My mpower is happily reporting voltage in .01 V intervals every 10 seconds or so.
I don't think i have an mpower active with nothing at all connected(I'm getting no results out of the one sitting at the office that I think has nothing on it), but i do have a few with devices connected.. worst case you can attach a regular boring desk lamp. Mine is reporting wattage, whether the port is on or off, the power factor, total kwH, voltage and current, for each port.
Power consumption is just voltage * amps, after all, so if it's coming up with a wattage used, it had to have voltage and amps at some point.
Granted, the mpower doesn't seem to do frequency.
You can also setup events that if X falls over or under Y, do Z.
