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Screw compressor running cost: how to calculate yours
The short answer: yearly running cost = power drawn (kW) × hours run × your electricity tariff. For a screw compressor, you work it out twice, once for the hours it spends loaded (making air) and once for the hours it spends unloaded (turning, but making no air), and add the two together.
Electricity is about 80% of what a compressor costs over five years. The purchase price and servicing make up the rest. So when two quotes are close on price, the running cost is the number that decides which machine is cheaper to own. Below is the method we use with buyers, a worked example on one of our machines, and the three places the money usually leaks away.
What you need before you start
You need four numbers. Most of them are on the compressor itself.
- Motor power in kW. It is on the nameplate and in the data sheet. A Colt CS 50 has a 37 kW (50 HP) motor, for example.
- Loaded hours and total running hours. Most screw compressor controllers record both; check the running-data screen on yours. Take the reading today and again after a week or a month, and the difference gives you a real pattern for your plant.
- Your electricity tariff in ₹ per kWh, including demand charges spread over the units. ₹8 is a fair figure for many industrial connections; use your own bill.
- Free air delivery (FAD) in CFM at your working pressure, from the data sheet. You need this to work out the cost of the air itself.
The calculation, step by step
- Loaded energy (kWh) = motor kW × loaded hours. When a screw compressor is loaded, it draws close to its motor rating. For an exact figure, use the package power from the data sheet, or have an electrician take a clamp-meter reading.
- Unloaded energy (kWh) = motor kW × 0.25 × unloaded hours. An unloaded screw compressor still turns and still draws 20 to 30% of its full power, while making no air at all. We use 25% here.
- Yearly cost (₹) = (loaded kWh + unloaded kWh) × tariff.
- Cost of air = yearly cost ÷ air delivered. Air delivered = FAD × 60 × loaded hours, in cubic feet. Quote it per 1,000 cubic feet, so you can compare machines and plants.
Worked example: a 50 HP fixed-speed screw compressor
A plant runs two shifts, 16 hours a day for 300 days, with a Colt CS 50 FS: 37 kW, 242 CFM at 8 bar. The controller shows 3,600 loaded hours and 1,200 unloaded hours over the year.
| Line | Working | Result |
|---|---|---|
| Loaded energy | 37 kW × 3,600 h | 1,33,200 kWh |
| Unloaded energy | 37 kW × 25% × 1,200 h | 11,100 kWh |
| Total energy a year | 1,44,300 kWh | |
| Running cost a year | 1,44,300 kWh × ₹8 | ₹11.5 lakh |
| Air delivered a year | 242 CFM × 60 × 3,600 h | about 5.2 crore cubic feet |
| Cost of air | ₹11.5 lakh ÷ 52,358 thousand cubic feet | about ₹22 per 1,000 cubic feet |
| Running cost over five years | ₹11.5 lakh × 5 | about ₹58 lakh |
Two things stand out. First, over five years the electricity costs several times what the machine does. Second, ₹88,800 a year, the cost of those 11,100 unloaded kWh, buys no air at all.
Where the money leaks away
1. Unloaded running. In the example, a quarter of the running hours produce nothing. If your controller shows a lot of unloaded time, your demand swings through the shift. A VFD compressor follows the swings by slowing down instead of unloading. On plants that average 50 to 70% of the compressor's capacity, that typically saves 25 to 35% of the energy bill. Our brochure figure is up to 35%. Our VFD screw compressor page has a fixed-speed vs VFD comparison on a 75 HP machine.
2. Leaks. Leaks run 24 hours a day and the compressor pays for every one of them. If this plant leaks 30 CFM, about one-eighth of the compressor's output, then one-eighth of the loaded energy goes into leaks: roughly ₹1.3 lakh a year. Here is a simple check. On a Sunday with no production, run the compressor and watch how often it loads. Any loading at all is leaks. Fittings, hoses, quick couplings and drain valves are the usual culprits.
3. Pressure set higher than the plant needs. As a general rule, every 1 bar of extra discharge pressure costs roughly 6 to 7% more power. If this plant runs at 8 bar but its machines work happily at 7, bringing the pressure down 1 bar saves about ₹69,000 a year. Check the pressure your machines need at the point of use. Then check the pressure drop across your dryer, filters and piping, and fix the drop rather than raising the setting.
Fixed speed or VFD: what the running cost tells you
Run the calculation with your own controller hours.
- Loaded close to all the time (say, more than 85% of running hours): your demand is steady, and a fixed-speed machine is the economical choice.
- Large unloaded hours, or a compressor that loads and unloads every few minutes: a VFD will pay back its extra price, usually within the first year or two on a two- or three-shift plant.
- Some plants do best with both: a fixed-speed machine carrying the steady base load, and a VFD machine taking the swings.
If you are choosing a new compressor rather than checking an old one, size it to your real demand. Add up the CFM of everything that runs at the same time and add about 20%. An oversized compressor spends more hours unloaded, and that shows up straight away in the running cost. Our screw air compressor page has the full model table and a quick selection guide.
Frequently Asked Questions
Roughly motor kW × tariff while it is loaded. A 37 kW machine at ₹8 per kWh costs about ₹296 an hour loaded and about ₹74 an hour unloaded. Your yearly figure depends on how those hours split, which the controller tells you.
Yes. Multiply HP by 0.746 to get kW: 50 HP is about 37 kW. Use the motor rated power, not the compressor air output.
Yes, it is the fairest way to compare two compressors, or the same compressor before and after a change. It takes the plant demand out of the comparison and shows how efficiently each one turns electricity into air.
Yes. A clogged air filter, an old oil separator element or a dirty cooler makes the compressor work harder and run hotter for the same air. Servicing on schedule keeps the power drawn close to the data-sheet figure.
Send us your compressor's kW, loaded and unloaded hours and your tariff through our contact page, and our engineers will work out your running cost and show what a VFD or a pressure change would save.