The present invention relates to a method for driving an emergency generator using an energy storage system in which, when the power of an emergency generator is applied, an energy storage system is allowed to handle the load demand, thereby driving the emergency generator at maximum efficiency. For this purpose, the method for driving an emergency generator using an energy storage system (ESS), including, after an emergency generator has been applied and until the emergency generator reaches a preset maximum efficiency driving state, handling, by an ESS, load demand using previously stored power, and, when the emergency generator reaches the preset maximum efficiency driving state, maintaining the driving of the emergency generator in the maximum efficiency driving state.