According to Article 6.6 in the latest “code for design of photovoltaic power station GB50797-2012”: Calculation of power generation :
1. The power generation forecast of photovoltaic power station shall be calculated and determined according to the solar energy resources of the station site and considering various factors such as photovoltaic power station system design, photovoltaic array layout and environmental conditions.
2. The annual average power generation EP of photovoltaic power station is calculated as follows:
Ep=HA×PAZ×K
Where:
HA — is the total annual solar radiation on the horizontal plane (kW˙h/m2）；
Ep — on-grid power generation (kW˙ h）；
PAZ — system installation capacity(kW);
K — comprehensive efficiency coefficient.
The comprehensive efficiency coefficient K is the correction coefficient after considering the influence of various factors, including:
1) PV module type correction factor;
2) The inclination and azimuth correction coefficient of photovoltaic array;
3) Availability of photovoltaic power generation system;
4) Light utilization;
5) Inverter efficiency;
6) Loss of collector line and step-up transformer;
7) Surface pollution correction coefficient of photovoltaic module;
8) Correction factor for conversion efficiency of photovoltaic modules.
This calculation method is the most comprehensive one, but for non-senior photovoltaic practitioners, the grasp of the comprehensive efficiency coefficient is a test. Generally speaking, the value of K2 is between 75% – 85%, depending on the situation.