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efficiency of solar panels

related country: Nigeria

We intend to install solar panels to power borehole water pump, and have been advised to enable switch to alternatives when panels produce less power eg rainy season and harmattan. What %age of optimum output is expected without direct sunlight? We are advised to enable use of mains electricity (often absent for weeks) and diesel generator as the alternatives.


have a look also at other posts on this platform - the efficiency is affected by the position etc. Solar panels will change in rain / under a full moon. You probably also want to consider the quality of the panels (manufacturer) and maintenance (cleaning etc).

Liz Palmer gravatar imageLiz Palmer ( 2020-01-28 15:05:09 )

8 Answers

lisa feldmann

Hello Helen,

check by the World Bank, it shows photovoltaic power potential data on different regional and national levels.

Maybe these resources from energypedia could also be of help:

Best, lisa


Can you get any data from local weather stations ? The PV output will vary approx the same as the solar irradiance - ie if that drops by 50% the output will drop by 50%. If the interuptions are likely to be short, you could provide a larger water tank (say 3 days supply or something) to compensate. But if the harmattan goes on for weeks (?) then you probably will need an alternative supply. There is quite a bit of data here which covers Africa and should show the reduction in the wet season, but I doubt if it includes things like the Harmattan.


Hello Helen,

This is not really my area of expertise but have you considered a wind generator as your back-up power source? I believe that the winds are fairly consistent during the Harmattan but the wind-borne dust and sand particles could be a problem by causing erosion of equipment. You could also consider battery back-up but if you have reduced sunlight for prolonged periods recharging the batteries would be as much a problem as powering your pump.




hello. In the wind pump area there are those really strong wind pumps made of steel plates. here you have a link just to show what I'm talking about there is also the panemona savonius which can be built easy amb they are stunning strong. Both can be built low tech

adolf gravatar imageadolf ( 2020-01-28 17:25:53 )

Hi Helen The European solar website noted above ( is a fantastic tool for a quick solar PV design. I used it loads of times in the past when I was an energy consultant ( now retired) If you pretend that your system is an off grid solar you can see how storage affects it. You can get hourly data for your site in Nigeria from the map. It will optimise solar panel angle and orientation for max efficiency Although I don't know about irrigation systems, in general I assume that water storage tanks are a lot cheaper than batteries, so you just need a big enough tank to cover your needs for the longest period where there is insufficient sun. Practical Action have designed loads of solar irrigation systems for Zimbabwe and have loads of practical detail somewhere on their website Good luck!


There are more ways to address this than just solar power.

How is it powered at the moment? A mechanical pump may need a manual back-up to cope with break-downs. This may reduce the level of service but can at least provide short term minimum supply.

The other thing is storage. Increasing storage will help smooth out supply and demand unbalances. If the downtime is going to be about a day, then have a day's extra storage. Having several smaller tanks will help manage the storage and allow for maintenance. Also have storage at home - that can also provide a good buffer

hope that helps


EWB USA Energy

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.

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2020-01-25 21:32:53
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