Why log power and flow together?
Electricity is usually the largest cost of owning a compressor over its life, yet most plants know only the motor rating and the nameplate free air delivery (FAD). Neither tells you what the machine uses or delivers in your plant today. A compressor that spends long periods unloaded, cycles too often or has a worn airend can draw much more power for each unit of air than its datasheet suggests.
Logging power alone shows when energy is used, but not what it buys. Logging flow alone shows demand, but not what it costs. Logged together, with synchronised clocks and matching boundaries, they show the energy-to-air relationship hour by hour: how efficiently the station runs at base load, at peaks, at part load and during non-production.
What power and flow logging can show
| Finding | How it appears in the data | Typical follow-up |
|---|---|---|
| Unloaded running | Significant power with little or no flow from that machine | Review pressure band, run-on timers and sequencing |
| Too many machines online | Several compressors part-loaded together | Base/trim strategy or a master controller |
| Poor part-load efficiency | Specific power rising sharply as flow falls | VSD trim machine, storage or a smaller trim unit |
| Performance loss | Lower flow for the same power compared with earlier logs | Inspect inlet filter, separator, coolers and airend |
| High non-production use | Steady flow and power when the plant is idle | Leak survey and zone isolation |
| Excess pressure | Generation pressure well above what users need | Pressure reduction; about 7% less power per bar lowered |
Defining the electrical boundary
Decide first what the power reading represents: the main motor only, the complete compressor package including fan and controls, or the whole compressor house including dryers, cooling pumps and ventilation. Each is a valid boundary for a different question, but they must not be mixed.
- Main motor: useful for diagnosing the compressor itself.
- Package input: the right basis for comparing compressors with each other or with datasheets that quote package power.
- Station input: the right basis for overall energy performance and for comparing before and after a system change.
Motor shaft output, motor electrical input and package input are different quantities, and a nameplate kW rating is not a measured input. Keep the boundary unchanged throughout a before-and-after comparison, or reconcile any change explicitly.
Measuring real power correctly
Current alone does not give power. Real power depends on voltage, current and power factor, and on the waveform. A three-phase power analyser or a suitable energy meter measures true kW and kWh.
Variable speed drives draw a distorted current, so the instrument and measuring point must suit that waveform; measure on the supply side of the drive for package input. Installation and removal of electrical measuring equipment must be done by qualified personnel under the site’s permit system. Record the instrument, current transformer ratio, connection type, logging interval and calibration status, and check phase association and signs during commissioning so an error is not carried through the whole campaign.
Defining the delivered-air boundary
The flow meter location must match the power boundary. A meter before the dryer includes air later used for dryer purge; a meter after treatment shows air actually sent to the plant. For a package comparison, measure the compressor’s own delivery. For a station figure, every supply path must pass through the meter, or the result must be limited to the measured path.
State the flow basis: actual volume at line conditions, a normalised volume at a stated reference, or a FAD-equivalent. Datasheet FAD and a site meter reading in normalised units are not directly comparable without a conversion using absolute pressure and temperature. Check the physical piping for bypasses, temporary hoses and second compressors feeding downstream of the meter.
Running a logging campaign
- Agree the question. Machine comparison, station efficiency, control review or before-and-after verification.
- Set boundaries. Mark the power and flow measuring points on the system diagram.
- Choose instruments. Power analysers or meters, flow meters suited to pipe size and range, pressure transmitters and machine-state signals.
- Synchronise clocks. Set all loggers to the same time source and note any drift at the end.
- Set the interval. One to ten seconds for control behaviour, summarised to one-minute values for analysis.
- Log a representative period. Usually at least a working week with a non-production period.
- Keep a diary. Production changes, maintenance, alarms, set-point changes and weather.
- Check data early. Review the first day’s data for gaps, over-range and channel errors.
Log discharge and header pressure alongside power and flow. Specific power depends strongly on pressure, so a comparison at different pressures is misleading unless the difference is accounted for.
Calculating and using specific power
Specific power is electrical input power divided by delivered airflow, normally stated in kW per m³/min at a stated pressure. Calculate it for matched time intervals, not by dividing a daily energy total by a peak flow. A simple worked example shows the method:
- A compressor package draws an average of 82 kW over a one-hour interval.
- Over the same hour the meter after the compressor records an average of 13.0 m³/min at the stated basis.
- Specific power for that hour is 82 ÷ 13.0, or about 6.3 kW per m³/min at the logged pressure.
- Energy for that hour is 82 kWh. Over a year at similar duty the plant can multiply by running hours and apply its own tariff.
For a multi-compressor station, the same calculation at station boundary uses total input power of all running machines and auxiliaries against total delivered flow. Comparing the station figure with the best individual machine figure shows how much energy is lost to control overlap and part-load running rather than to the compressors themselves. That gap is often where the cheapest improvements lie, because it can be narrowed with settings rather than new equipment.
The worked example illustrates the arithmetic only. Plotting specific power against flow for every interval shows how efficiency changes across the load range. A flat curve suggests good part-load control; a curve that climbs steeply at low flow shows where controls, storage or a different trim machine could help.
