Though solar power systems are generally very reliable and virtually maintenance free (no moving parts), over time your panels may become damaged through incidental wear or part of your system might fail. In these circumstances, you might experience a loss of some or all of your power generation - and in extreme cases exposed wires can create an electrical arc, which can cause fires. Regular inspection should be carried out to ensure against these kinds of problems.
Remember: safety should always be your first priority. Don't get up on the roof unless you have the necessary training and safety gear, and never try to fix electrical systems unless you're trained and qualified to do so.
Know your energy output
The key to identifying a system failure early on is knowing how much power your system is capable of generating on a good day, and compensating slightly if the weather is poor. That way, if your system starts to drop in power gain, you'll know immediately if there’s a problem and that it’s time to have your system checked out.
The most obvious problem is if when a panel breaks from impact with an object like a cricket ball, a hailstone, or as the result of a bird showing off to his girlfriend by dropping something heavy on it. This may not be as costly to repair as it sounds, as there is a good chance it's only the facing glass which has been damaged. If you catch a broken panel before water can get in, there's every chance it'll be quite easy to replace and remount. If the cells underneath have been damaged though, it will depend on how extensive the damage is, and whether individual cells can be replaced or if the whole panel needs to be changed.
If the power appears to be dropping for no reason, some or all of your cells may not be responding. Covering up certain panels and/or individual cells to see if the power output changes will help diagnose whether it's localised to a single cell or the entire panel. If the power output drops, then the panel is active; if it doesn’t change, then the panel is not responding and will need closer inspection.
Most other common problems relate to wiring - any inspections on wiring work should be carried out by a licensed electrician. If you notice any power shorts or corroded wiring, shut off the system immediately and call to have it professionally repaired.
Who should do the replacement?
In nearly all cases, you should refer back to the original company you’ve purchased the equipment from to do a professional inspection and carry out any repairs. Your warranty will cover any faults related to installation or to the goods themselves, so it's worth getting as long a warranty as you can with a company you trust to be able to back it up.