Island grids operate autonomously, i.e. they are not connected to the interconnected grid, and supply a spatially confined area. This can be an island, West Berlin at the time of partition, remote mountain huts or ships. The electricity is usually provided by a diesel generator, but photovoltaic systems, wind turbines or battery storage are also used in some cases. Emergency generators are installed in hospitals, data centers and some companies to take over the supply in the event of a power failure and ensure island operation. Since the gensets require a certain start-up time, UPSs (uninterruptible power supply / batteries) are installed to keep the voltage stable during this time.