also consider the module manufacturer’s warranted minimum power rating. After modules are placed into service, their power output will decrease over time. Besides initial photon degradation due to the physical process that generally occurs within the first few hours of a PV module’s operation, the long-term effects of weather and photon degradation influence module performance over its lifetime. One report estimates that initial degradation will be 0% to 3.9% of a crystalline PV module’s performance, while continuous degradation can reduce performance from 0.1% to 1.0% per annum (see Access). Reported degradation values will vary.
Most modules are warranted for minimum peak power output within two different time frames—90% of minimum peak power for 10 years, and 80% for 20 to 25 years. While you can’t do much to prevent module degradation, you can select wisely. Before you buy, compare the rated power tolerance for various modules. Most modules have a tolerance of ±5% (or better) of STC-rated power. For example, if a 100 W module has a specified power tolerance of ±5%, then the minimum peak power value for this module will be 95 W, and the module warranty will be based on this value rather than the STC rated power of 100 W. The tighter the tolerance, the more you can be assured that you’re getting the wattage that you paid for.
Lastly, because module degradation can result in lower voltage output, be careful when matching a PV array to a particular inverter’s input voltage range. Say a grid-tied inverter input window will accept eight to twelve modules in series. After many years in the field, the voltage of the PV array could degrade to the point that on a hot, sunny day, eight modules in series no longer stay within the inverter’s input voltage window. To avoid this problem, aim for the higher end of an inverter’s input voltage window when you’re determining the number of modules in series strings.
Module Mismatch. Manufacturing tolerances mean that modules of the same make and model will have slightly different current-voltage characteristics resulting in a decreased efficiency when the modules are connected together—you can figure in a loss of up to 2% because of mismatch. “Module mismatch,” as discussed here, is not referring to modules of differing make or models being wired together. This is a separate issue—and if mixing modules is done incorrectly it can result in much more significant power loss. If modules are wired in series, then all within the series string should be of the same model and with the same tilt and orientation.
Inverter Inefficiencies. The next consideration in the system loss lineup is inverters, which convert the PV array’s DC into AC for household use. Unlike modules, inverters should be installed out of direct sun. Too much heat is a deadly enemy of all electronics, and inverters are no exception. Installing in a high temperature environment can cause a unit to operate less efficiently and may lead to premature component failure. Even inverters that have weather protection and are