The short answer. A compressor’s monthly electricity cost is its input power, not its nameplate power, multiplied by the hours it runs loaded and unloaded, the working days and your tariff. A typical 22 kW screw compressor working 14 loaded and 4 unloaded hours a day for 26 days uses about 9,450 kWh a month, which at ₹8.50 per kWh is roughly ₹80,300 a month and ₹9.6 lakh a year. The calculator below works out your own figure in seconds, and the sections that follow show where the money goes and which hours you can win back.
Who this page is for: factory owners, plant engineers, maintenance heads and finance teams who pay for compressed air and want to know what one compressor really costs to run before they decide on repairs, upgrades, a new machine or an energy audit.
Key facts
- Electricity is usually the largest cost of owning an industrial compressor over its life, far larger than the purchase or service cost of the machine.
- Input power is higher than nameplate power: a 22 kW motor at 92 % efficiency draws about 23.9 kW from the supply at full load.
- A load/unload screw compressor running unloaded still draws typically 20–35 % of its loaded power while producing no air. A variable speed machine idles far lower.
- Every 1 bar (14.5 psi) of unnecessary pressure adds roughly 6–7 % to the loaded energy of a screw compressor.
- Compressed air is one of the most expensive utilities in a factory: only a small part of the electrical energy put into the compressor reaches the tool as useful work. The rest leaves as heat.
- The cost per loaded hour is the most useful single number for judging leaks, pressure settings and idle running, because it prices the air you actually make.
Calculate your monthly running cost
Enter your own compressor’s figures. The defaults describe a typical 22 kW screw compressor in a one-and-a-half-shift factory; they are only a starting point, so replace every value with your machine’s nameplate data, your hour-meter readings and the energy charge on your electricity bill.
Loaded input kW = rated kW ÷ (efficiency ÷ 100). Unloaded kW = loaded input kW × unloaded %. Daily kWh = loaded kW × loaded hours + unloaded kW × unloaded hours. Monthly kWh = daily kWh × working days. Monthly cost = monthly kWh × tariff; yearly cost = monthly cost × 12. Cost per loaded hour = monthly cost ÷ (loaded hours × working days). These are your electricity costs, not equipment or service prices.
The method, step by step
The calculator uses the same arithmetic a service engineer uses on site. Doing it once by hand makes the result easy to trust and easy to explain to a finance team.
- Loaded input power. Divide the motor’s rated output by its efficiency: loaded input kW = rated kW ÷ (efficiency ÷ 100). A 22 kW motor at 92 % draws 22 ÷ 0.92 = 23.9 kW.
- Unloaded input power. Multiply the loaded input by the unloaded percentage: unloaded kW = loaded input kW × unloaded % ÷ 100.
- Daily energy. Daily kWh = (loaded kW × loaded hours) + (unloaded kW × unloaded hours).
- Monthly energy. Monthly kWh = daily kWh × working days per month.
- Monthly and yearly cost. Monthly cost = monthly kWh × tariff in ₹/kWh. Yearly cost = monthly cost × 12.
- Cost per loaded hour. Monthly cost ÷ (loaded hours × working days). This spreads the whole bill, idle running included, over the hours that actually make air.
The method deliberately keeps to the main motor. Add fan motors, the dryer and any auxiliary drives separately if they are significant; on many packaged screw compressors the cooling fan is 2–5 % of the main motor rating and the refrigerated dryer a few percent more.
Nameplate kW, input kW and why they differ
The figure on the motor nameplate is shaft output: what the motor delivers to the airend. The electricity meter records input, which is larger because the motor loses some energy as heat. Efficiency for standard industrial motors in the 7.5 to 75 kW range typically sits between about 90 and 95 %; premium efficiency motors sit at the upper end, older rewound motors often lower.
Two further effects push real input power above the simple calculation:
- Service factor loading. Many compressor packages run their motor slightly above nameplate output at full pressure, using the motor’s service factor. The package’s rated input power on the data sheet is then higher than the nameplate kW divided by efficiency.
- Drive and package losses. Belt drives, gearboxes and variable speed drives each lose a few percent. A variable speed drive typically loses 2–3 % at full speed.
If you have the package data sheet, use the total package input power at your working pressure instead of nameplate kW and set efficiency to 100 % in the calculator. If you can measure, a clamp-on power meter reading over a full shift beats every estimate.
