Backup generator

An emergency power system is an independent source of electrical power that supports important electrical systems on loss of normal power supply. A standby power system may include a standby generator, batteries and other apparatus. Emergency power systems are installed to protect life and property from the consequences of loss of primary electric power supply.

They find uses in a wide variety of settings from homes to hospitals, scientific laboratories, data centers,telecommunication equipment and ships. Emergency power systems can rely on generators, deep cycle batteries, flywheel energy storage or hydrogen fuel cells.

Emergency power systems were used as early as World War II on naval ships. In combat, a ship may lose the function of its boilers, which power the steam turbines for the ship's generator. In such a case, one or more diesel engines are used to drive back-up generators. Early transfer switches relied on manual operation; two switches would be placed horizontally, in line and the "on" position facing each other. a rod is placed in between. In order to operate the switch one source must be turned off, the rod moved to the other side and the other source turned on.

Mains power can be lost due to downed lines, malfunctions at a sub-station, inclement weather, planned blackouts or in extreme cases a grid-wide failure. In modern buildings, most emergency power systems have been and are still based on generators. Usually, these generators are Diesel engine driven, although smaller buildings may use a gasoline engine driven generator and larger ones a gas turbine. However, lately, more use is being made of deep cycle batteries and other technologies such as flywheel energy storage or fuel cells. These latter systems do not produce polluting gases, thereby allowing the placement to be done within the building. Also, as a second advantage, they do not require a separate shed to be built for fuel storage.

...
Wikipedia