Why would you "blow through" usage limits?
Your numbers are nuts.
If you drive an electric car a thousand miles a month (remember, it has a short leash - if you drive much more, you're depending on public chargers at your destinations to supplement your range), you're still only using 256 kWh, since the Leaf and the BMW both get about 3.9 miles to the kWh. With the minimal charging inefficiencies not withstanding, that means you've got $38.90 in extra power each month for a thousand miles of electric driving at the CA average of 15.2c/kWh. [Aside, with the national average cost of gasoline at about $3.60/gal -- more in CA -- you should be getting about about "100eMPG" in converting cents on the electric meter to cents on the pump.]
Your Volt looks like it has a 16.5kWh battery. It should cost you $2.50 a day (on average) to charge --- especially since anyone with a fish-tank timer on their charging plug can get 11 cents/kWh charging from midnight to 9am.
Of course you CAN pay more, but why? Your corner case with overages and daytime rates are atypical.
If your monthly power bill doesn't have room to some artificial cap for $1.29 a day in electricity, then you might want to consider unplugging your toaster.