Less expensive, off-line UPS are usually considered less sophisticated than other UPS products. During normal operation the power will flow straight through the unit, and if the input voltage fails or fluctuates outside pre-set tolerances the UPS will then feed battery power via an inverter. When the mains power returns, the inverter is turned off and the output is switched back to mains. Typically there will be a break of between 4 and 10 milliseconds during transfer to and from the battery mode.