Certianly not certified accounting here, but in the first full of having the EV, I monitored my utility monthly billing cycle... here is what I learned:
I drove about 1,000 miles, and charged every night using a 240v charger. My electric bill increased ~$50 over the prior month when I did not have the vehicle.
The next month, I drove about the same number of miles and noticed only a $30. increase over the base month...
So, somewhere between $30-$50/month for me. Obviosly, there is other variability in my electrical use (although nothing major that I can think of, like use of A/C, etc.)... And in my 3rd full month I noticed an incerase over the base month of only ~$20, although my miles driven that month was only ~700.
I am on an Electric Vehicle rate plan, and set the EV to only charge at night (delayed departure for 6am)...
So at the high-end, it's about $0.05 (5 cents) per mile driven in electricity costs. Compare than to my previous gas vehicle, which was about $0.28 per mile in fuel cost alone, and it's a terrific deal!