Solar or PV panels will need cleaning at some point. Its common sense that the cleaner the surface the more efficent the panel will perform. Regarding frequency of cleans required will depend on the location. Some areas suffer from more airbourne dirt than others. I think you'll know when it requires a clean. Normally this would be every 6 months to 1 year. Rain water will leave deposits on the panel along with airbourne dirt and these bake fast onto the surface.
What method is best?? In my own experience I would say Pure Water as it has no chemicals and calcium and chlorine particles have been removed. Pure Water is an excellent cleaning agent and is used to clean windows by most professional window cleaners. I would contact your local professional window cleaner by using this link http://www.bwca.co.uk/directory2.php These window cleaners have been professionally trained and will use professional equipment such as the Reach and Wash System by Ionic Systems, probably the best known compnaies in it field.
Depending on the pitch of your roof and accessability the Reach and Wash System will be a perfect solution to your many years of environmentally friendly solutions.
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