Modern solar electric systems have been shown to be very reliable. With no moving parts, the reliability hinges on warranties of key components.

Most solar photovoltaic modules have warranties of 25 years, at which time their output is warranted to be 80% of the day they were installed. Solar photovoltaic systems are given service lifetimes between 30-50 years. 

Modern inverters also have minimum warranties of 5 years with some manufacturers increasingly offering 10 year warranties. Realistically, there will be some solar modules that fail "out of the box", but these are spotted immediately by the installer. Some inverters have also failed quickly, but are also quickly replaced. As more systems get "real experience" on rooftops, then we'll get more definite information about solar reliability.

Comparing PV Panels

The power rating shown on the nameplate of a PV panel reflects performance under Standard Test Conditions (STC) of 1000 W/m² solar irradiance and 25^oC PV module temperature.

As part of the California Solar Initiative, many panels are also tested under  PVUSA Test Conditions (PTC).  PTC watt rating is based on 1000 Watt/m² solar irrandiance, 20 degree Celsius ambient temperature, and 1 meter/second wind speed. The PTC watt rating is lower than the STC.