# What does a variable frequency drive save on an Alwar oil mill compressor?

Canonical: https://senseible.earth/answers/alwar-vfd-on-compressor-unload-power

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](https://senseible.earth/green-loan-check)
- [Screw compressor for an Alwar oil mill](https://senseible.earth/answers/alwar-screw-compressor-for-four-reciprocating)
- [VFD on an ice plant compressor](https://senseible.earth/answers/vfd-on-ice-plant-compressor-saving)
- [BEE project reports: grid factor behind CO2](https://senseible.earth/answers/bee-project-reports-co2-grid-factor-differs)

Sources: [BEE, Detailed project report: variable frequency drive on compressor, Alwar oil mill cluster (SAMEEEKSHA)](https://sameeeksha.org/pdf/dpr/Alwar_oil_mills_cluster/Energy_cost_reduction_with_variable_frequency_drive_over_compressor.pdf), [CEA, CO2 Baseline Database for the Indian Power Sector](https://cea.nic.in/cdm-co2-baseline-view?mappingId=234&lang=en)

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.