That depends on so many variables!
- daily miles driven
- whether you also charge at work
- time of day you charge
- utility rate plan
- whether you have solar on your home
- other electrical demands insure your home
My utility offers an EV rate plan, but I had to sign up for it using my i3's VIN. Even though it gives me a better cost structure for charging, it hiked the rate I pay for appliances I run in the afternoons.
Previously I remained on a low overall tier, 24 hours a day, for most of the billing cycle. So the time of day we used our appliances didn't affect the bill, until late in the cycle when we might exceed the Tier 1 kWh threshold. At that point, costs would begin to be calculated at the Tier 2 rate.
But charging the i3 ate through Tier 1 quickly. I switched plans. Now with the peak pricing EV plan, my electric costs are lower before 3:00 pm, and they double after 3:00.
All in all, my electric bill has risen from $35 to $95. I drive 45 miles per day on average. I get over 300kWh from my employer each month 16A x 120v x 8 hrs/day x 20 days/month), which is about 700 "free" miles (4.5 miles per kWh driving efficiency).
My public charging costs at this point are basically zero.