Common mistakes to avoid
- Using nameplate motor kW and rated FAD instead of measured values.
- Dividing package power by a station flow that includes another compressor.
- Logging current only and calling the result power.
- Comparing machines at different discharge pressures without adjustment.
- Different logging intervals or unsynchronised clocks on power and flow.
- Changing the boundary between the before and after campaigns.
- Logging a short, unrepresentative period and annualising it.
How Reddy Compressors helps
We carry out power and flow logging on single compressors and complete compressor houses, for any make. Our engineers agree the boundary with your team, install power, flow, pressure and machine-state logging, keep the production diary and report specific power across the load range, unloaded running, control behaviour and the measures that would reduce energy use.
As an independent multi-brand specialist that sells, rents and services ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes, we can act on what the data shows: service or overhaul a machine that has lost performance, adjust pressure bands and sequencing, recommend a VSD or a different compressor mix, or provide rental cover while changes are made. Repeat logging after the work gives you a verified before-and-after comparison.
Our office is at Tarnaka, Secunderabad, and our service teams cover all of Telangana and Andhra Pradesh. Call +91 99590 60718 or +91 92468 88812.
What to send us
- Make, model, motor rating and nameplate photo of each compressor and dryer.
- Running pressure and how machines are controlled or sequenced.
- Single-line diagram or a description of the compressor house electrical supply.
- Pipe sizes at the compressor outlet and after treatment.
- Shift pattern and available non-production windows.
- The decision you need to make, and any previous energy or audit data.
Frequently asked questions
How do I measure the power consumption of an air compressor?
Use a three-phase power analyser or a suitable energy meter that measures true kW from voltage, current and power factor, installed by a qualified electrician under the site’s procedures. Log over a representative period, not a spot reading, because power changes with load, unload and pressure. Decide whether you are measuring the motor, the package or the whole compressor house, and keep that boundary consistent.
What is a good specific power for a screw compressor?
It depends on pressure, size, cooling and control type, so there is no single figure. Compare a machine with the manufacturer’s data at the same pressure and boundary, or with its own earlier logs. A rising value at the same pressure and flow suggests performance loss. A value that climbs sharply at part load suggests that controls or the compressor mix should be reviewed.
Can I estimate compressor energy from current readings?
Current logging is useful for seeing when a compressor is loaded, unloaded or stopped, and for a rough estimate where voltage and power factor are known to be stable. It is not true power, especially with variable speed drives or poor supply quality. For energy decisions and before-and-after verification, measure real power with a suitable instrument.
How long should power and flow logging run?
Long enough to cover every operating pattern relevant to the decision, which for most plants means at least a working week including a weekend or non-production period. Shorter tests can compare two machines under a controlled, steady duty. For verification of an improvement, the before and after campaigns should cover comparable production patterns and similar lengths.
Why does my compressor use power when it is unloaded?
An unloaded screw compressor keeps the motor and airend turning while the inlet is closed and the separator is blown down, so it still draws a significant share of its loaded power. Frequent or long unloaded running wastes energy. Narrowing the pressure band, adjusting run-on timers, better sequencing, extra storage or a VSD trim machine can reduce it.
Does a flow meter need to be installed permanently?
Not always. A temporary insertion meter or clamp-on ultrasonic meter can serve a logging campaign. Plants that track energy performance continuously, or that run several compressors under a master controller, often benefit from permanent flow and power metering so specific power can be trended and problems spotted early.
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.
Motor input from volts, amps and power factor
Screen a compressor’s electrical input from voltage, current and measured power factor in the loaded and unloaded states, and see how far a wrongly assumed power factor overstates unloaded power.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Supply: Three-phase, balanced (line-to-line voltage)
- Voltage (V): 415
- Loaded: line current (A): 150
- Loaded: measured power factor: 0.86
- Unloaded: line current (A): 70
- Unloaded: measured power factor: 0.45
Result
- Loaded apparent power: 107.8 kVA
- Loaded real power: 92.7 kW
- Loaded reactive power: 55 kVAr
- Unloaded apparent power: 50.3 kVA
- Unloaded real power: 22.6 kW
- Unloaded reactive power: 44.9 kVAr
- Unloaded power as a share of loaded power: 24.4 %
- Unloaded kW if the loaded PF were wrongly assumed: 43.3 kW, overstated by 91% against the measured value
Method. Three-phase screen (balanced supply, line-to-line voltage V_L): S (kVA) = √3 × V_L × I ÷ 1,000; P (kW) = S × PF; Q (kVAr) = √(S² − P²). Single-phase: drop the √3 and use phase voltage. The power factor must be measured in the same operating state: an induction motor’s PF falls sharply when the compressor unloads, so applying the loaded PF to unloaded amps overstates unloaded kW. Not valid on the supply side of a VSD (harmonics) or with unbalanced phases: use a true-RMS power analyser there. State the measurement boundary (motor only, or whole package) with every result.
What to take from it. Judge unloaded consumption from measured kW or a measured power factor, never from amps with the full-load power factor.