Why does the Vapi 600 kilogram boiler report save 63 tonnes of wood while the 400 kilogram report saves 49?

Mostly hours. The Bureau of Energy Efficiency (BEE) Vapi 600 kilogram boiler report assumes 12 hours a day for 200 days, 2,400 hours, against 9 hours for 180 days, 1,620, for the 400 kilogram one. Hourly wood use is almost the same, 84 and 83.5 kilograms.

Same wood an hour, different hours a year

• 400 kilogram report: 83.5 kilograms of wood an hour, boiler efficiency 41.4%, 9 hours x 180 days = 1,620 hours, 135 tonnes a year. New boiler at 65% saves 49 tonnes.
• 600 kilogram report: 84 kilograms an hour, efficiency 44.41%, 12 hours x 200 days = 2,400 hours, 200 tonnes a year. New boiler at 65% saves 63 tonnes.
• Check on the hours: 200 tonnes / 2,400 = 83.3 kilograms an hour, in line with the 84 tested.

Per hour, the smaller one saves more

49 tonnes over 1,620 hours is 30.25 kilograms of wood saved an hour. 63 tonnes over 2,400 hours is 26.25. The 400 kilogram report saves more each hour because its old boiler was worse, 41.4% against 44.41%. The 600 kilogram one saves more in the year only because it is assumed to run longer.

That matters if your boiler runs fewer hours than the report's. A lakh is 100,000. Rupees (Rs) 6.13 lakh of cost for 2.76 years of payback at 2,400 hours becomes about 4.1 years at 1,620 hours (our arithmetic in the example below).

Steam made against the size of the boiler

• 600 kilogram report: 225 kilograms of steam an hour at 6 kilograms per square centimetre (kg/cm2), which is 37.5% of the rating.
• 400 kilogram report: 210 kilograms an hour, which is 52.5% of the rating.
• Both reports say their boilers run at less than 20% of capacity. The printed steam gives higher shares. Our reading is that the sentence was copied between reports; the tests are the better guide.

Printed slips in the 600 kilogram report

• Only one unit was tested, Nandosal Chem Industries, where the 400 kilogram report tests three. The test: 84 x 3,800 kilocalories (kcal) = 319,200 in, 225 x 630 = 141,750 out, 44.41%.
• The enthalpy rows (the heat each kilogram of steam or water carries) are swapped: 'enthalpy of steam at 9 kg/cm2' shows 30 and feed water shows 660, while the pressure printed is 6. 660 minus 30 = 630 is used as printed.
• The summary prints the internal rate of return (IRR) as 21.17%, and the sensitivity table prints 21.72%. In the same table the 5% decrease row shows a return on investment of 34.37% against 24.72% for the normal case, which looks like a typing slip (our reading).
• Own money 25% is Rs 1.53 lakh and the loan Rs 4.60 lakh on a cost of Rs 6.13 lakh.

Each boiler at the other one's hours (our arithmetic)

• 600 kilogram boiler at 1,620 hours: 26.25 kg an hour x 1,620 = 42.5 tonnes x Rs 3,500 = Rs 1.49 lakh. Payback 6.13 / 1.49 = 4.1 years
• 400 kilogram boiler at 2,400 hours: 30.25 kg an hour x 2,400 = 72.6 tonnes x Rs 3,500 = Rs 2.54 lakh. Payback 5.10 / 2.54 = 2.0 years
• Printed: 2.76 years at 2,400 hours and 2.96 years at 1,620 hours

Wood at the reports' Rs 3,500 a tonne, 2010 prices. The swap holds each boiler's own saving an hour and changes only the hours.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so your wood purchases by month show how much wood you burn. Divide by the wood an hour you weigh to get the hours your boiler works. Steam made is not read from invoices, so the steam reading is taken on site.

• Green loan readiness check
• Vapi 400 kg boiler: the three unit tests
• Boiler efficiency: direct and indirect method
• Vapi reports: wood counted as a cut or zero

Sources: BEE, Detailed project report: energy efficient boiler 600 kg/hr, Vapi dyes and chemicals cluster (SAMEEEKSHA), BEE, Detailed project report: energy efficient boiler 400 kg/hr, Vapi dyes and chemicals 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.