My WiFi access point is remote from the cable modem and router. They have a traditional AC UPS (with large batteries that can power them for several hours). I wanted to also provide back-up power for the AP but an AC UPS was too large and felt inefficient. I found an old app-note from Maxim that made clever use of a reset supervisor IC to detect power failing and fashioned my own circuit with parts on-hand. The circuit is fairly efficient and only switches on the local DC-DC converter when power has failed. Otherwise it supplies power from the AP's original AC-DC power supply.