A device or system that provides electrical power without interruption in the event that commercial power drops to an unacceptable voltage level. A UPS comprises circuitry and batteries that may provide power just long enough to shut down a computer or other system gracefully, without loss of data, or perhaps for many hours of normal operation in the event of a catastrophic commercial power failure. A typical UPS system operates in a hot standby, or offline, mode, continuously charging its batteries from a commercial power source and constantly prepared to assume responsibility within a few milliseconds for powering the client system. A more expensive online UPS actively filters commercial power, running it through the battery packs and an inverter, smoothing out the electrical waveforms and correcting for any power spikes and dips. See also inverter and waveform.