Often, certain loads deemed critical by a customer are connected to the utility supply through an emergency power supply (also called backup or standby power supply) to provide continuity of service in the event of failure of the utility supply. This article describes the basic requirements and components of an emergency power supply. Since the purpose of an emergency power supply is essentially to enhance reliability of service to a load point, this article also shows how to assess service reliability and how to size the emergency power supply to provide a desired level of reliability.