Evaluation of PV system performance

In order to do so, performance indices as defined and explained in the international standard IEC 61724 can be used. Those indices were implemented so that the performance of  “installations of different sizes, operating in different climates, and providing energy for different uses” could be easily compared (IEC, 1998). They can thus be used to clearly determine advantages of a particular design or specific operating conditions.

Daily definitions of the normalized performance indices (yields, losses, efficiencies) given in the following sections are taken from IEC 61724; identical symbols have been used in order to ease the understanding of the different quantities (IEC, 1998)

Daily mean yields are the quotient of energy quantities over installed array’s rated output power P₀ and indicate the amount of time during which the array would be required to operate at P₀ to provide a particular monitored energy quantity (i.e. actual array operation relative to its rated capacity).

The array yield Y_A is the daily array energy output per kW of installed solar module panels array:

Y_A represents the number of hours per day that the array would need to operate at its rated power P₀ to contribute to the same daily array energy to the system as was monitored.

The final PV system yield Y_f is the share of the daily net energy output of the entire PV system, which was supplied by the array per kW of installed PV array.

Y_f represents the number of hours per day that the array would need to operate at its rated output power to equal its monitored contribution to the net daily load.

In standalone PV systems, the balance of system (BOS) efficiency corresponds to the efficiency with which the energy from all sources (net energy E_in,t from array and storage over the reporting period) is transmitted to the load (net energy E_use,tto the load and storage over the reporting period).

The reference yield Y_r is calculated by dividing the total daily in-plane irradiation by the module’s reference in-plane irradiance G_{I, ref} (kW.m^-2).

Y_r represents the number of hours per day during which the solar radiation would need to be at reference irradiance in order to contribute to the same incident energy as was monitored. If G_{I, ref}=1kW.m^-2, then Y_r corresponds to the number of peak sun-hours per day.