Generally, owners of photovoltaic systems are most concerned about the final output of the photovoltaic system. Installers should provide them with an estimate, taking into account the capacity of the photovoltaic array, the amount of solar radiation received, and the loss of the system.

Case 1: Sydney, Australia

The capacity of the photovoltaic array is a string of 17 modules, and the rated power of the photovoltaic array is 3145Wp.

By consulting the Australian Bureau of Meteorology, the amount of solar radiation in the field can be calculated; the average daily solar peak hours in Sydney is 4.5. This number is multiplied by 365 to determine the annual radiation amount as 1643kwh/㎡/year.

By using the derating factors listed above, system losses must be considered. The photovoltaic array is installed on the roof using a bracket installation method, so it obeys the roof orientation and inclination angle, that is, the true north deviates by 20 and the inclination angle is 40°. The final system efficiency is determined using the table on the Australian Clean Energy Council website.

The total derating factor caused by the system loss can be obtained by multiplying all the derating factors, so it is 0.88845×0.9×1×1×0.98×0.95×0.95=0.71. For the conditions specified above, the system efficiency will be 71 %, the system loss is 29%.

Through this figure, the overall output power of the system can be calculated: 1643kwh/㎡/year x3.145kwx0.71=3669kWh/year, so the total output power of the system is 3669kwh/year.

Source of loss | Derating factor | describe |

temperature | 0.88845 | As calculated in the previous section |

Dust/pollution | 0.9 | Sydney is a coastal city, so the pollution loss is assumed to be about 10% |

Vendor tolerance | 1.0 | The data sheet is not provided, so it is not considered |

Occlude | 1.0 | Assuming that the array is not obstructed |

Orientation and inclination | 0.98 | See table |

Voltage drop | 0.95 | Assumed voltage drop 5% |

Inverter efficiency | 0.95 | The inverter efficiency given in the data sheet is 95% |

Case 2: German Garden Berlin

The photovoltaic system in Berlin is a string of 16 modules, so the rated power of the photovoltaic array is 2960wp. The output power of the system can be calculated using the PVGIS online tool, which requires several inputs. The program requires various losses of the system, and the orientation and inclination are optimized by the program, and the temperature is calculated by the PVGIS system. PVGIS will also calculate the power loss caused by angular reflection effects. Figure 2 shows the input of the PVGIS online tool.

The total derating factor taking into account the system loss is 0.95 × 1 × 1 × 0.95 × 0.95 = 0.8574, which means that the system efficiency is about 86% under specified conditions.

The percentage of total system loss can be reduced by 1 minus the derating factor and multiplied by 100: 1-0.8575 = 0.1426, so the total system loss is 14.26%.

Source of loss | Derating factor | describe |

temperature | 0.918 | Such as PVGIS calculation |

Angular reflection effect | 0.97 | Such as PVGIS calculation |

Dust/pollution | 0.95 | Berlin is an urban area, so the pollution loss is assumed to be about 5% |

Vendor tolerance | 1.0 | The data sheet is not provided, so it is not considered |

Occlude | 1.0 | Assuming that the array is not obstructed |

Voltage drop | 0.95 | Assumed voltage drop 5% |

Inverter efficiency | 0.95 | The inverter efficiency given in the data sheet is 95% |

Temperature loss (not included in the above system loss calculation) can be calculated by PVGIS based on the system location and photovoltaic module type. In this example, the PVGIS tool calculates a temperature loss of 8.2% based on Berlin’s temperature data and component type (in this example, crystalline silicon). The corresponding temperature derating factor is 0.918. The reason for the different calculated values is that two different temperature derating factor calculation methods are used. These methods are based on different assumptions, such as PVGIS using data from general crystalline silicon photovoltaic modules.

The PVGIS tool software shows that the solar radiation received by the module is about 1140kwh/㎡/year. By using this number, the total output power of the system is

1140k wh/㎡/year×2.960kWp×0.7635 =2576kWh/year

The system efficiency is 76%. The total system output power is 2576kWh per year, which is about 70% of the output power of the Sydney case system. The difference in solar radiation between the two sites is the main reason for the large difference in system output.