A main and an auxiliary energy supply device supply energy to a load normally and during disturbances of the main supply respectively while, for bridging brief disturbances in the load supply, for example naturally brief disturbances or during the short time when a disturbance has begun and the auxiliary supply has not yet come into operation, there is provided an energy store which can receive and store energy from the main supply and make this stored energy quickly available to the load. A control logic system controls the main and auxiliary supply devices and the storage means to ensure substantially constant energy supply to the load. The disturbances in question may comprise voltage rises or reductions and/or waveform distortions for example.