I measure AC voltage and amperage at the wall socket, and just multiply them together to get watts. I understand that's not a perfect method for AC, but it's more than close enough for practical purposes.

For the MythTV box, I confess I haven't used that method recently. Instead, I'm relying upon the real-time wattage display on the UPS it is plugged into. Kinda fun to watch the displayed values change in response to system activity!

And the figure there includes some external devices (antenna pre-amps) that are also powered from the same UPS.

Cheers