Operating power converters. At least some of the example embodiments are methods including: storing energy in a field of a transformer arranged for flyback operation, the storing by making conductive a primary switch coupled to a primary winding of the transformer; ceasing the storing of energy when a primary current through the primary winding reaches a predetermined value; measuring on time of the primary switch, the measuring creates a value indicative of on time; transferring energy from the field of the transformer; measuring discharge time of the energy from the field of the transformer during the transferring, the measuring of the discharge time creates a value indicative of discharge time; calculating a value indicative of input voltage of the power converter using the value indicative of on time and the value indicative of discharge time; and then compensating the predetermined value used in a subsequent storing energy step.