How do the 5,000 and 7,000 litre a day solar water heater reports for a dairy compare on furnace oil saved and payback, and which oil figure for the plant is right?

A 7,000 litre a day solar water heater saves 13,230 kilograms of furnace oil a year in the Bureau of Energy Efficiency (BEE) Gujarat reports, against 9,450 for 5,000 litres, and pays back a little sooner: 2.15 years against 2.30. The 7,000 litre report quotes the plant's yearly oil use two different ways.

What the system is, in plain words

It is a larger version of the water heater on a house roof. Flat panels heat water, and an insulated tank keeps it hot. The dairy uses that water for cleaning cans and tankers in place (CIP) and as warm feed water for its oil-fired hot water generator. The generator then only tops up the heat, so it burns less furnace oil. A system is sized in litres a day (LPD) of hot water.

The two reports side by side

• 5,000 LPD: heat captured 5,000 litres x 43 degrees (27 up to 70) = 215,000 kilocalories a day. Oil saved 31.5 kilograms a day, 9,450 over 300 sunny days. At 30 rupees (Rs) a kilogram that is Rs 2.84 lakh a year, where a lakh is 100,000. Cost Rs 6.54 lakh, payback 2.30 years. The report shows 32 tonnes of carbon dioxide (CO2) saved.
• 7,000 LPD: 301,000 kilocalories a day. Oil saved 44.1 kilograms a day, 13,230 a year. Rs 3.97 lakh a year. Cost Rs 8.56 lakh, payback 2.15 years. The report shows 45 tonnes of CO2 saved.
• Both assume the oil-fired generator is 65% efficient and the oil gives 10,500 kilocalories a kilogram. Oil saved is 215,000 / (0.65 x 10,500) = 31.5 kilograms, and 301,000 / (0.65 x 10,500) = 44.1.
• The saving is in step with size: 44.1 / 31.5 = 1.4 = 7,000 / 5,000. The cost is not: Rs 8.56 lakh / 7,000 = Rs 122 a litre of capacity against Rs 6.54 lakh / 5,000 = Rs 131, so the larger system pays back a little sooner.

Which oil figure for the plant is right?

Both reports use the same chilling centre, taking 150,000 litres of milk a day and using 974,664 kilowatt hours (kWh) a year. The 5,000 report says its oil use is 63,036 kilograms a year, 175 a day. The 7,000 report says 63,036 in its opening paragraph and then, in the baseline table, 1,225 kilograms a day, 442,020 a year.

The two are seven times apart. The 442,020 is also the figure for the 400,000 litre dairy in the Gujarat 210 tonne of refrigeration desuperheater report, so it looks carried over from another report. That is our reading; the report does not say.

It matters for the share saved. 13,230 is 21% of 63,036 but only 3% of 442,020. Use your own oil invoices for the plant's yearly use, not either figure.

A cost table slip, the same in both

Each report's cost table prints the civil work line ten times larger than the figure its total uses. The 5,000 table shows Rs 0.10 lakh and its total of Rs 6.54 lakh needs Rs 0.01 lakh. The 7,000 table shows Rs 0.2 lakh, and its total of Rs 8.56 lakh needs Rs 0.02 lakh: 7.35 + 0.3 + 0.8 + 0.02 + 0.09 = 8.56. Ask for a quote with civil work priced for your roof.

The carbon dioxide line

• Both reports work to 3.4 kilograms of CO2 for each kilogram of oil: 9,450 x 3.4 = 32.1 and 13,230 x 3.4 = 45.0.
• Working from kilograms at the IPCC default of 3.13 kilograms of CO2 for each kilogram of furnace oil, 9,450 x 3.13 = 29.6 tonnes and 13,230 x 3.13 = 41.4 tonnes. Both are a little below the reports' 32 and 45.
• Do not mix the two bases. Senseible's 2.97 kilograms a litre is built on 0.95 kilograms a litre of oil, so putting it on the reports' density of 0.9 would overstate the litres' carbon by about 5.6%. Say which factor you used.

The 7,000 LPD case, line by line

• Heat: 7,000 litres x (70 minus 27) = 301,000 kilocalories a day
• Oil: 301,000 / (0.65 x 10,500) = 44.1 kg a day x 300 days = 13,230 kg. At Rs 30 = Rs 3.97 lakh. 8.56 / 3.97 = 2.15 years
• CO2: 13,230 kg x 3.13 = 41,410 kg, or 41.4 tonnes (the report shows 45)
• Share of the plant's oil: 13,230 / 63,036 = 21%; 13,230 / 442,020 = 3%

These are the reports' printed figures with simple arithmetic on top. The share lines and the carried-over reading are our own. Prices are the reports' 2010 prices.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records, so the oil bought before and after the solar system is on record, bill by bill. Hot water metering can be added as a custom service, on request. Meanwhile keep a tank log with the bills.

• 5,000 litre solar water heater in a dairy: oil saved
• Desuperheater on a 100 tonne of refrigeration dairy plant: oil saved
• Dairy plant energy per kg of milk
• Green loan readiness check

Sources: BEE and SAMEEEKSHA detailed project report: solar water heating system 7,000 LPD, Gujarat dairy cluster, BEE and SAMEEEKSHA detailed project report: solar water heating system 5,000 LPD, Gujarat dairy cluster, BEE and SAMEEEKSHA detailed project report: waste heat recovery by desuperheater, 210 TR, Gujarat dairy cluster, IPCC 2006 Guidelines for National Greenhouse Gas Inventories, volume 2 (default factor for fuel oil)

Last reviewed 2026-11-07. Costs and paybacks are at the reports' 2010 prices. The carried-over reading and the share of plant oil are our own arithmetic on the printed numbers; use your own oil invoices and a current quote.