Shading - vegetation can grow and compromise generation
Debris – twigs, leaves, lichen, moss, bird mess
Environmental conditions such as sea spray, pollution or agriculture can all apply a dirty film to panels
Connections & cables – connectors can suffer from water ingress and cables in some sites are vulnerable to damage by vermin or birds.
Component failure – Solar panels very rarely fail (almost never) however inverters, like all white goods, have a finite working life. Inverter failure is the single most common and impactful issue a solar array can experience.