Does a 1,000 litre a day solar water heater really pay back in 2.1 years in the Tiruppur manual?
The Bureau of Energy Efficiency (BEE) Tiruppur manual prices a 1,000 litre a day solar water heater at rupees (Rs) 1.61 lakh (1 lakh = 100,000) and books 21,000 kilograms (kg) of wood saved, Rs 0.75 lakh, payback 2.1 years. Rebuilt from its own lines, wood saved is 10,000 to 15,000 kilograms and payback is 3 to 4.5 years.
The report's table
- Capacity 1,000 litres a day, water in at 30 degrees Celsius (C) and out at 70, 150 days a year.
- Present hot water generation efficiency 15%. Wood Rs 3.6 a kilogram.
- Printed: temperature difference 60 degrees, heat required 600,000 kilocalories (kcal) a day, wood saved 21,000 kilograms a year (the manual labels this column kg/day / year; we read it as kilograms a year), Rs 0.75 lakh. Cost Rs 1.61 lakh, payback 2.1 years.
- Cost table: Rs 1.55 lakh equipment, 0.02 erection and 0.04 miscellaneous, total Rs 1.61 lakh. That ties.
What does not tie
- Temperature difference: 70 minus 30 is 40 degrees, not the 60 printed.
- Heat: 1,000 litres x 60 degrees is 60,000 kcal a day, and x 40 degrees is 40,000. The printed 600,000 is ten times the first and fifteen times the second.
- Wood at 15% efficiency and 4,000 kcal a kilogram, the value in the manual's chulha table (its unit tables work out at 3,800 to 4,000): at 60 degrees that is 60,000 / 0.15 / 4,000 = 100 kilograms a day, 15,000 a year. At 40 degrees it is 66.7 kilograms a day, 10,000 a year.
- The printed 21,000 kilograms matches neither. The Rs 0.75 lakh follows from it: 21,000 x 3.6 = Rs 75,600.
- On 15,000 kilograms the saving is Rs 54,000 and the payback 1.61 / 0.54 = 3.0 years. On 10,000 it is Rs 36,000 and 4.5 years. We could not find what produces 21,000, so treat 3 to 4.5 years as our rebuilt range, not the manual's figure.
What else the manual says about it
- Average daily solar radiation on a collector tilted at 25 degrees is 5.08 kilowatt hours (kWh) a square metre a day, 1,854 a year.
- Hot water at 70 degrees C for about 5 months of the year and 55 degrees C for 2 to 3 months.
- It says a subsidy of up to 20% of total cost was available from the central government if no loan is taken from a financial institution. That is a study-year statement; check the current scheme with your bank or the Ministry of New and Renewable Energy before counting on it.
How to read it
A solar heater still saves wood, but at the manual's printed 25 months it is already the second slowest of its ten measures, and the rebuilt figure is slower. A boiler swap or heat recovery from hot drained water pays back sooner. Fit it where wood fired hot water for bleaching or soaping runs most days, and weigh that wood for a month first.
Rebuilding the wood saving from the water line
- Heat per day: 1,000 litres x 60 degrees (as printed) = 60,000 kcal; x 40 degrees (70 minus 30) = 40,000 kcal
- Wood heat at 15% efficiency: 60,000 / 0.15 = 400,000 kcal; or 40,000 / 0.15 = 266,667 kcal
- Wood at 4,000 kcal a kilogram: 100 kg a day x 150 days = 15,000 kg; or 66.7 x 150 = 10,000 kg
- Money at Rs 3.6: Rs 54,000 or Rs 36,000; payback 1.61 lakh / 0.54 lakh = 3.0 years, or / 0.36 lakh = 4.5 years
- Printed: 21,000 kg, Rs 0.75 lakh, 2.1 years
The 4,000 kcal a kilogram is the manual's chulha figure, used here for its solar table, which prints no wood value. The rebuilt range is our own arithmetic.
What Senseible does here
Senseible reads fuel invoices and electricity bills into monthly records, so the wood bought each month is on file to test a savings claim against. A solar heater's output comes from a meter on the heater; Senseible keeps the wood bills.
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- Tiruppur improved chulha, 56 units total checked
- Tiruppur manual: payback table in months or years
- Green loan readiness check
Last reviewed 2026-11-05. Figures are from the manual's study year (around 2010 to 2011). Solar heater prices, subsidies and wood prices have changed; get a current quote and check the scheme.