A differential generating voltmeter was developed to operate with the electrostatic accelerator of the Universidade de Sa˜o Paulo. It has an energy spread of 0.2% for 1 MeV protons. The calibration reproducibility of the system is 0.15% over all the accelerator voltage range and for a time period of eight months. The linearity of the energy calibration curve is 0.1% over the whole voltage range of the accelerator.