This work describes the analysis of factors which affect the results of estimation of the electron accelerating potential (kVp) applied to an X-ray tube, through determination of the end point of the energy spectrum of the emitted radiation beam. Measurements have been performed utilizing two spectrometers each with a silicon PIN photodiode: one operating at room temperature, and the other, a high resolution spectrometer, with a Peltier cooler. Both were directly irradiated by different X-ray beams. Both systems work at low voltage and without liquid nitrogen cooling, thus avoiding the drawbacks presented by germanium detectors. Each kVp value was determined by linear regression of the end of the spectrum, so as to give, simultaneously, the best fit to the experimental data and low standard deviation for the kVp value. Detector energy resolution and calibration, counting statistics and high voltage waveform ripple have been investigated in order to establish better experimental conditions and to optimize measurement time. Results of measurements carried out with X-ray tubes connected to single-phase, three-phase or constant potential units, using additional filtration of Cu, Al or Mo (for mammographic beams), are presented. The variations resulted in kVp uncertainties up to 0.1 kV.