If you are willing to part with $10,000 and some luggage space, eDrive of California will upgrade your Prius to a PHEV-35 (can go 35 miles in EV mode) and get 100mpg. Article does not say if power from the grid is factored in the 100mpg estimate, although it must be saving energy in the big picture. Also note that lithium batteries degrade over time
, regardless of use.
From one perspective, this could be viewed as a cheaper way to drive EV most of the time.