come from its own verified data.
What a specific-power comparison covers
| Mode | Result |
|---|---|
| Single operating point | Specific power for one machine at one pressure and flow |
| Two or more offers | Side-by-side comparison on a common basis (compare more offers two at a time) |
| Part-load curve | Run the tool once for each demand point |
| Annual load profile | Run the tool once for each load band and add the kWh |
| Measured audit point | Measured package kW divided by measured flow |
What must match before you compare
| Item | Why it matters | What to do |
|---|---|---|
| Flow basis | FAD, standard flow and displacement differ | Convert all to FAD with the FAD conversion tool |
| Discharge pressure | About 7% more power per extra bar | Compare at the pressure you will actually run |
| Power boundary | Fan, VSD losses and dryers add load | Use total package input power for every offer |
| Operating state | Full load and part load behave differently | Compare full load with full load, and part load at matched demand |
| Accessories | An integrated dryer adds kW but also does work | Include or exclude consistently |
| Site conditions | Inlet temperature affects output | Note the reference conditions of each sheet |
If one offer is stated at 8 bar(g) and another at 7 bar(g), correct for pressure or, better, ask both suppliers for data at the same pressure. The rule of about 7% per bar is a useful approximation for screw compressors in the normal industrial range, not an exact conversion.
Worked example: two offers at full load
A plant needs 10 m³/min (about 353 cfm) at 7.5 bar(g) (about 109 psig) and runs 6,000 hours a year at near full load.
- Offer A: 10.2 m³/min at 7.5 bar(g), package input 65 kW. Specific power ≈ 6.37 kW per m³/min.
- Offer B: 10.0 m³/min at 7.5 bar(g), package input 60 kW. Specific power = 6.00 kW per m³/min.
At the plant’s 10 m³/min, the difference is (6.37 − 6.00) × 10 ≈ 3.7 kW. Over 6,000 hours that is about 22,000 kWh a year. Multiply by your own tariff to see the annual cost, and by the expected life of the machine to see the lifetime difference. Even small differences in specific power become large energy totals on machines that run most of the year.
Part load: where most compressors really live
Few compressors run at full load all the time. A fixed-speed machine on load/unload control may spend much of its time unloaded, still drawing a significant part of full-load power while delivering no air. A variable speed drive machine reduces speed with demand, but its specific power changes across the speed range and is often best in the middle of the range rather than at the extremes.
For a fair comparison on variable demand:
- Build the demand profile: hours per year at each flow band. The air demand duration curve tool helps.
- For each machine, obtain package input power at each flow band, including unload power for fixed-speed units.
- Multiply power by hours in each band and add them up for annual kWh.
- Divide annual kWh by annual air delivered to get a weighted average specific power.
This weighted figure is the one that predicts your energy bill. A machine that wins at full load can lose on a plant with long low-demand periods, and the reverse is also true. The load unload cycle energy calculator covers unload losses in detail.
Worked example: part load
The same plant runs 3,000 hours at 10 m³/min and 3,000 hours at 6 m³/min.
- Fixed-speed machine: 60 kW loaded; at 6 m³/min average demand it runs loaded about 60% of the time and unloaded the rest, drawing roughly 25 kW unloaded. Average power ≈ 0.6 × 60 + 0.4 × 25 = 46 kW.
- VSD machine: 62 kW at 10 m³/min; at 6 m³/min it draws about 37 kW (drawn from its stated part-load data).
Annual energy: fixed-speed = 3,000 × 60 + 3,000 × 46 = 318,000 kWh; VSD = 3,000 × 62 + 3,000 × 37 = 297,000 kWh. The VSD uses more power at full load but about 21,000 kWh less a year on this profile. The numbers here are illustrative; use your own measured profile and each maker’s data.
Measuring specific power on an existing compressor
For a machine already installed, specific power can be measured during an air audit: log true electrical input with a power analyser at the package supply, log delivered flow with a suitable flowmeter, and record discharge pressure and inlet temperature at the same time. Compare against the original data sheet. A rising figure over time often points to airend wear, a restricted inlet filter or separator, fouled coolers, worn inlet valves or poor control settings. See our specific-power assessment and power and flow logging pages.
