The paper deals with photovoltaic (PV) systems able to generate reactive power and to curtail active power generation. Based on measurements performed on an existing PV system over 10 years its typical yearly operation profile is determined. It is used to evaluate the ability of the PV system to generate reactive power. After that the efficiency characteristic of a micro-inverter is determined experimentally. It is described as a function of generated active and reactive powers. This function is afterwards used in an optimization procedure, where measured time dependent load and power generation profiles are applied to determine optimal reactive power generation in PV systems installed in an existing low voltage network. Based on calculations performed over one year, the impact of reactive power generation on yearly production of electrical energy in PV systems is evaluated.