I’m trying to think of a useful way to come up with a comparative costs for fueling an electric car versus a gas one. How much does it actually cost to “fill up” a Tesla battery, for instance?

If you can think of more useful means of stating what I’m trying to get at, please enlighten me and elaborate.

Gasoline has 33.41 kwh (kilowatt-hours) of energy in it (cite.) Obviously, your local price for electricity varies, but if you pay a dime per kwh, that’s $3.34 worth of energy.

HOWEVER!

There are massive and inherent losses in converting that chemical energy into kinetic energy (ie, moving your car), and internal combustion engines are pretty inefficient. Plus, if that electricity came from fossil fuels, the inefficiencies of energy transition are already taken into account. So, whereas those 33 kwh of energy may move your ICE car 30 miles, it would move a Tesla about 100 miles.

According to wikipedia, gasoline has an energy density of 34.2 MJ/liter, so a US gallon of gasoline has 129 MJ = 129*10^6 watt-second = 35,800 watt-hour (35.8 kilowatt-hour). US average electricity rate is about $0.12/kWh, so 35.8 kWh would cost $4.30.

Which may sound high until you remember that electric cars are far more energy-efficient. (Or you could say, the inefficient part of converting fossil fuel to usable energy happens at the power plant, and you are filling your electric car with a more pure, usable form of energy.) The Tesla Model-S has an efficiency of about 34 kWh / 100 miles, which means it goes about 105 miles with the equivalent energy of 1 gallon of gas.

A gallon of gasoline contains 36.6 KWh of energy. My local power company charges 12.8 cents per KWH. This leads us to a price of $4.68 per gallon equivalent. However, electric motors are about 3 times more energy efficient than gasoline powered ones. Dividing this price by three gives us $1.56.

According to this, a gallon of gas is reckoned by the EPA to be equivalent to 33.7kWh of electricity. And according to this, the average American price of electricity is $0.12/kWh, so naively, a gallon of gas is equivalent to about $4 worth of electricty.

FWIW, all the energy densities for gasoline posted vary slightly due to differences in gasoline formulation (as well as the definition of a gallon!) So feel free to adjust as appropriate.