Solar Panel - Module Performance and Aging

Module Performance and Aging

Module performance is generally rated under standard test conditions (STC): irradiance of 1,000 W/m², solar spectrum of AM 1.5 and module temperature at 25°C.

Electrical characteristics include nominal power (P_MAX, measured in W), open circuit voltage (V_OC), short circuit current (I_SC, measured in amperes), maximum power voltage (V_MPP), maximum power current (I_MPP), peak power, W_p, and module efficiency (%).

Nominal voltage refers to the voltage of the battery that the module is best suited to charge; this is a leftover term from the days when solar panels were only used to charge batteries. The actual voltage output of the panel changes as lighting, temperature and load conditions change, so there is never one specific voltage at which the panel operates. Nominal voltage allows users, at a glance, to make sure the panel is compatible with a given system.

Open circuit voltage or V_OC is the maximum voltage that the panel can produce when not connected to an electrical circuit or system. V_OC can be measured with a meter directly on an illuminated panel's terminals or on its disconnected cable.

The peak power rating, W_p, is the maximum output according under standard test conditions (not the maximum possible output). Typical panels, which could measure approximately 1x2 meters or 2x4 feet, will be rated from as low as 75 Watts to as high as 350 Watts, depending on their efficiency. At the time of testing, the test panels are binned according to their test results, and a typical manufacturer might rate their panels in 5 Watt increments, and either rate them at +/- 3%, +/-5%, +3/-0% or +5/-0%.

Solar panels must withstand rain, hail, and cycles of heat and cold for many years. Many crystalline silicon module manufacturers offer a warranty that guarantees electrical production for 10 years at 90% of rated power output and 25 years at 80%. The output power of many panels slowly degrades at about 0.5%/year.