If you are starting to see a thin yellowy film / residue building up on the solar panel, it is a good idea to clean, as this will affect its sunlight collection. You can clean the solar panel with a normal cloth using soap and water. Be sure to clean off any soap residue with water once completed. If the staining is persistent, try white cleaning vinegar (do not use commercial or cooking vinegar; their high acetic acid concentration can damage the panel). You can also use a microfiber cloth to dry the solar panel to ensure fine particulates and fingerprints are removed.