 It consisted of a motor, connected in parallel with the mains, driving a generator, in series with the mains.The motor ran at the depleted mains voltage of 100 V and the generator added another 10 V to restore the voltage to 110 V. This was an inefficient system and was made obsolete by the development of alternating current mains, which allowed for high-voltage distribution and voltage regulation by transformers.Again in the days of direct current mains, power stations often had large lead-acid batteries for load balancing.