Why do the Bureau of Energy Efficiency (BEE) Solapur reports show solar water heaters of 500, 1,000 and 2,000 litres a day (LPD) paying back in 2.1 to 2.8 years?

BEE's Solapur reports price solar water heaters at rupees (Rs) 1.04 lakh for 500 litres a day, Rs 1.61 lakh for 1,000 and Rs 3.47 lakh for 2,000. Payback is printed as 2.12, 2.64 and 2.75 years. The 1,000 litre one replaces kerosene stoves, the other two wood chulhas.

The three reports side by side

• 500 LPD, replaces wood chulhas: audited chulhas run at 4.5%, 4.6% and 5.0% efficiency. Wood saved 19.6 tonnes a year x Rs 2,500 = Rs 0.49 lakh, on 251 days of 8 hours. Cost Rs 1.04 lakh. Payback 2.12 years, internal rate of return (IRR) 22.22%, net present value (NPV) Rs 0.31 lakh.
• 1,000 LPD, replaces kerosene stoves at 10% efficiency: 1,500 litres a year x Rs 40.66 = Rs 0.61 lakh, on 240 days. Cost Rs 1.61 lakh in the summary, Rs 1.60 lakh in the cost table. Payback 2.64 years, IRR 11.44%, NPV Rs 0.05 lakh. It has 4 flat plate collectors and a 1,000 litre tank.
• 2,000 LPD, replaces wood chulhas: audited at 6.25%, 5.04% and 6.2%. Wood saved 42 tonnes x Rs 3,000 = Rs 1.26 lakh, on 240 days. Cost Rs 3.47 lakh. Payback 2.75 years, IRR 14.51%, NPV Rs 0.43 lakh.
• Each treats the solar heater as replacing all of the old fuel: the proposed fuel line reads nil. Each heats to 60 degrees Celsius (C). Implementation takes 8 to 10 weeks in each report's text, and 10 weeks in each annexure schedule.

Cost per litre and saving per litre

Cost per litre a day of capacity: Rs 208 for the 500, Rs 161 for the 1,000, Rs 173.50 for the 2,000. Wood saved per litre of capacity: 39.2 kilograms a year for the 500 but 21.0 for the 2,000, a gap. Our reading: it probably comes from the baseline units, not the heaters. The 2,000 litre case is also priced with wood at Rs 3,000 a tonne against Rs 2,500 in the 500 litre one. Judge each by your own fuel use, not by its litres.

Printed figures that do not tie

• The 2,000 LPD report's specification table prints a capacity of 1,000 LPD, a 1,000 litre tank and 8 collectors. The 1,000 LPD report lists 4 collectors for 1,000 litres. Ask the supplier what is actually quoted.
• 1,000 LPD baseline: section 1.2 says a typical unit burns 2,200 litres of kerosene a year for dyeing. The report saves 1,500, which is 68%. But Annexure 1 audits a stove at 20 litres a day, 4,800 over 240 days. The two baselines do not agree, and the 1,500 is not explained.
• Cost: summary Rs 1.61 lakh against Rs 1.60 lakh in Table 4.1 (1.55 + 0.02 + 0.03).
• 1,000 LPD results: the summary prints savings of Rs 0.61 lakh, payback 2.64 years and NPV Rs 0.05 lakh. Section 4.3.2 prints savings of Rs 0.62 lakh, payback 2.58 years and NPV Rs 0.06 lakh.
• The 1,000 LPD report also says the unit gives an output of 1,000 LPD, which replaces about 2,000 LPD of hot water generation in the conventional system. That is the report's stated assumption, so ask what baseline your own units would show.

The carbon dioxide (CO2) lines

Wood is counted at 1.4 tonnes of CO2 a tonne: 27.5 tonnes for the 500 LPD (19.6 x 1.4 = 27.4) and 59 for the 2,000 (42 x 1.4 = 58.8). Our reading, as on the linked pages: CO2 from burning wood is biogenic and reported apart from Scope 1, so these are not Scope 1 cuts.

Kerosene is a fossil fuel, so the 1,000 LPD saving is a real Scope 1 cut. The report prints 4 tonnes of CO2 for each kilolitre, 6 tonnes for 1,500 litres. On the report's own 8,500 kilocalories a litre (35.59 megajoules) and the Intergovernmental Panel on Climate Change (IPCC) default of 71,900 kilograms (kg) of CO2 per terajoule for other kerosene, a litre is 2.56 kilograms, and 1,500 litres is about 3.8 tonnes. The printed factor is about 56% higher.

Before you buy

• Measure the hot water you really use a day and its temperature. A heater bigger than the demand saves nothing more.
• Check the fuel price. The reports use Rs 2,500 or Rs 3,000 a tonne of wood and Rs 40.66 a litre of kerosene, at 2010 prices.
• A solar heater does not run on cloudy days, so keep the old stove as backup and read your fuel bills, not the report, for the real saving.

The 1,000 LPD kerosene saving, rebuilt

• Money: 1,500 litres x Rs 40.66 = Rs 0.61 lakh. Payback Rs 1.61 lakh / Rs 0.61 lakh = 2.64 years
• Kerosene heat: 8,500 x 4.1868 = 35.59 megajoules a litre
• CO2 a litre: 35.59 x 71.9 kg per gigajoule / 1,000 = 2.56 kg
• Saved: 1,500 x 2.56 = 3.8 tonnes of CO2 a year, against 6 printed

Printed lines are the report's own at the report year's prices. The kerosene factor is the IPCC 2006 default; the calorific value is the report's own. Our arithmetic, an estimate.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so wood, briquette and kerosene purchases by month are on file before a heater goes in. It has no wood factor; keep wood tonnes beside the figure.

• Solar water heater in a Pali petcoke unit
• Solapur textile cluster wood fuel carbon
• Efficient chulha in dyeing: wood saved
• Green loan readiness check

Sources: BEE, Detailed project report: solar water heating system (500 LPD), Solapur textile cluster (SAMEEEKSHA), BEE, Detailed project report: solar water heating system (1000 LPD), Solapur textile cluster (SAMEEEKSHA), BEE, Detailed project report: solar water heating system (2000 LPD), Solapur textile cluster (SAMEEEKSHA), 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 fuel at the report year's prices. Check today's fuel prices, your hot water use and a current quote before you rely on the payback.