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Inverter

The inverter’s main task is to converter the direct current generated by the PV generator into grid-ready alternating current. This conversion process from direct to alternating current entails a (minor) loss of energy. Therefore, the greater the efficiency of the inverter (current maximum is approximately 94.3%), the greater the yield. The inverter is also the information centre (display and storage of system data) and performs protective functions (protection against surges and immediate shut-down in the event of grid loss).
Inverter costs are around 7 to 13% of total system price depending on manufacturer, performance and functions (source: engesaar 2009, solarfuture 2009).

Figure 1: Fronius inverter

Source: fronius 2009

Optimal operating point

The PV generator and inverter must be fine-tuned to each other. In order to export the maximum amount of power to the grid, the inverter must operate at the optimal working point (maximum power point – MPP) of the PV generator. Modern inverters have an integrated MPP tracker.

Figure 2: Current-voltage curve with MPP of a solar cell

Source: BW_wirtschaftsministerium

Dimensioning

Because average and lower insolation is most common at Central and West European latitudes, the inverter is usually slightly undersized. For example, a 4.8 kW inverter is used for an installed capacity of 5 kWp. This is so that the inverter does not have to operate at partial-load low efficiency as often during the year (source: Woyte et al.).

Last page update: 25.01.2011