Large-power PV irrigation systems (PVIS) from 40 kWp to 1 MWp are increasingly being introduced onto the market since the main technical barriers have been removed and some real-scale demonstrators have shown cost savings up to 80% when compared with the electricity costs from the grid or diesel generators. The electricity compatibility between the PV generator and the frequency converter limits the number of PV modules in series to keep the PV generator voltage below the maximum input voltage of the frequency converter. This limitation can produce losses in terms of PV production and water pumped. As the maximum voltage delivered by the PV generator depends, among others, on the solar cell temperature, some electronic devices are being offered in the market to change the number of PV modules in series depending on this temperature, but they are complex and reduce the reliability of the system. This complexity is not justified because an in-depth analysis of the actual impact of these losses has never been carried out. This paper analyses and quantifies these PV energy losses for the two main applications in the market (pumping to a water pool at a variable frequency, and direct pumping to the irrigation network at constant power) which have different mechanisms of losses, and for locations with different typical meteorological years. The results show that, in the case of pumping to a water pool, the losses are irrelevant if the maximum yearly mean ambient temperature of the location is less than 21.5 °C and the voltage required by the pump is less than 568 V. In the case of constant power pumping, the losses are irrelevant if the mentioned temperature is less than 25.8 °C and the grid voltage is less than 587 V. In the rest of the cases, the losses are minimized just adding one more PV module in series. As a result, a general methodology to help designers selecting the number of PV modules in series in PV irrigation systems is presented.