As it says at the bottom of each Yahoo message:
EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
If you search for "custom" you will find Meter:Custom is described on page 24 (PDF pags 69) of the Input Output Reference.
I would try the "generic" fuel category and see if it will let you add a "watt" variable. But it may not, the variables, as I remember, need to be "summed" variables like all the other Meter variables.
Linda