What do the Bureau of Energy Efficiency (BEE) Surat reports say a 600 or 800 kilowatt (kW) gas engine with a heat recovery boiler saves, and what is missing from their carbon figure?

BEE's Surat reports price a 600 kW gas engine with a waste heat boiler at rupees (Rs) 187.15 lakh and an 800 kW one at Rs 213.30 lakh. They save Rs 56.19 and 75.65 lakh a year on grid power and coal, with paybacks of 3.30 and 2.82 years.

The two reports side by side

• Both run 12 hours a day, 350 days, at 75% load. 600 kW: 450 kW x 4,200 hours = 1,890,000 kilowatt hours (kWh) a year. 800 kW: 600 kW x 4,200 = 2,520,000 kWh.
• The waste heat boiler makes 300 kilograms (kg) of steam an hour on the 600 kW set and 431 on the 800 kW. At a steam to coal ratio of 7 to 1 that replaces about 43 and 62 kg of coal an hour. The printed yearly coal is 180 and 258 tonnes: 300 / 7 x 4,200 hours = 180 tonnes, and 431 / 7 x 4,200 = 258.6.
• Engine power costs Rs 2.96 a kWh (0.24 standard cubic metres of gas x Rs 12.32), against Rs 5.60 from the grid, a gain of Rs 2.64 a kWh. 600 kW: Rs 49.89 lakh from power plus Rs 6.30 lakh from coal, Rs 56.19 lakh. 800 kW: Rs 66.52 lakh plus Rs 9.05 lakh.
• Cost, 600 kW: Rs 117.65 lakh equipment, Rs 61.00 lakh design, engineering, cabling and switches, Rs 8.50 lakh erection, Rs 187.15 lakh. 800 kW: Rs 131.30 + 73.00 + 9.00 = Rs 213.30 lakh. Per kW, Rs 0.31 lakh against Rs 0.27 lakh.
• Printed indicators, 600 kW against 800 kW: internal rate of return (IRR) 17.67% and 22.29%, net present value (NPV) Rs 62.53 and 118.35 lakh, 10 days of downtime each.

Why the larger set pays back faster

It is mostly the bigger kWh base for a lower cost per kW. The 600 kW set would replace about 96% of the typical unit's 1,971,084 kWh of grid power. The 800 kW set would replace 75% of the typical unit's 3,357,000 kWh. Our reading: a plant that cannot use all the engine's output at the engine's hours gets less than either report shows. The reports size the 600 kW set for plants making 50,000 to 80,000 metres a day and the 800 kW set for more than 50,000.

Printed figures that do not tie

• 800 kW: the printed power line, Rs 66.52 lakh, plus the printed coal line, Rs 9.05 lakh, is Rs 75.57 lakh, not the printed Rs 75.65 lakh.
• 600 kW: payback is 187.15 / 56.19 = 3.33 years, printed as 3.30. Its "benefit per unit" of Rs 2.903 should be Rs 2.973 (56.19 lakh over 1,890,000 kWh).
• The 800 kW coal line rounds 62 kg an hour; the printed 258 tonnes and Rs 9.05 lakh match 431 / 7 kg, not 62 x 4,200.

What the carbon figure leaves out

The reports print 800 and 1,103 tonnes of carbon dioxide (CO2) a year and give no factors. They say gas has less CO2 than coal fired power and that coal burnt in the boiler falls. Neither says how much CO2 the gas itself adds. We could not rebuild either figure from the printed inputs.

Our reading of the three lines, kept separate. Gas burnt: 0.24 x 1,890,000 = 453,600 standard cubic metres, and 604,800 for the 800 kW set. Grid power no longer bought is lower Scope 2. Coal no longer burnt is lower Scope 1, and gas burnt is added Scope 1. If the engine is a four-stroke lean-burn type, its methane slip adds more; the reports do not say what type it is.

Before you decide

• The 600 kW report lists as a barrier: "Shortage of natural gas supply in the region and frequent change of policies by gas companies on the tariff." The whole case rests on gas at Rs 12.32 a cubic metre.
• Take your own gas rate, grid rate and engine hours from your bills and rerun Rs 2.64 a kWh.
• The 600 kW report also says the project is "highly capital intensive". Ask for the maker's guaranteed gas use per kWh at your load.

The three CO2 lines for the 600 kW set (estimate)

• Grid power avoided: 1,890,000 kWh x 0.675 = 1,276 tonnes (Scope 2)
• Coal avoided: 180 tonnes x 1.76 = 317 tonnes on Indian coal, or x 2.44 = 439 on imported coal (Scope 1)
• Gas added: 453,600 x 2.05 = 930 tonnes (Scope 1), before any engine methane
• Lean-burn methane if it applies: 453,600 x 0.65 = about 295 tonnes of carbon dioxide equivalent (CO2e)
• Comparison only: 1,276 + 317, less 930, = 663 tonnes, or 785 on imported coal, against 800 printed

Grid at 0.675 kg CO2 a kWh (Central Electricity Authority, financial year 2025-26). Coal at 1.76 and 2.44 kg CO2 a kg and gas at 2.05 kg a standard cubic metre are Senseible's standard factors; the reports do not say which coal. Methane is the Intergovernmental Panel on Climate Change default for lean-burn engines worked out on the linked page. Report separate lines in your own records; the sum is only to set beside the printed cut.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so your gas, coal and grid bills sit side by side before and after an engine. It does not add engine methane on its own; add the slip worked out on the linked page.

• Gas engine methane slip
• Steam turbine on a pressure reducing valve
• Old report CO2 versus today's grid factor
• Green loan readiness check

Sources: BEE, Detailed project report: natural gas co-generation system (600 kW), Surat textile cluster (SAMEEEKSHA), BEE, Detailed project report: natural gas co-generation system (800 kW), Surat textile cluster (SAMEEEKSHA), Central Electricity Authority (CEA), CO2 Baseline Database for the Indian Power Sector, IPCC 2006 Guidelines, Volume 2, Energy (default emission factors and net calorific values)

Last reviewed 2026-11-05. Report figures are from BEE's Small and Medium Enterprises (SME) programme project reports, with gas at Rs 12.32 a cubic metre and grid power at Rs 5.60 a kWh. Check both prices and a current quote before you rely on the payback.