Watts = Volts * Amps

1440 Watts = 12 amps (for 120 volt charger) * 120V (normal 120 household voltage)

1.44 KWH = 1440/1000 (energy needed to charge the Volt for an hour using a 120V outlet)

18.72 KWH = 1.44 KWH * 13 hours (to charge the battery fully - assumes 100% efficiency)

$2.43 = 18.72 KWH * $0.13 (my cost for electricity for a KWH - not too far from the average cost)

So it costs $2.43 to go 53 miles or $ .047 per mile (Gas would be $2.60 per gallon here and cost $ .049 per mile - updated based on flyingsherpa comment)

The EPA estimates a little less mileage (perhaps based on cheaper average cost of a KWH):

50.5 miles/full charge = 2.7 miles/KWH (EPA estimate for G2 Volt) * 18.72 KWH for a full charge

What am I missing? There are claims by GM that the Volt running solely on electricity get 92 miles per gallon (close to double what I estimate). A non-plugin Prius get about 55 MPG. Is the plugin Volt really much of an improvement?