Abstract Emulation of the rotational inertia of a synchronous generator (SG), in the form of virtual inertia, is possible through the virtual synchronous generator (VSG). This is a power-electronics based device, whose purpose is to damp the frequency oscillations due to disturbances in the power balance of a power system, similarly to the rotational inertia of a SG. This paper identifies Virtual Inertia as a property of the VSG and provides mathematical expressions that relate it to the rotational inertia of an SG using classical mechanics and machine theory. The ideal VSG is introduced and used as reference for comparison with the SG analogue. Two different algorithms that emulate the rotational inertia of a SG are tested and adjusted accordingly to match the performance of the ideal VSG. The comparison shows that slow frequency-tracking limits the performance of both algorithms.