Limits and common errors
- Using motor nameplate kW instead of measured or stated package input.
- Comparing flows on different reference bases.
- Ignoring the dryer and filter pressure drop, which forces a higher discharge pressure.
- Treating a full-load figure as the annual average.
- Forgetting that data sheet values carry tolerances under the test standard, typically ISO 1217.
- Comparing a used machine on its original sheet without testing it.
Lowering system pressure is often the cheapest improvement of all. If the plant can run at 6.5 bar(g) instead of 7.5 bar(g), compressor power falls by roughly 7%. Read our page on system pressure optimisation.
Specific power across a multi-compressor station
In a station with several compressors, the specific power that matters is the station figure: total input power of all running machines, including dryers, divided by total air delivered to the plant. A station built from efficient machines can still perform poorly if two machines trim at the same time, if a large fixed-speed unit runs mostly unloaded, or if the set pressures overlap badly. Sequencing the machines so that base units run fully loaded and one machine trims usually improves the station figure without buying anything. The part load fleet dispatch calculator tests different running orders against your demand profile.
Turning the result into a decision
Specific power is one input, not the whole decision. Once the offers sit on a common basis, weigh the energy difference against the other factors that decide lifetime cost and reliability.
- Annual energy difference in kWh, from the weighted part-load calculation.
- Purchase and installation cost, including any dryer, receiver and pipework changes.
- Service cost and intervals over the expected life.
- Serviceability and parts availability for the make and model in your region.
- High-ambient capability, since compressor rooms in Telangana and Andhra Pradesh can run hot through the summer.
- Fit with the existing fleet, so base, trim and standby machines work together under one control strategy.
The lifecycle cost comparison calculator brings these together over the life of the machine.
How Reddy Compressors helps
We are an independent multi-brand specialist at Tarnaka, Secunderabad, serving industry since 2002. We sell new and used or refurbished compressors, rent from our own fleet and service ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes, without being tied to any one brand. We put competing offers on one basis, build an annual energy comparison from your demand profile, and measure specific power on your existing machines during an air audit. Where a machine has drifted, our engineers check the causes and plan service or overhaul. Service teams cover Telangana and Andhra Pradesh, and we supply across India.
What to send us
- Data sheets of the machines you are comparing
- Required pressure at the point of use and the current set pressure
- Running hours per year and the demand pattern by shift
- Nameplate photos of existing compressors and dryers
- Electricity bills or metered compressor kWh, if available
Frequently asked questions
What is a good specific power for a screw compressor?
It depends on size, pressure and design, so there is no single good number. Oil-injected screw compressors around 7 bar (101.5 psig) often fall in the region of 6–7 kW per m³/min at full load, with smaller machines higher. Use that only as a sense check. Compare actual machines on verified data at the same pressure and power boundary.
How do I convert kW per 100 cfm to kW per m³/min?
One m³/min is about 35.3 cfm, so 100 cfm is about 2.83 m³/min. Divide a kW per 100 cfm figure by 2.83 to get kW per m³/min, or multiply kW per m³/min by 2.83 to go the other way. Check that both figures use free air delivery at the same pressure before comparing them.
Does higher pressure increase compressor power consumption?
Yes. For screw compressors in the normal industrial range, each extra bar of discharge pressure typically costs about 7% more power for the same flow. Running a plant at 8 bar when users need 6.5 bar wastes energy every hour. Fix the pressure drop in dryers, filters and pipework first, then lower the set pressure in steps while watching the critical users.
Is a VSD compressor always more efficient?
No. A VSD compressor saves energy where demand varies, because it avoids unload losses. On a steady full-load duty, a good fixed-speed machine can match or beat it, since the drive adds small losses. Compare weighted annual specific power across your real demand profile, not only the full-load figure, before deciding.
How can I measure the specific power of my existing compressor?
Log true package input power with a power analyser and delivered flow with a suitable flowmeter at the same time, with discharge pressure and inlet temperature recorded. Divide average kW by average flow in m³/min. Compare the result with the original data sheet at the same pressure. A rising figure usually means wear, restriction or poor control settings worth investigating.
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
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 7 November 2026. How our engineering content is reviewed.