Solar can remove routine fuel purchases, but “no fuel cost” is not the same as “no cost”. Panels, pump, controller, tank, installation, financing, security, maintenance and eventual replacements all belong in the decision. A petrol pump also has more than fuel cost: oil, spark plugs, servicing, repairs, travel to buy fuel and downtime matter. The fairest comparison makes both systems deliver the same useful water volume at the same total dynamic head.
Define an equal irrigation service
Write the annual or seasonal water duty: gross cubic metres delivered, TDH, pressure, irrigation months and operating days. Confirm that both proposed systems can perform that duty from the same source. If solar requires a tank or larger pipe to operate effectively, include those assets. If the petrol option already owns reusable hoses or a tank, distinguish sunk assets from new investment.
Acreage is not an equal-service measure. One system may apply less water, run fewer days or fail under the real head. Use the one-acre sizing method and TDH guide to define the hydraulic service before pricing it.
Build the petrol baseline from records
Measure litres of petrol consumed over several representative pumping sessions and meter or estimate the useful water delivered. Record head and runtime. Fuel use per hour without water output can reward an inefficient or partly blocked system in the model. Ideally calculate litres of fuel per cubic metre delivered at the relevant duty.
Use the current official price for the farm’s location and period. Kenya’s Energy and Petroleum Regulatory Authority publishes maximum retail petroleum prices for defined pricing cycles. The price changes; cite the exact bulletin and validity dates rather than writing a permanent number into an evergreen article. For a forecast, test low, base and high prices.
Annual petrol operating cost can be modelled as fuel litres per day × pumping days × KSh/litre, plus scheduled maintenance, expected repairs and fuel-acquisition cost. The last item can include transport and paid labour but should not invent a value. Use farm records or show it separately.
Build the solar cash flow without hiding costs
Year zero includes pump, controller, panels, mounting, pipe changes, tank, civil work, transport, installation, electrical protection, security and taxes or duties actually payable. If financed, model the deposit, fees, interest and payment schedule rather than comparing the sticker price with cash fuel expenditure.
Annual solar costs include inspection, cleaning labour, planned service, repairs, insurance or security and any paid monitoring. Put controller, pump or panel replacement in the year reasonably expected under the supplied warranty and service plan. A warranty is valuable only if terms, exclusions, local support, return logistics and downtime are workable.
Simple payback is additional upfront solar investment divided by annual cash operating savings. It is easy to understand but ignores timing after payback. A multi-year net cash flow is better: for each year, subtract solar costs from avoided petrol costs and include replacements. Discounted analysis can account for the time value of money, but the discount rate must be stated.
Illustrative calculation—not a current price quote
| Input | Illustrative assumption | Replace with |
|---|---|---|
| Petrol use | 2.0 L/pumping day | Measured farm average |
| Pumping days | 180/year | Crop calendar and records |
| Fuel price | KSh 190/L | Current EPRA location bulletin |
| Petrol maintenance | KSh 12,000/year | Receipts and service plan |
| Additional solar capital | KSh 180,000 | Comparable landed quotations |
| Solar annual cost | KSh 6,000/year | Service/security estimate |
In this illustration, annual petrol fuel is 360 litres and costs KSh 68,400. Adding KSh 12,000 maintenance gives KSh 80,400. Subtracting KSh 6,000 solar annual cost produces KSh 74,400 annual cash savings. Simple payback on KSh 180,000 additional capital is about 2.4 years. Every number is hypothetical. At 90 pumping days, the fuel saving halves and payback lengthens materially; a controller replacement or loan interest can move it again.
Do not add the entire crop revenue as a solar benefit if the petrol system could deliver the same water. If solar expands reliable irrigation beyond the previous service, count only the supported incremental contribution margin—extra revenue minus extra seed, fertilizer, labour, packaging, transport, losses and market risk. Keep that scenario separate from energy savings.
Run sensitivity before trusting the answer
- Utilization: low annual pumping makes capital recovery slower.
- Fuel price: test official current, lower and higher scenarios.
- Hydraulic performance: compare useful water, not runtime.
- Financing: include all cash payments and timing.
- Downtime: value only evidenced lost margin or replacement-pump hire.
- Asset life: test early controller/pump replacement and warranty response.
- Water constraint: cheaper pumping can encourage over-abstraction; authorized volume and source sustainability remain limits.
FAO’s solar-powered irrigation materials note both the opportunity to replace conventional energy and the need for economic, water-governance and environmental assessment. Removing a marginal fuel cost can increase pumping, so flow measurement and responsible water allocation belong in the investment plan.
Method and limitations
This article provides a total-cost and cash-flow structure, not financial advice or a current market quotation. The example is deliberately illustrative. It excludes tax treatment, inflation, exchange rates, grants, salvage value and farm-specific credit risk. Verify EPRA prices, supplier terms and cash flows on the decision date.
Frequently asked questions
What is a good payback period?
There is no universal threshold. Compare payback with asset risk, financing term, crop-cycle cash flow and alternative uses of capital. A short calculated payback based on exaggerated use is not good evidence.
Should labour savings be included?
Include only cash labour genuinely avoided or time with a defensible alternative value. Do not count unpaid family time as a cash saving and then also count it as additional profit without a clear method.