The Bureau of Energy Efficiency report for Punjab foundries puts a variable frequency drive on a 22 kilowatt screw compressor: why do its loading and unloading hours look swapped?
The Bureau of Energy Efficiency (BEE) report counts 9,375 kilowatt hours (kWh) saved a year for rupees (Rs) 1.00 lakh, a payback of 2.13 years. Its savings sum is right at 7.5 kilowatts for 5 hours a day over 250 days. Its annexure labels the 1,260 unloaded hours as loading hours.
What a variable frequency drive does to an idling compressor
A screw compressor fills the air tank, and when the tank is full it keeps spinning without making air. That is called running unloaded. It is like leaving a car idling at a red light with the engine still burning fuel. A variable frequency drive (VFD) is a speed controller. It slows the motor to match the air you use, so it does not idle.
The report fits one to a 140 cubic feet a minute compressor with a 22 kilowatt motor. The motor draws 22 kilowatts when loaded and 7.5 when unloaded.
The figures as printed
- Running 3,600 hours a year, which is 250 days. The report says the compressor is loaded 9.3 hours a day and unloaded 5 hours a day.
- Saving: 7.5 kilowatts x 5 hours = 37.5 kWh a day, x 250 days = 9,375 kWh a year. At Rs 5 a unit that is Rs 46,875, printed as Rs 0.47 lakh. A lakh is 100,000.
- Cost Rs 1.00 lakh: Rs 0.95 lakh of equipment, which the report says takes a 40% discount from vendor quotes, and Rs 0.05 lakh of other cost. Payback 1.00 / 0.47 = 2.13 years. Internal rate of return 29.57%.
- Carbon dioxide (CO2): 7.6 tonnes a year printed, with no factor. That is 0.81 kilograms a kWh. On the grid factor of 0.675 kilograms per kWh for the 2025-26 financial year the same 9,375 kWh is 6.33 tonnes.
Where the report's own labels disagree
- Annexure 2 lists 1,260 hours as loading and 2,340 as unloading. Table 1.4 says unloading is 35% of the time, and 35% of 3,600 hours is 1,260. 5 hours x 250 days is 1,250. So the 1,260 hours are unloaded hours and the labels are swapped.
- Section 1.4 says unloading is about 35% of the load time. By its own numbers 5 hours is 35% of the 14.3 hours the compressor runs in total (9.3 + 5), not 35% of the 9.3 loaded hours.
- The same annexure prints the money saving as Rs 5.00 lakh. That looks like the Rs 5 rate repeated in the wrong row; the report gives no explanation. The sum is Rs 0.47 lakh.
- Using the 1,260 unloaded hours, 7.5 x 1,260 = 9,450 kWh, close to the printed 9,375, which uses 5 hours x 250 days.
What the saving assumes
The report says the drive would completely avoid the unloading time, so it counts the whole 7.5 kilowatts as saved. That is how the report words it. My reading, not the report's: a compressor on a drive still draws some power at its lowest speed, so ask the maker for that figure before you take the saving as given.
On the corrected hours (2,340 loaded, 1,260 unloaded) the compressor uses about 22 x 2,340 + 7.5 x 1,260 = 60,930 kWh a year, so 9,375 kWh is about 15% of its electricity. That is arithmetic on the report's figures, not something the report states.
Example: your own compressor, same method
- Loaded draw 30 kilowatts, unloaded draw 10 kilowatts, unloaded 4 hours a day, 270 days (example numbers)
- Unloaded energy: 10 x 4 x 270 = 10,800 kWh a year
- Take 70% of it as saved if the maker's minimum speed draws about 3 kilowatts: 10,800 x 0.70 = 7,560 kWh
- CO2 avoided: 7,560 x 0.675 = 5,103 kilograms, about 5.1 tonnes
The 70% is an example for a drive that does not bring the idle draw to zero. The report itself assumes the whole unloaded draw is saved. Read your hours off the compressor controller.
What Senseible does here
Senseible reads fuel invoices and electricity bills into monthly records, so the plant's units are on file before and after a drive goes in. The standard product reads bills; compressor meters or hour counters can be added on request.
- Estimate a compressor's kWh from load and unload hours
- Drive on an Alwar oil mill compressor
- Compressed air leaks: cost and emissions
- Green loan readiness check
Last reviewed 2026-11-07. Report figures are from BEE's Small and Medium Enterprises (SME) programme, at its Rs 5 a unit tariff and an implied grid factor of about 0.81 kilograms a kWh (7.6 tonnes over 9,375 kWh), which is older than today's. Check your tariff and a current drive quote before you rely on the payback.