Loaded, unloaded and stopped: where the hours go
A fixed-speed screw compressor on load/unload control alternates between three states. Each costs a different amount.
| State | What the machine is doing | Typical input power | Air delivered |
|---|---|---|---|
| Loaded | Inlet valve open, compressing to set pressure | 100 % of loaded input | Full rated flow |
| Unloaded | Inlet closed, motor turning, sump blown down | Typically 20–35 % of loaded | None |
| Stopped (auto standby) | Motor off after the run-on timer | Near zero, controller only | None |
| Variable speed, part load | Motor slows to match demand | Roughly in proportion to flow, down to minimum speed | Matched to demand |
The hour meter on most controllers shows total running hours and loaded hours. Unloaded hours are simply total minus loaded. Read both meters on the same day, a month apart, and you have the two numbers the calculator needs. Where a machine shows very high unloaded hours, typically more than a quarter of total running hours, there is usually money to recover through better control, a larger receiver or a smaller trim machine. See our guide to unloaded and idle running and the load/unload cycle energy calculator for the detailed cycle analysis.
Reading your tariff correctly
Use the energy charge per kWh from your bill, including any fuel and power purchase cost adjustment and electricity duty that are charged per unit. For industrial consumers on high-tension supply, the demand charge is billed separately per kVA of maximum demand; it is not part of this calculation, although a large compressor starting at the wrong moment can raise it.
If your factory has time-of-day tariffs, work out a weighted average rate for the hours the compressor runs, or run the calculator twice, once for peak and once for off-peak hours, and add the results. Where part of the supply comes from rooftop solar or a captive source, use the blended cost per unit actually paid.
Worked example (illustrative)
A packaging plant in Hyderabad runs a 37 kW fixed-speed screw compressor. The hour meters, read a month apart, show 16 loaded and 6 unloaded hours per working day over 26 days. The motor is 93 % efficient, the unloaded draw is 28 % of loaded, and the energy charge is ₹9.20 per kWh.
The calculator carries every figure at full precision and rounds only the results. The steps below show rounded figures, so redoing them from the rounded numbers can differ from the calculator by a few rupees.
- Loaded input power = 37 ÷ 0.93 = 39.785 kW (about 39.8 kW).
- Unloaded power = 39.785 × 28 ÷ 100 = 11.140 kW (about 11.1 kW).
- Daily energy = (39.785 × 16) + (11.140 × 6) = 636.6 + 66.8 = 703.4 kWh.
- Monthly energy = 703.4 × 26 = 18,288 kWh.
- Monthly cost = 18,288 × 9.20 = 1,68,249.6, rounded to ₹1,68,250. Yearly cost = monthly cost × 12 = ₹20,19,000.
- Cost per loaded hour = 1,68,250 ÷ (16 × 26) = 1,68,250 ÷ 416 = ₹404.4.
Where the savings are. The 6 unloaded hours a day cost 11.140 kW × 6 h × 26 days = 1,738 kWh, or about ₹15,990 a month, for no air at all. Bringing unloaded running down to 2 hours a day, with a correctly sized receiver and a tighter run-on timer, saves 11.140 × 4 × 26 = 1,159 kWh, about ₹10,659 a month. Separately, if the pressure can come down by 1 bar, from 8.0 bar(g) (116.0 psig) to 7.0 bar(g) (101.5 psig), without starving any machine, the loaded energy of 39.785 × 16 × 26 = 16,551 kWh falls by roughly 7 %, the top of the typical range, another 1,159 kWh or about ₹10,659 a month. Together that is more than ₹2.5 lakh a year from one compressor, before a single leak is repaired.
All figures are illustrative; use your own meter readings and tariff.
Five ways the bill goes up without anyone noticing
| Cause | What you see on the log | Effect on the bill | First action |
|---|---|---|---|
| Leaks | Loaded hours rise for the same production; compressor loads on Sundays | Often 20–30 % of output in older plants | Price them with the leakage cost calculator, then tag and repair |
| Pressure set too high | Set point raised “to be safe” after a complaint | About 6–7 % per bar on loaded energy | Find the real pressure need at the farthest user |
| Excessive unloaded running | Unloaded hours above a quarter of total hours | 20–35 % of loaded power for zero air | Check receiver size, pressure band and run-on timer |
| Fouled coolers and old separator | Discharge temperature and separator pressure drop rising | Higher power for the same flow, then trips | Clean coolers; change the separator at the maker’s limit |
| Worn airend | Same loaded hours deliver less air; pump-up test slower | Specific power rises steadily | Run a pump-up test and compare with the data sheet |
From monthly cost to specific power
Monthly rupees tell you how much you spend; specific power, the kW needed per m³/min of free air delivered, tells you how well you spend it. Two compressors with the same bill can differ widely in the air they deliver. A healthy modern oil-injected screw compressor at 7 bar(g) (101.5 psig) typically sits in the region of about 6 to 7.5 kW per m³/min of free air, at full load, according to its data sheet; older or worn machines, and machines running unloaded for long hours, sit noticeably higher.
