Solar pump versus diesel: reading the payback correctly
A cooperative runs a 5 hp diesel pump set for about 1,200 hours a year. The engine consumes 0.9 litres of diesel per hour, diesel costs Rs 92 per litre, and combustion releases about 2.68 kg of carbon dioxide per litre. A solar pump of matched duty - 3.5 kW motor, 4.8 kWp array, no battery, daylight only - costs Rs 4.2 lakh installed. A 50% capital subsidy is available, and the balance can be borrowed at 9% simple interest over seven years. The grid reaches the village but supplies power for only four to six hours a day, at hours the farmers do not control, while the cooperative irrigates in the late evening and at night. Annual maintenance of the diesel set is Rs 9,000 and rising. The subsidy is capped and the cooperative must decide by March.
1. Calculate annual diesel use, fuel cost, carbon dioxide emissions and the simple payback on the cooperative's own money.
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Fuel: 1,200 hours x 0.9 litres per hour = 1,080 litres a year. Cost: 1,080 x Rs 92 = Rs 99,360 a year, about Rs 0.99 lakh. Carbon dioxide: 1,080 x 2.68 = 2,894 kg, about 2.9 tonnes a year.
Solar: Rs 4.2 lakh capital, 50% subsidy, so the cooperative pays Rs 2.1 lakh. Borrowing Rs 2.1 lakh at 9% simple interest over seven years adds roughly Rs 0.66 lakh of interest, taking total outgo to about Rs 2.76 lakh, or about Rs 39,000 a year across seven years.
Simple payback on the cooperative's share: 2.1/0.9936, about 2.1 years; including interest, about 2.8 years. Add the Rs 9,000 a year diesel maintenance that disappears, less perhaps Rs 3,000-5,000 a year for module cleaning and an eventual inverter replacement, and the arithmetic looks strong. Over fifteen years the avoided fuel is about 16,200 litres and roughly 43 tonnes of carbon dioxide, before counting emissions embedded in manufacturing the modules. The arithmetic is not in doubt; the question is whether it is the right question.
2. Is payback the decisive argument? Identify the practical constraints that could make the solar pump fail despite the arithmetic.
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No. Payback answers when the money returns, not whether the pump does the job.
Timing mismatch is the biggest risk. A battery-free pump delivers power only in daylight, but the cooperative irrigates late evening and at night - the very reason it owns a diesel set. Unless the irrigation schedule moves to daytime, or a day storage tank is added, the cooperative keeps the diesel set and the solar pump idles: payback of never. The honest comparison is solar plus a day tank or a small hybrid inverter, which changes capital cost.
Duty match across the year matters next. A 4.8 kWp array does not deliver 3.5 kW in December haze or in the monsoon, when output can fall by a third or more - exactly when a crop may need water. Size on worst-month output or use a variable-frequency drive; a 3.5 kW motor started direct-on-line may need a soft starter.
Third, in this setting often decisive: free daytime energy removes the marginal cost of pumping, so the cooperative may irrigate more, not just differently. The water-saving effect of expensive diesel disappears, which matters in a block with a falling water table.
3. What would you check before signing, and would you always recommend the full 4.8 kWp system?
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Technically: the pump's duty point against actual head and discharge, and the array's worst-month output against that duty; whether a day storage tank is included and how it is sized; the motor starting method; warranty periods for modules, inverter and motor, and who services them within 50 km.
On water: the pre-monsoon water-table trend in the cooperative's wells and whether the block is notified or critical. A subsidised pump in an over-drafted block is a policy contradiction worth raising in writing.
Financially: whether the 50% subsidy is paid upfront or as reimbursement. If the cooperative must find Rs 4.2 lakh first and wait a year for Rs 2.1 lakh, the effective cost is well above the payback figure, and a March deadline may force a costlier loan.
Contractually: who owns the asset, and whether theft or vandalism insurance is possible.
On sizing, no - not always. A smaller 2 kW array for daytime-only irrigation, with the diesel set retained for night watering, may serve the same command area at lower capital and lower risk than an oversized replacement. Decide on whole-system cost per cubic metre delivered, not on the pump's sticker price.