The inverter sizing factor describes the ratio between the maximum direct current (DC) output of all installed photovoltaic modules and the maximum alternating current (AC) output of the entire PV system. It serves as the basis for determining the size and number of inverters required. These inverters convert the solar power into grid-compliant alternating current (AC), which is suitable for connection to the public power grid or for direct consumption within the company. The sizing factor is calculated as the ratio of the inverter’s DC power to the PV power. If the system is undersized, the value is above 100; if it is oversized, the value is below 100. A sizing factor of 100 % therefore means that, for example, with an installed PV capacity of 700 kWp, the DC inverter capacity is also 700 kW. However, such a configuration is not always the optimal or most economical one. Therefore, the sizing factor must be recalculated individually for each commercial photovoltaic system during the planning phase.
Inverters and module strings
The planning of larger photovoltaic systems for businesses becomes complex because only solar modules with the same actual power output should be connected to an inverter. For such a string, all modules must therefore have the same orientation, inclination, and shading so that the inverter can efficiently convert direct current into alternating current.
80 different strings for dimensioning at 700 kWp
However, one usually plans different Hall roofs, Inverters, solar carports, solar canopies, or open-space fields with different orientations, tilts, and shading. The calculation takes place again every time a module string with a new MPP tracker of the inverter or an MPP tracker of a multi-string inverter is completely planned. To remain with a system size of 700 kWp, the planner already calculates about 80 strings for a simple roof PV system of this dimension if the system is to have an east-west orientation, as the number of modules per string also cannot be arbitrarily large: The most powerful inverters today can convert a maximum of 1,000 volts per string, which corresponds to a power of about 20 modules.
Most of the time, inverters are in partial load operation
With a dimensioning factor for inverters of 100 %, it is assumed that under ideal conditions, i.e. with optimum solar radiation without clouds and at temperatures below 25°, each string with its inverter delivers the best possible electricity yield. However, such conditions rarely exist and most of the time an inverter is operated at partial load. And this is precisely where the problem lies: all inverters no longer work as efficiently under partial load. This effect sets in from a 20 percent power drop in the string and an 80 percent partial load operation of the inverter. Apart from the higher investment costs of large inverters, undersizing is therefore also economical.
A dimensioning factor for inverters greater than 100 % is economical
When planning the economics of an entire photovoltaic system for a business, the calculation and sizing of the inverters play a crucial role. If they are undersized, power peaks can only be curtailed under ideal conditions. This energy loss over the course of a full year for a PV system is generally marginal and must be weighed against the higher investment costs for larger inverters. For a 700 kWp system that, depending on the location, could produce approximately 630,000 – 700,000 kWh annually—if it were designed with a sizing factor of 100 %—the energy loss in our latitudes, with an undersizing of 120 %, averages 4,500 kWh. In contrast, inverters with a higher rated output cost several tens of thousands of euros. Depending on solar irradiance, sizing factors between 110 and 130 % are considered particularly cost-effective in Europe. In Germany alone, this value can vary by +/- 5 % depending on the location.