We have a detailed blog post on solar panel cleaning here.
But the short answer is: You will probably never have to clean them. They should self clean in the rain.
The exception to this is:
1. If your panels are flat.
2. You live in a very dry area.
3. You have a really stubbornly soiled panel due to moss, dead possums, particularly sticky bird poo etc.
If your solar electricity production does drop - then there may be something on the panels that requires cleaning. The best way to find out is a visual inspection.
If you don’t feel confident on the roof and poking around near electrical wiring, get a professional; solar systems produce deadly voltages. Many window cleaners are now trained to clean solar panels, and solar panel cleaning companies are springing up all the time. I pay $4 per panel to have mine cleaned.
Cleaning solar panels yourself can be a little tricky – they are up on your roof, after all! This means that proper safety precautions must be taken (including, but not limited to, a safety harness) if you want to get up onto your roof to clean the panels yourself.
If you DO want to be a cleaning hero and get up there yourself, all you really need is warm water and dishwashing soap to remove any dirt or grime – clean the panels like you’d clean your car.
In terms of cleaning frequency, you should only need to clean your panels annually (unless flocks of birds happen to see your panels as a particularly attractive bull’s-eye!)