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Power supply to the city and the southern suburbs are handled largely by the Bombay Electric Supply and Transport (BEST). The western suburbs, north of Bandra, are served by the Bombay Suburban Electric Supply (BSES). The rest of the Bombay Metropolitan Region is serviced by the Maharashtra State Electricity Board (MSEB).
These two companies supply Bombay with its current  demand of about 2 GWatts of power. Of this, between 1 and 1.5 GWatts is obtained from the Tata Electric Companies (TEC). The rest comes from the Maharashtra State Electricity Board (MSEB). TEC purchases 300 to 400 MW per day from the MSEB. Statewide, there is a difference of 2 GWatts in demand between peak and off-peak hours.
The power supply system in Bombay is linked to the Maharashtra State grid, which in turn is part of the western grid-- connecting Maharashtra, Gujarat, Madhya Pradesh and Goa. The grid is fed mainly by generators belonging to the Central Government undertakings-- the National Thermal Power Corporation (NTPC) and the Nuclear Power Corporation (NPC). The MSEB currently  has a quota of 31% of the power generated for this grid.
Due to shortfalls in supply, the MSEB also plans to buy power from privately owned power projects. Most power failures to date have been due to failures in the system, rather than shortage. However, projections of future demands indicate a rapid rise in power usage. Over the last few years  there has been public debate about a larger role for privately owned power plants.
In case of a grid failure, power supply within the city limits of Bombay is ensured by an "islanding system". This means that if the MSEB supply to the city gets cut off, all the power generated by TEC gets automatically diverted to the city. This priority is regarded to be due to the need of ensuring power to the Mantralaya, the Central and Western Railways, and major hospitals.