I've got an electric mower too (actually, it's a cordless -- replacing a corded one I had for 15 years). It costs about 1/3 as much to run as a gas mower. We have hydro electric here -- but, even coal fired generating plants are much more efficient than mower engines (line losses included). You can't run a car on coal -- every drop of gas that an electric mower saves is available for someone's car. Plus: the electric is much quieter; starts instantly every time; doesn't require storing fuel on-site; never, ever needs any maintenance (except for sharpening the blade).
"You can't run a car on coal"
Well, actually, you can, and it was done during fuel shortages in WWII:
http://www.lindsaybks.com/bks/producer/index.html Producer Gas for Motor Vehicles
by John D and Martin G Cash
reprinted by Lindsay Publications
It does appear to be a lot of trouble and hardware for the results, however.
The best suggestion in the book is to mount everything on a trailer and drag it along, so you can disconnect it when switching back to gasoline.