A method for simplifying the analytical treatment of single phase systems is introduced. This method, called monophasor theory, makes it possible to apply the same calculation methods and control strategies used for three-phase systems. This way, the spacial phasors of three-phase systems are translated into “monophasors” for single-phase system. The presented approach is a generalisation of already existing attempts of single-phase system descriptions. The Monophasor theory is presented, together with an application example of single phase voltage source inverter.