A diesel pump and a solar pumping system can deliver the same water to the same tank and differ in lifetime cost by a factor of three. The comparison is rarely made honestly, because the diesel pump hides its price inside a fuel bill that never ends.
Year zero: the sticker
Diesel wins the first invoice, clearly. A decent diesel set costs a fraction of a solar array, controller and pump combined. That is why diesel is everywhere, and why the comparison must not stop at the purchase order.
The running meter: fuel
A small pump burning 1 litre per hour, six hours a day, consumes over 2,000 litres a year before haulage to site. In remote areas, delivered diesel commonly costs a third more than the pump price suggests. Over ten years, the fuel alone typically exceeds the entire solar system's cost, several times over.
The hidden meter: maintenance
The diesel needs oil, filters, injectors, belts, and an engine overhaul inside the decade, plus operator time every single day it runs. The solar system needs periodic panel cleaning and a pump service. That is nearly the entire maintenance schedule.
A worked example
Pump 20 m³ a day from 40 metres, six days a week. The diesel machine is modestly priced, then consumes roughly 2,000 litres a year plus servicing: a running bill that outgrows the solar system's price before year four. The solar alternative costs more on day one for the array, controller and submersible, then runs on free sunlight for twenty years or more.
When diesel still makes sense
To be fair, seasonal peak demand, emergency backup, or a site already drowning in cheap fuel can justify an engine. But for every-day pumping in sun-rich country the arithmetic no longer argues. The cheapest diesel litre is the one you never buy.