The amount of the electrical energy used for the interior lighting of medium and large buildings is generally considerable. The European Standard EN15193 was devised to establish conventions and procedures for the estimation of energy requirements of lighting in buildings by an energy performance numeric indicator. This methodology is based on the three derating factors that consider the influence of the daylight exploitation, the occupancy behavior and, if present, of a constant illuminance sensor. The factors are evaluated by a statistical approach on the basis of general reference data tabulated by the same Standard, not considering more detailed parameters of the control system that can impact severely in the effective energy savings. The Standard methodology appears extremely useful for a preliminary evaluation. For a more accurate evaluation, this paper suggests an improvement of the procedure that considers the effective operation time and occupancy behavior, the type of control and lamps, the number of control groups, the technique of modulation (dimming or switching), and the delay in turning off. The suggested procedure is compared with the Standard one to highlight the improvements.