Abstract: Normal line voltage, such as 115 volts AC available on a standard home or office receptacle (or single or three phase 230 volts AC), is surge protected and filtered prior to monitoring for voltage deviations outside of a voltage "window" such as 105-125 volts AC. Deviations outside of this voltage "window", are sensed and a deviation output signal is generated. The filtered AC supply and an auxiliary AC supply are selectively connected to the user equipment through a power distribution panel, with the selection being determined by the deviation output signal. By the method and apparatus of the invention, the computer equipment is supplied with primary input power from the utility company or the like as long as the voltage remains within this adjustable voltage window of 105-125 volts AC.