Power input selection by priority is often needed in battery-powered portable instruments and handheld devices where the wall adapter or USB power sources have higher priority than the internal battery. In turn, the main battery has higher priority than a backup battery, where available. High-availability servers have a main and auxiliary supply rail, with the auxiliary rail not being used as long as the main power rail is operational. To ensure a graceful shutdown, memory-based systems employ a last-gasp capacitive power backup when the main supply goes down. Therefore, a hierarchy of priority exists, especially in systems with three or more input supplies. Such applications require a prioritized method for selecting the input supply.