Two solar pumping systems can be built from identical panels on identical wells, yet one delivers a third more water than the other. The difference is rarely the array. It is usually three letters long: MPPT, Maximum Power Point Tracking.
A solar panel does not deliver a fixed voltage. Its output slides up and down constantly with sunlight, temperature and load, and for every condition there is exactly one voltage where the panel produces its maximum power. Run at that voltage and you harvest everything the sun offers. Run a few volts off it and a quarter of your array is silently doing nothing.
A very fast negotiator
An MPPT controller is effectively a very fast negotiator. Dozens of times per second it nudges the operating voltage up and down, measures the result, and settles wherever the product of volts and amps is greatest. A cloud passes, the sun climbs, a shadow crosses: the tracker finds the peak again and again through the day.
At noon on a clear day almost any controller does well, which is why cheap systems look fine in demonstrations. MPPT earns its keep at the margins: weak morning light still yields useful water instead of nothing, recovery after broken cloud is instant instead of lost minutes, and cold winter panels running at higher voltage are followed rather than wasted.
When you compare solar pumping packages, look past panel wattage. A smaller array behind a good MPPT controller will outperform a larger array behind a crude one. In pumping, efficiency is not an abstract spec on a datasheet. It is water in the tank, every single day.