Energy storage system (ESS), as the name suggests, is a system used to store energy, which can refer to the conversion of electrical energy in a power system into some form of device that can store the energy so that it can be converted back to electrical energy when needed. Uninterruptible power supply (UPS) is an input power source that provides emergency power to the load when the main power supply of the electrical equipment fails, which is used to protect the equipment that has high requirements for the stability of the power supply. In the case of failure, it provides backup AC power for the electrical load equipment without interruption to maintain the normal operation of the electrical equipment. These failures include power outages, undervoltage, interference or surges.

ESS as a whole is different from the UPS system.

 Firstly, the use of the UPS system is an emergency plan that occurs when the main power supply fails. In addition to emergency, the power supply in ESS is used for peak load in many cases, and it is used for "peak cutting and valley filling" with other types of energy, such as nuclear power, wind power, solar energy, etc. Power can be transferred to reduce costs, or to help those discontinuous energy sources to be steadily exported. secondly, UPS, as an emergency system, is usually to maintain the stability of some key equipment, so that they have time to shut down normally or until the emergency power is turned on, so the duration is generally not very long, may only be ten minutes or even a few minutes, and then turn on the emergency power, the emergency power may come from the ESS system or other standby generators.Finally, the UPS system also has energy storage equipment, some UPS systems use flywheel system to store energy, some UPS systems use battery systems to store power, but also in peacetime to charge the energy storage system, when the main power system encounters a failure, the UPS starts to provide uninterrupted power supply.