I have an APC 750 ES UPS with a special labeled plug on it that it uses to sense power. If power consumption drop (IE, the computer turns off or goes to sleep), it will cut power to a few of the other plugs. I found that when I had my main monitor on a plug that would lose power, the USB hub dropping off was enough to make the Mac wake back up from sleep. I believe my UPS shipped with this turned on by default, so it might be something to double check on yours. Overall I find it handy since it does kick in when the Mac is put to sleep or turned off completely.