From a Fuel Receipt to a Cost per Cubic Metre: A Half-Hour Calculation You Can Do Yourself

A farmer asking "is solar worth it?" cannot answer his own question until he knows what a cubic metre of lifted water costs him today. That figure appears on no invoice, but it can be extracted from papers every farm already has: fuel receipts and the running-hours book.

Start at the fuel end. Total the litres of diesel bought for the irrigation pump over a known period - a full month is better than a week - and multiply by the price actually paid. Add oil, filters and the routine maintenance attributable to that pump over the same period. The result is the operating cost for the period. Ignore the capital cost of the pump at this step: we are computing a cost to run, not depreciation.

Then work out the volume pumped. If you have a meter the figure is the difference between two readings. If you do not, estimate it: discharge rate multiplied by running hours. But be careful - the discharge rate on the pump's nameplate is measured at a specified head, and an older pump working against a higher head delivers considerably less. A nameplate estimate usually overstates volume, which makes the cost per cubic metre look lower than it is. This is one reason we insist on a meter: the estimate errs in the direction that comforts rather than the direction that helps.

Divide cost by volume. The result is EGP per cubic metre. Keep that number: it is the only unit of account that makes every subsequent decision comparable. Saving 2,800 cubic metres a season means nothing on its own; multiplied by your cost per cubic metre it becomes a sum whose size you know.

Now use it in both directions. Towards rationing: what does converting from flood to scheduled drip save? Volume saved multiplied by your cost. Towards energy: a solar pumping system costs close to nothing to run once installed, so the annual saving is approximately your current fuel and maintenance spend, and the payback period is the installed cost divided by that saving - computed on your farm's own figures rather than a national average.

Note that the order of those two decisions is not neutral. Ration first and the required volume falls, which shrinks the solar array you need and what it costs. Installing an array sized against flood consumption and then converting to drip means you bought panels you do not need. Measure, then ration, then electrify - in that order.

Finally, record the figure with a date and recompute it in six months. A rise in cost per cubic metre while the fuel price holds steady is not a coincidence: it usually means impeller wear or a falling water table. A number that began as an investment appraisal tool becomes an early maintenance indicator at no extra cost.