An uninterruptible power supply, often referred to simply as a UPS, is a device that is able to supply power to electrical equipment during a power outage for a certain amount of time. UPSs are often used in data centres to ensure the operation of essential components, such as servers, routers or telephone systems to use. Additionally, UPSs are also available for individual computers. Depending on the type and size of the system, the maximum backup time needed varies. Large professional data centre systems are usually dimensioned so that the power supply is assured until petrol or diesel emergency generators have time to kick in. Some systems provide the equipment with power for the time it takes them to shut down. This may be implemented through a mechanism that automatically shuts down the computer as soon as the UPS comes into action. UPS units for individual PCs will provide power for longer, depending on their battery capacity.