The headline difference is simple: a solar pumping system has a higher capital cost and almost no running cost, while a grid pump is cheaper to install and increasingly expensive to run.
For a typical 5 kW system pumping 40 m³ a day, grid electricity and standby generator diesel commonly total KES 25,000–40,000 a month. Over ten years that dwarfs the incremental cost of the solar array.
Solar also removes outage risk. In rural counties where supply is intermittent, a solar array with adequate storage keeps tanks full without anyone touching a generator.
The one caveat is night-time and heavy overcast pumping. We size storage tanks to cover 1.5 days of demand, or fit a hybrid controller where continuous supply is critical.