UPower is a piece of middleware for power management on Linux systems. It enumerates power sources, maintains statistics and history data on them and notifies about status changes. It consists of a daemon (upowerd), an application programming interface and a set of command line tools. The daemon provides its functionality to applications over the system bus. PolicyKit restricts access to the UPower functionality for initiating hibernate mode or shutting down the operating system (freedesktop.upower.policy). The command-line client program upower can be used to query and monitor information about the power supply devices in the system. Graphical user interfaces to the functionality of UPower include the GNOME Power Manager and the Xfce Power Manager.