Give them feedback on how they are driving.

Washingtonpost article on how Prius owners are trying to outdo each other by driving more fuel efficiently.

Some feedback systems are easy to use, having the mpg meter on the console of the vehicle, which the driver is already monitoring works.

But having a small dial near the power outlet?  That may not be as useful.  Nor is trying to monitor your electric meter on an hourly basis.  While feedback is always useful to have, not everyone pays attention when they can.
This is not so much a technology problem, as it is a usability issue. If it is a part of what people already do, then it is fine.  But if you are requiring a new way of doing things for marginal benefit then it will probably fail.