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.