Emergency electrical power is defined as an independent reserve source of electric energy which, upon failure or outage of the normal source, automatically provides reliable electric power within a specified time.

A reliable and adequate source of electric power is necessary for the operation of active industrial operations. Power must also be available at to provide water for fire protection, energy for automatic fire alarms, light for security purposes, heat for preservation of critical industrial equipment, and for other operations.

Power, supplied by either the local utility company or generated on-site. The source of distribution may be subject to brownout, interruption or extended outage. Safety, and health requirements may require an uninterruptible power supply (UPS) or standby/emergency supply for specific critical loads. Justifiable applications for emergency electrical power are:

1. Hospitals (life support, operating room, emergency lighting and communication, refrigeration, boiler plant, etc.).
2. A class B plant (considered a standby long term power source) is used where multiple commercial power feeders are not available or extended and frequent power outages may occur.
3. Airfields (control tower, communications, traffic control, engine start, security, etc.).
4. Data processing plant systems.
5. Critical machinery
6. Communication and security.