This paper presents a single-phase inverter with multilevel topology adopted to interface photovoltaic systems with the electrical power grid, and at the same time, to eliminate harmonics currents and to compensate reactive power, operating as shunt active power filter. Multilevel inverters have been attracting increasing attention in the past few years as power converters of choice in high voltage and high power applications. Various topologies to implement these inverters have been introduced and studied recently. The theory of instantaneous reactive power applied to compensation of single-phase circuits is also presented in this paper. This theory is a particular case of the well-known p-q theory, which was originally developed for three-phase circuits. Simulation results demonstrate this control algorithm applied to multilevel single-phase topologies.