When you are comparing machines, choosing a replacement or checking a repair, move from rupees to specific power. Our specific power comparison calculator and the guide to specific power take that step.
Monthly running cost record
Fill this in from the hour meters and the electricity bill on the same day each month. Three months of entries show trends that a single bill never will.
| Month | Total hours (meter) | Loaded hours (meter) | Unloaded hours | Unloaded share % | Working pressure (bar g) | Calculated kWh | Tariff ₹/kWh | Monthly cost ₹ | Cost per loaded hour ₹ |
|---|---|---|---|---|---|---|---|---|---|
Warning signs: unloaded share above 25 %, loaded hours rising while production is flat, or cost per loaded hour rising faster than the tariff.
Exceptions and cautions
- Variable speed machines. Loaded hours on a variable speed compressor are hours at varying speed, not at full power. Use the average measured input power instead of the rated figure, or the controller’s own energy reading if it has one.
- Multiple compressors. Calculate each machine separately; base-load and trim machines have very different loaded shares.
- Diesel portable compressors. This page covers electric machines. Diesel portables are costed by litres of fuel per hour at the duty load, not by tariff.
- Estimated efficiency. If you do not know the motor efficiency, 92 % is a reasonable starting point for a mid-size motor, but a measured input power is always better.
- Demand charges are outside this calculation. Staggered starting of large compressors can protect your maximum demand.
Common mistakes
- Multiplying nameplate kW by total hours and calling it the bill. It ignores efficiency, overstates loaded running and hides the cost of idle hours.
- Assuming an unloaded compressor costs nothing. At 20–35 % of loaded power, long unloaded hours are often the cheapest saving in the plant.
- Using the total bill rate including fixed and demand charges as the per-unit tariff, which inflates the result.
- Reading the hour meters on different days or forgetting the unloaded meter.
- Judging a replacement compressor on purchase price alone when ten years of electricity will cost many times more.
How Reddy Compressors helps
Reddy Compressors has been a multi-brand compressor specialist since 2002, supplying, renting, servicing and supporting spare parts for all major makes. Our service engineers read hour meters, measure input power and pressure on site, and turn the figures into a clear list of savings, from pressure settings and control timers to cooler cleaning and receiver sizing. Where a machine is worn or oversized, we can compare repair, overhaul and replacement on lifecycle cost. We work across Telangana, Andhra Pradesh and every district within about 300 km of Hyderabad, with supply across India.
What to send us
- A photograph of the compressor and motor nameplates.
- Total and loaded hour-meter readings, taken a month apart on the same date.
- The working pressure setting (load and unload pressures) and the pressure at your farthest user.
- A copy of the energy charge section of your electricity bill.
Send them through our equipment and energy enquiry, or Call/WhatsApp 99590 60718 or 92468 88812.
Frequently asked questions
How much electricity does a 22 kW air compressor use per month?
With 92 % motor efficiency, 14 loaded and 4 unloaded hours a day at 30 % unloaded power, and 26 working days, it uses about 9,450 kWh a month. Your figure depends on your loaded and unloaded hours, which you can read from the hour meters.
Why is my electricity bill higher than nameplate kW times hours?
Nameplate kW is shaft output. The motor draws more from the supply because of its efficiency, many packages run the motor slightly above nameplate at full pressure, and fans, dryers and drive losses add more.
Does a compressor use power when it is unloaded?
Yes. A fixed-speed screw compressor running unloaded typically draws 20–35 % of its loaded power while delivering no air. Long unloaded running is one of the most common sources of avoidable cost.
How much can reducing pressure save?
Roughly 6–7 % of loaded energy for every 1 bar (14.5 psi) reduction on a screw compressor, provided every machine still receives the pressure it needs at the point of use.
Which tariff should I use in the calculator?
Use the energy charge per kWh from your bill, including per-unit adjustments and duty. Leave out fixed and maximum demand charges, which are billed separately.
Is a variable speed compressor always cheaper to run?
Not always. It saves most where demand varies widely. At steady full load, a variable speed machine can use slightly more energy than a fixed-speed one because of drive losses of about 2–3 %.
How accurate is this calculator?
It is as accurate as its inputs. With hour-meter readings and the correct tariff it gives a sound monthly estimate. A clamp-on power measurement over a full shift gives the most accurate loaded and unloaded power figures.
Related pages
- Load/unload cycle energy calculator
- Specific power comparison calculator
- Leakage cost calculator
- Specific power explained
- Unloaded and idle running
- Compressor pump-up test and capacity check
- Lifecycle cost comparison calculator
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 5 November 2026.