Operating points and characteristics

The power output of the PV module can be determined from the irradiation on the inclined surface of the PV generator and the calculated module temperature. For each irradiation and temperature, there are any number of operating points at which the PV module can be operated. These operating points describe the electrical characteristic of the module.

Figure 1 shows the characteristics of a module with 280 W nominal power for different irradiation values and a module temperature of 25 °C. In principle, the module can be operated at any point on the characteristic curves shown. The power output depends on the product of the voltage and the resulting current.

Example: The module is irradiated with 1000 W/m², the module temperature is 25 °C. This results in the upper characteristic curve from figure 1. If the module is operated at 44 V, it supplies a current of 0 A, the module power is 44 V $\cdot$ 0 A = 0 W. The module is therefore in idle mode. If the module is operated at a voltage of 0 V (i.e. in short circuit), it can supply more than 8 A, but the module also produces no power, since 0 V $\cdot$ 8.2 A = 0 W.

Figure 1: Characteristic curves of a 280 W module with different irradiations.

For any other point on the characteristic between 0 V (short circuit) and 44 V (no-load), however, the resulting power is greater than 0 W. If each of these powers is applied above the selected voltage, the characteristic curves are obtained from figure 2.

Here it becomes clear that the power output of the module initially increases as the voltage of the module is increased. At about 36 V the power then reaches a peak of about 280 W, after that the power drops again. At 44 V the power returns to 0 W.

Figure 2: Power curves for a 280 W module at different irradiations.

The point at which the module delivers the highest power is called MPP (maximum power point).

For the operation of the PV system it is therefore important to operate the modules at maximum output, in the MPP, in order to maximise the yield. The task of finding this MPP and operating the modules at this point is performed by MPP trackers installed in inverters.