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
Bei der wirtschaftlichen Planung einer gesamten Photovoltaikanlage für ein Unternehmen spielt die Berechnung und Dimensionierung der Wechselrichter dadurch eine entscheidende Rolle. Sind sie unterdimensioniert, lassen sich die Leistungsspitzen nur dann kappen, wenn ideale Bedingungen herrschen. Dieser Energieverlust über einen gesamten Jahresverlauf einer PV-Anlage ist in der Regel marginal und muss dann den höheren Investitionskosten für größere Wechselrichter entgegengesetzt werden. Bei einer 700 kWp-Anlage, die jährlich je nach Standort etwa 630.000 – 700.000 kWh produzieren könnte, wenn sie mit einem Dimensionierungsfaktor von 100 % geplant würde, beträgt der Energieverlust in unseren Breitengraden bei einer Unterdimensionierung von 120 % durchschnittlich 4.500 kWh. Demgegenüber stehen mehrere zehntausende Euro für Wechselrichter, die eine höhere Nennleistung aufweisen. Je nach Sonneneinstrahlungsintensität gelten in Europa Dimensionierungsfaktoren zwischen 110 und 130 % als besonders wirtschaftlich. Alleine in Deutschland kann dieser Wert um +/- 5 % je nach Standort variieren.
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