Worldwide, photovoltaic installations are making an increasing contribution to electric energy generation. These are power-unstable sources due to the rapid and frequent change in insolation. As a result, a common problem noted in low-voltage power grids is that the permitted voltage values at the source connection point are exceeded. There are several methods of limiting the voltage values present at the inverter. One of them is the generation of reactive power in a photovoltaic installation. In the literature, one can find many relationships that allow one to determine the increase in voltage caused by the change in reactive power, where the imaginary part of the voltage loss is omitted as insignificant. The authors’ research has shown that this can lead to significant errors. Omitting the imaginary value causes the determined values to be even more than 4.5 times smaller—these differences increase with the length of the line. The analyses carried out by the authors show that the determination of voltage increments with and without taking into account the imaginary part of the voltage loss in the calculations differs from the values determined via computer simulation (failure to take into account the imaginary part results in calculated values of voltage increase being lower than the values determined via a computer by about 40% on average).