Most of my (nomal commute, non-spirited) driving I average about 300 Watt-hours per mile, maybe a tad over. Add in a little extra for non-locomotion energy usage (lights, climate control, vampire losses) and battery and charging inefficiency; call it 350 Watt-hours pulled from the wall per mile driven.
From there you just have to multiply by the number of miles you drive per month. 1000 miles x 350 Watt-hours per mile = 350,000 Watt-hours = 350 kWh per month. Adjust for your driving habits accordingly.