What does a variable frequency drive save on an Alwar oil mill compressor?
A Bureau of Energy Efficiency (BEE) report for Alwar fits a variable frequency drive (VFD) to an 18 kilowatt compressor that idles at 6 kilowatts for 40% of its loaded hours: 11,460 kilowatt hours (kWh) a year, Rs 0.55 lakh, for Rs 1.2 lakh. Payback is 2.1 years.
How the saving is worked out
- An 18 kilowatt (kW), 100 cubic feet per minute (CFM) compressor draws 6 kW while running unloaded.
- It runs unloaded for 40% of its loaded time, and loaded hours are 4,775 a year.
- The report assumes the drive removes the whole 6 kW idle draw: 6 x 0.4 = 2.4 kW on average, its "net saving".
- 2.4 kW x 4,775 hours = 11,460 kilowatt hours (kWh); at Rs 4.80 a kWh that is Rs 55,008.
A thin return
- Cost Rs 1.2 lakh. Payback 2.1 years (1.2 / 0.55 is 2.18).
- Net present value (NPV) Rs 0.096 lakh, internal rate of return (IRR) 15%, debt service coverage ratio (DSCR) 1.09. A DSCR of 1.09 means cash flow covers loan repayments by only 9%.
- Downtime 5 hours. The report's carbon dioxide (CO2) saving is 10.4 tonnes, which is 0.91 kg a kWh.
Check before you buy
Read the unload hours off the compressor controller for a month. The 40% is the report's assumption. A compressor that rarely idles gains little from a drive, and a worn-out compressor may be better replaced. For that case see the screw compressor report.
Carbon dioxide at today's grid factor (estimate)
- 11,460 kWh x 0.675 = 7.7 tonnes CO2 a year
- The report's 10.4 tonnes uses about 0.91, higher than the Central Electricity Authority (CEA) financial year 2025-26 factor
Report figures from the Bureau of Energy Efficiency (BEE) Alwar oil mill project report; the 0.675 factor is CEA version 22.0, an estimate.
What Senseible does here
Senseible reads electricity bills into monthly kilowatt hours (kWh), so the months before and after a drive are on record with their bill dates. It cannot see how long a compressor idles; that comes from the compressor controller. The green loan check shows what a lender asks for.
- Green loan readiness check
- Screw compressor for an Alwar oil mill
- VFD on an ice plant compressor
- BEE project reports: grid factor behind CO2
Sources: BEE, Detailed project report: variable frequency drive on compressor, Alwar oil mill cluster (SAMEEEKSHA), CEA, CO2 Baseline Database for the Indian Power Sector
Last reviewed 2026-11-04. Report figures are from a Bureau of Energy Efficiency (BEE) Small and Medium Enterprises (SME) programme project report; costs and paybacks are at the report year's prices. Check today's prices and a current quote before you rely on the payback.