The dimensioning factor for inverters

The dimensioning factor for inverters describes the ratio between the maximum direct current (DC) that can be generated by all installed photovoltaic modules and the maximum alternating current (AC) output power of the entire PV system. It forms the basis for calculating the size and number of inverters required to convert the solar power into grid-compliant alternating current that is suitable for connection to the public grid or for direct consumption in the company. The dimensioning factor is calculated from the quotient of the DC power of the inverter and the PV power. In the case of undersizing, the value is above 100 and in the case of oversizing below 100 %. A dimensioning factor of 100 % therefore means that, for example, with an installed PV output of 700 kWp, the DC inverter output is also 700 KW. However, such a constellation is not the optimum or most economical in all cases. Therefore, the dimensioning factor must be recalculated individually when planning each commercial photovoltaic system.  

Inverters and module strings

The planning of larger photovoltaic systems for companies is made more complex by the fact that only solar modules with the same actual output should be connected to an inverter. In such a string, all modules must therefore have the same orientation, inclination and shading so that the inverter can efficiently convert the direct current into alternating current. Therefore, if different Hall roofssolar carports, solar canopies or open fields with different orientations, inclinations and shading, a module string must be calculated each time and connected to an MPP tracker of the inverters or an MPP tracker of a multi-string inverter. In order to remain at a system size of 700 kWp, around 80 strings must be calculated when planning a simple roof-mounted PV system of this size if the system is to have an east-west orientation, as the number of modules per string cannot be arbitrarily large either: the most powerful inverters today can convert a maximum of 1,000 volts per string, which corresponds to an output of around 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

The calculation and dimensioning of the inverters therefore plays a decisive role in the economic planning of an entire photovoltaic system for a company. If they are undersized, the power peaks are only capped when ideal conditions prevail. This energy loss over the course of a PV system's entire year is usually marginal and must then be set against the higher investment costs for larger inverters. For a 700 kWp system, which could produce around 630,000 - 700,000 kWh per year depending on the location, if it were planned with a dimensioning factor of 100 %, the energy loss in our latitudes amounts to an average of 4,500 kWh with an undersizing of 120 %. This compares with tens of thousands of euros for inverters with a higher rated output. Depending on the intensity of solar radiation, dimensioning factors between 110 and 130 % are considered particularly economical in Europe. In Germany alone, this value can vary by +/- 5 % depending on the location.