In this document
We will explain in detail the calculation of the PV system losses in Solargis Evaluate
Solargis Evaluate offers a comprehensive calculation of PV system losses in summarized yearly and monthly figures. How to read the PV system losses page in Solargis Evaluate is described in a different article. In this article we will explain in detail how all of the presented figures are calculated.
Yearly losses
Yearly losses offer an easy-to-read summary of performance of the PV system and its inefficiencies. The yearly figures are a good starting point for system performance investigation and optimization. You will be able to identify the largest sources of inefficiency within the system from the yearly summary.
Losses table
The losses table is a detailed breakdown of PV system losses as calculated by Solargis Evaluate. The table for a sample PV system is shown below. Based on this table we will explain the calculation of the figures within.
Note: All values shown in the table are rounded, so sums may not add up precisely. This is expected, as sums are calculated internally with higher accuracy.
Energy output / PVOUT specific [kWh/m²]: Value calculated by the Solargis Evaluate PV Simulator.
Energy loss [kWh/m²]: The energy output in this step minus the energy output.
Energy loss partial [%]: The proportion of the energy output in this step to the energy output.
Energy loss relative to previous [%]: The proportion of the energy loss in this step to the energy output.
Energy loss relative to input [%]: The proportion of total energy loss is the difference between the energy output at the current step and that at the first step of the section, relative to the energy output of the first step.
Performance ratio cumulative [%]:
Optical losses: Proportion of the energy output in this step to the first step in the section.
Electrical losses: Proportion of the PVOUT specific in this step to the total GTI value calculated as noted.
Note: The fact that the Global tilted irradiation on Front side and GTI effective are both equal to 1,470 kWh/m2 is coincidental. Such coincidences can naturally happen between other values, too.
Relative loss baseline
Losses are calculated in three categories: optical, electrical, and unavailability losses. They are evaluated separately because they differ in units (optical vs. electrical) and methodology: optical and electrical losses are calculated within the simulation, while unavailability losses are calculated post-simulation. As a result, the column “Energy loss relative to input” is referenced to a different baseline in each section:
Optical losses: Global tilted irradiation (front side and rear side separately)
Electrical losses: Conversion of irradiation to DC in the modules
Unavailability losses: relative to the previous step (i.e. no common input value)
Total GTI calculation
For the purposes of the Performance Ratio calculation in the Electrical losses section, the value of total GTI including rear side efficiency derating is used as the reference value. It is calculated as:
Where
GTIfront, horizon is the GTI on the front side of the module with horizon shading included.
GTIrear, horizon is the GTI on the rear side of the module with horizon shading included.
cb is the bifaciality coefficient, specific to the PV module used in the system design. The coefficient expresses the efficiency of the rear side of the module in converting incoming irradiation into electric current relative to the same efficiency of the front side of the module. The value of this coefficient is typically 0.8 (80%).
This input GTI reflects the real-world conditions, where the horizon-shaded irradiation represents the ideal system input, while accounting for the decreased efficiency of the rear side of the PV module.
Capacity factor
The capacity factor is calculated as the ratio of the “Total system performance considering technical availability and losses due to snow” to the theoretical maximum annual production, assuming each installed kWp produces 1 kW continuously, 24 hours a day, year-round. It is therefore obtained by dividing the total system performance (including availability and snow losses) by 8,760 kWh/kWp.
Loss diagram
The energy losses are summarized in a visual and easy-to-understand way in the Sankey loss diagram shown below for the same sample PV system. The calculation follows the values in column 4 of the loss table. The optical losses (grey) for bifacial systems are calculated with respect to the sum of the front and rear side GTI, without consideration of the bifaciality factor.
Note: The Shading losses (near) in this diagram are high, this is caused by the summing of optical losses for the front and rear side of the PV module. Rear side near shading losses can easily reach 50% of the rear GTI (46.5% in this case). This is why this diagram should always be read in conjunction with the energy conversion and related losses table.
Monthly losses
Seasonal variations in environmental conditions throughout the year, primarily changes in the sun's path and ambient temperature, affect the performance of a PV power plant.
A monthly breakdown of losses enables the analysis of seasonal patterns, supports the identification of the underlying causes of underperformance, and helps optimize system design. Additionally, cumulative graphs of monthly losses, aggregated over all simulation years, are provided as part of the monthly losses analysis (illustrated below). These graphs allow for the assessment of both seasonality and interannual variability in losses, supporting the establishment of realistic performance expectations for a PV plant under varying year-to-year conditions.
The calculation of monthly losses follows the same rules as the yearly losses explained above. The "Breakdown of energy losses" and "Seasonality and interannual variability of monthly PV losses" graphs show losses relative to the input GTI, corresponding to the 5th column in the yearly losses table.
In the Energy conversion and related losses table, you can select the input relative to which the figures are calculated. The three options correspond to the following columns in the yearly losses table:
Losses relative to GTI without shading (%): Used to compare the magnitude of different losses.
Losses relative to the previous step (%): A standard way of presenting PV system losses; this allows for the simple calculation of total loss as a sum of losses in individual conversion steps.
Losses relative to the previous step ($kWh/m^2$ or $kWh/kWp$): Useful for understanding the total energy lost in each conversion step.