Unfortunately very hard to tell because our electric company replaced our meters the month before I bought my Volt and everybody's electric bills went up by 20-30 dollars a month. My bill has gone up about 40 dollars a month since the meter change. However, I estimated it's about $45 a month if I do all my charging at home. But since I do half my charging at work for free the actual impact on my electric bill has been no more than $25 a month. I estimated the $45 impact by monitoring my KWh usage in the Volt, adding 10% for charging losses, and then multiplying by the 12.31 cents per KWh. That said, when I included maintenance and tire wear I was paying over $120 per month to commute in my Cruze. In my Volt it's slightly less than $40 per month, which includes the $900 I just spent on tires. My commute hasn't changed.
I haven't stopped for gas since Sep 30, driving 4,700 miles and burning a little less than 6.5 gallons of gas.
The other day I pulled up my vehicle operating costs sheet for the Cruze and compared them to the Volt. In the first 35K miles the Volt has been $2,400 cheaper to operate.