Solar PV systems have no moving parts and just sit there generating electricity as long as light is falling on the solar modules.
How do you know if they are working as efficiently as they were intended to? If the output from a solar PV installation is degraded for any one of a number of reasons you are unlikely to know unless you have a substantial incident like an inverter failure. It is difficult to quantify the reduction in output due to dirty modules. Bird droppings, dust and algae build up will all impact on the output of a system and its profitability.
On large systems where there can be six or eight strings to one inverter a faulty string can go in-noticed for a long time. There are numerous reasons why a single string might go down on a site which may have as many as 150 strings in a 1MW array. In our experience we think we have seen most of them with possibly the most bizarre being a family of field mice setting up home in a DC cable duct and then eating through the 6mm cable insulation just prior to their unfortunate and untimely incineration at 900V.
On most of our sites we are responsible for 24/7 monitoring of the site down to individual string level. If a fault develops we are automatically informed usually within 1 hour of it arising. From our office or anywhere we happen to be at the time we can then log into the site and drill down to the specific inverter or string and identify the problem. Very often the first thing our client knows is when we advise that we have been to site to correct an issue.