It is shown that the high-resolution electrostatic Mollenstedt energy analyser is capable of measuring high-voltage fluctuations. The sensitivity is shown to be better than 10^-6 at an accelerating voltage of 25 kV. The analyser can well be applied to ripple measurements. Since fluctuations in accelerating voltage affect the energy of the electrons, the high-voltage ripple may be evaluated from measurements of the energy distribution.