Reliability of power sources is an increasing challenge in many sectors and battery-backed uninterruptable power supplies (UPS) are one option to protect and keep electronic equipment operating in the event of grid power failure. The three major UPS configurations are offline (also called standby and battery backup), line-interactive and online double conversion. While online systems are the most complex and costly, they provide waveform conditioning during normal mains supply and are becoming bidirectional to connect to smart grids. To deliver all of this, UPS systems have become more reliable and offer greater availability while reducing energy losses and costs with the move to transformerless and modular systems.