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I was in Ghana in Feb 2020 reviewing solar pumping sites. The dust from the Harmattan was very heavy this year and we saw measured solar intensity levels often below 500 watts per Meter squared and some times in mid-afternoon below 250 watts/M2. Panel output is almost linear with sunlight intensity, and the published standard test condition ratings for panels are based on 1000 watts a square meter of sunlight and the panels cells relatively cool at 25 C. As the cells get hotter voltage and power production drops off.

While some types of panels do slightly better than others with diffuse solar radiation, none of them are going to give anywhere near rated power under poor conditions like that. At night, yes, panels will show some voltage under full moon, but power output is so minimal, don't count on any.

The charge-discharge cycle on batteries gives losses of about 15-20% for lead acid batteries, so storing water is more cost efficient than storing power. The life of a water storage tank generally exceeds the life of lead acid batteries by many times. If you design and build a water tank that leaks 15% you are in the wrong job.

Rather than designing based on full rated panel power numbers, it is better to look at the NOCT (Normal Operating Cell Temperature) rating. This are measured with the cells hot in the sun but at less than full laboratory sunlight levels. These numbers are often about 75% of published power ratings. In addition, deploying excess solar power resources leads to longer pumping power window each day. The pump starts earlier and pumps longer as there is still enough power for the pump even at poorer sun angles. Common designs in east Africa are for panels about twice the pump power requirements. But even with that there are days when you will not get full pumping needs met, thus the need for water storage. Now with irrigation systems, there may well be less need for irrigation during seasons when sunlight resources are limited by clouds and rain, but for domestic applications there usually less of a demand drop.