Why does the 120,000 kilocalorie gas hot air generator pay back in 1.02 years and the 60,000 in 2.35 in the Bureau of Energy Efficiency Ahmedabad reports?

Because the bigger one saves nearly three times as much and costs about a quarter more. In the Bureau of Energy Efficiency (BEE) Ahmedabad reports, the 120,000 kilocalorie (kcal) an hour unit saves rupees (Rs) 3.88 lakh a year for Rs 3.96 lakh, and the 60,000 saves Rs 1.36 lakh for Rs 3.19 lakh.

Side by side

• Wood replaced: 72 kilograms an hour (259.2 tonnes a year) against 144 kilograms (518.4 tonnes), both over 12 hours x 300 days.
• Gas used: 8.3 normal cubic metres (Nm3, gas measured at a standard temperature and pressure) an hour (29,880 a year) against 15.5 (55,800).
• Extra power: 14,760 kilowatt hours (kWh) a year against 21,480.
• Equipment: Rs 2.90 lakh against Rs 3.60 lakh. Total cost: Rs 3.19 lakh against Rs 3.96 lakh. A lakh is 100,000.
• Net saving: Rs 1.36 lakh against Rs 3.88 lakh. Payback: 2.35 years against 1.02.

Why the payback differs so much

Wood use doubles, but the machine does not cost double. 3.96 / 3.19 = 1.24 times the cost for 3.88 / 1.36 = 2.85 times the net saving.

Fuel saving is 7.78 minus 5.68 = Rs 2.10 lakh for the small unit and 15.55 minus 10.60 = Rs 4.95 lakh for the large one, 2.36 times. Power cost rises only 1.46 times, from Rs 0.738 lakh to Rs 1.07 lakh, so the net saving grows faster than the fuel saving.

How fragile each result is

• A 5% fall in fuel savings, small unit: internal rate of return (IRR, the yearly return on the money put in) 25.46% down to 12.53%, net present value (total gain over the project's life, counted at the value of money now) Rs 1.76 lakh down to Rs 0.27 lakh, cover ratio (how many times the year's cash covers the loan payments) 1.71 down to 1.20.
• Same fall, large unit: IRR 76.55% down to 61.54%, net present value Rs 10.50 lakh down to Rs 7.76 lakh, cover ratio 3.99 down to 3.23.
• Our payback at 5% less fuel saving: small 3.19 / 1.255 = 2.5 years; large 3.96 / 3.63 = 1.1 years. At 5% more: 2.2 and 0.96.

Sizing, and one assumption both share

• Both reports say the generator suits a dryer with 400 trays, the larger 'and more'. Our reading: choose on your own wood use an hour, 72 or 144 kilograms, not on tray count alone.
• The large report says typical units burn 120 to 130 kilograms an hour, 468 tonnes a year, yet sizes on 144, 518.4 tonnes. 144 x 2,700 kcal = 388,800 kcal an hour.
• Our check: 120,000 / 388,800 = 30.9%, and for the small unit 60,000 / (72 x 2,700) = 60,000 / 194,400 = 30.9%. Both reports assume the old wood unit delivers about 31% of the wood's heat, near the 32% they quote.

Carbon dioxide (CO2) lines that differ

• Wood CO2: 243 tonnes for 259.2 tonnes and 486 for 518.4, both 0.94 tonnes a tonne of wood.
• Gas CO2: 58 tonnes for 29,880 Nm3 and 119 for 55,800. Divided out, that is 1.94 and 2.13 kilograms an Nm3. The reports print no factor, so the two gas lines are not on one basis (our division).
• Net cuts printed: 185 and 367 tonnes. Power is not counted in either, and wood CO2 is biogenic, meaning the tree took it from the air as it grew, and the Greenhouse Gas (GHG) Protocol has it reported on a separate line outside Scope 1 (the emissions from fuel you burn on your own site), so neither cut is a Scope 1 cut (our reading).

Small printed slip

The large report's text gives the term loan as Rs 2.97 lakh, 75% of 3.96, and its financing table prints Rs 2.9 lakh. Own money is Rs 0.99 lakh.

Payback for both, from the reports' inputs

• Small: 259,200 kg x Rs 3 = Rs 7.78 lakh. 29,880 Nm3 x Rs 19 = Rs 5.68 lakh. Power 14,760 x Rs 5 = Rs 0.74 lakh. Net 7.78 minus 5.68 minus 0.74 = Rs 1.36 lakh. 3.19 / 1.36 = 2.35 years
• Large: 518,400 x Rs 3 = Rs 15.55 lakh. 55,800 x Rs 19 = Rs 10.60 lakh. Power 21,480 x Rs 5 = Rs 1.07 lakh. Net 15.55 minus 10.60 minus 1.07 = Rs 3.88 lakh. 3.96 / 3.88 = 1.02 years

Inputs are the reports' own, at the report year's prices. Gas at Rs 19 an Nm3 is the reports' assumption, not a current price.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so your fuel purchases by month show how much you burn. Divide by your dryer hours to get kilograms an hour, which decides which size fits. Gas pressure and supply at your site are taken on site.

• Green loan readiness check
• Ahmedabad small gas hot air generator report
• Wood to gas hot air generator: CO2 effect
• Efficient wood hot air generator saving

Sources: BEE, Detailed project report: energy efficient gas fired hot air generator 60,000 kCal/hr, Ahmedabad chemical cluster (SAMEEEKSHA), BEE, Detailed project report: energy efficient gas fired hot air generator 1,20,000 kCal/hr, Ahmedabad chemical cluster (SAMEEEKSHA)

Last reviewed 2026-11-07. Figures are from the report named above, written in 2010; costs, fuel prices and paybacks are at that year's prices. Check today's prices and a current maker's quote before you rely on the payback.