Energy Yield and Performance Ratio of Photovoltaic SystemsFor investors and operators alike, there are two fundamental questions:
Energy of incoming lightThe yearly sum of global irradiation, h, that hits the module is specific to the location and should be obtained from databases, measurements, or  in the first instance  from an irradiance map. It is measured in [kWh/m^{2}]. In Berlin, for instance, it is h ~ 1,200 kWh/m^{2}.Target YieldThe target yield is the theoretical annual energy production (on the DC side of the module), only taking into account eh energy of the incoming light and the module's nominal efficiency.Performance RatioThe Performance Ratio is the ratio between actual yield (i.e. annual production of electricity delivered at AC) and the target yield:The performance ratio, often called "Quality Factor", is independent from the irradiation and therefore useful to compare systems. It takes into account all preconversion losses, inverter losses, thermal losses and conduction losses. It is useful to measure the performance ratio throughout the operation of the system, as a deterioation could help pinpoint causes of yield losses. Energy Output per AreaThe energy, E, delivered by a system with area A can be estimated from:The preconversion efficiency reflects the losses incurred before the beam hits the actual semiconductor material, caused by shading, dirt, snow and reflection off the glass. The system efficiency reflects electrical losses caused by wiring, inverter and transformer.The module itself is defined by a nominal efficiency and relative efficiency as shown on previous page. Energy per Rated PowerSometimes, the energy yield is expressed in terms of the peak power of the module, which is independent from the area of the module. It is (with H_{0}= 1,000 W/m^{2}):This is a very useful ratio, since the energy yield E is a measure of the earnings potential while the peak power reflects the cost of the system. Note that the peak power in the above formula is the module's peak power, not the system's installed capacity, which is P_{sys} = P_{module}η_{sys}. 

Energy LossesSometimes it is more intuitive to think in terms of energy losses that occur at every step of the way rather than component efficiencies. Both concepts are the same, as losses = 1  efficiency, both expressed in percentage terms. Starting with the intensity of the incoming light (i.e. the energy that is actually available to the system), there are three major blocks of energy losses:




