I don't have a solution for you at this time but we are working on some issues related to electric vehicles which we hope to have in place later this year.
The best, most comprehensive data can be found in the ecomodder fuel logs - there the site engine converts every entry into US, UK, metric and energy equivalent too, so you can even read Wh/mi for a gas vehicle (no Wh/km, though).
But plug-ins are too tricky for them too. You should enter both the consumed liquid fuel and electricity with them, and it seems they can't do it.