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If gasoline has 37 KWH/gallon and electricity costs 10c/KWH, then the equivalent cost of electricity is $3.70/gallon. The site you linked assumes the equivalence to be $1.00. It looks like you don't gain much by plugging them in to charge, in fact you lose, since the juice costs more than the equivalent gas.

What you're missing is the efficiency of the engine. A gas engine is VERY inefficient in comparison to an electric motor. You'll be lucky to get 25% of the energy in a gallon of gas turned into useful work, whereas you should easily get at least 67% of the energy from electricity. That means that for 10 kWh of useful work, you're paying for about 1.1 gallons of gas or 15 kWh or electricity. That's $2.63 in gas at today's price, or $1.50 in electricity.