Two main things to check periodically are accumulation of leaves under the panels, and soiling on the front glass.
A buildup of leaves inhibits airflow under the panels, resulting in a higher operating temperature which reduces system efficiency. In the wet season, masses of leaves hold moisture against the roof which, over time, can compromise roof integrity.
Solar panel front glass gets dirty primarily from wind-blown dust and pollen. A light dusty film may only cause a 5 percent output decline, while heavier buildup can cause a 20 percent decline. A good rainstorm will usually wash away most of the accumulated soiling. Summer is a good time to clean your panels (if you need to at all).
The old saying “if it ain’t broke, don’t fix it” applies to most residential installation scenarios where cleaning just isn’t worth the bother – or the potential danger of accessing your roof. Unless dirt is clearly visible or performance is noticeably impacted; simply let nature do the job for you :).
Factors that may warrant cleaning solar panels:
Array tilt. Panels at 15 degrees or less are more prone to dirt accumulation.
Local environment. Bird droppings, heavy pollen, and soiling from nearby construction/agriculture.
Personal. You love your panels and want them operating and looking their best!
DIY NOTE: Do not attempt to access your rooftop to maintain your panels unless you have the appropriate safety equipment and training. If you don’t; hire a suitably qualified professional instead.
Cleaning tips: Early morning can be a particularly good time for cleaning as dew will likely have softened the dirt on the panels; meaning you’ll need to use less water and less energy to clean your solar panels. Clean water and a little scrubbing with a soft brush should remove the most stubborn grime. Rinse and squeegee dry to avoid mineral deposits. A widely available and effective tool is an adjustable pole fitted with a brush/squeegee head.