Performance Ratio

The Performance Ratio (PR) is a measure of the efficiency with which a PV plant is operating. The calculated yield of the plant is compared to the annual POA normalized by the reference irradiance, \(G_{\text{ref}}\), 1000[W/m²].

$$Yield = \frac{\Delta t\left\lbrack h \right\rbrack\sum_{}^{}E_{grid,act}\lbrack W\rbrack}{total\ DC\ capacity\lbrack Wp\rbrack}$$

$$Capacity\ Factor = \frac{Yield\lbrack kWh/kWp\rbrack}{8760\lbrack h\rbrack}$$

$$PR = \frac{\text{Capacity\ Factor}}{\frac{\Delta t\sum_{}^{}G_{\text{poa}}\lbrack W/m^{2}\rbrack}{\left( 1000\lbrack W/m^{2}\rbrack*8760\lbrack h\rbrack \right)}} = \frac{Yield\lbrack kWh/kWp\rbrack}{\frac{\Delta t\sum_{}^{}{G_{\text{poa}}\lbrack W/m^{2}\rbrack}}{\left( 1000\lbrack W/m^{2}\rbrack \right)}}$$

Bifacial Performance Ratio

The Bifacial Performance Ratio is similar to PR and acts as a measure of efficiency reflecting the operation of PV plants with bifacial module technology. The concept was introduced in standard IEC 61724-1:2021, and integrates the bifacial irradiance factor (\(BIF\)) to correct the irradiance terms. \(BIF\) is a dimensionless factor that permits calculating the effective irradiance reaching a bifacial device from both the front and rear-side collectivelly. It is calculated using the maximum power bifaciality factor (\(\phi\)) of the PV module and the in-plane rear-side irradiance ratio (\(\rho_i\)).

$$BIF = 1 + \phi \cdot \rho_i$$

In SolarFarmer, if the PV plant is defined with several bifacial modules in the layout, the bifacial irradiance factor is estimated by means of a weighted average of the several bifaciality factors using the collector surface area for each module type as weights. Integrated in the PR definition, the bifacial PR formulation results in:

$$PR_{\text{bifacial}} = \frac{Yield\lbrack kWh/kWp\rbrack}{\frac{\Delta t\sum_{}^{}{G_{\text{poa}}\lbrack W/m^{2}\rbrack} BIF}{\left( 1000\lbrack W/m^{2}\rbrack \right)}}$$

Note that the bifacial PR will generally be lower than the PR value. The bifacial PR will not exceed 100% as it can occur with PR values where the rear-side contribution acts as a larger gain. For monofacial systems (bifaciality factor equal to zero), the PR and bifacial PR would be equal as illustrated by the formulation.