Energy Costs of Driving Electric Cars

2009 Renault Twizy Z.E. concept 2009 Renault Twizy Z.E. concept[/caption]Electric cars consume less expensive energy. Driving costs are lower. However, electric cars are responsible for emitting more carbon dioxide and more pollutants when electricity is generated from coal. They have shorter driving ranges and need long times for recharging. They are also more expensive. A car driving 25 miles on a gallon of gasoline must pay $16 at the pump versus an electric car, which pays only $5.28 to the utility.

The media are full of news about electric cars. The $100,000 Tesla electric sports car is going to be sold soon, automakers are falling over each other to announce new electric vehicles, and entrepreneurs across the world are jockeying for venture capital.

Petroleum prices have jumped during the last year and everybody is feeling the pain at the pump. Not a single soul is concerned about filling up the batteries of his next electric car with electricity. After all, you just plug the car into the next receptacle and there seem to be no appreciable costs.

Joe owns a Corvette, a high performance car manufactured by General Motors, which according to the car manufacturer drives 25 miles per gallon.

Joe’s monthly commute is 1000 miles, he drives very carefully, and actually achieves 25 MPG on his daily trip going to work and running errands. At $4.00 per gallon at the pump he is paying $160 for gasoline every month.

Joe is thinking about buying the Tesla, an electric sports car. He tried to find the cost of electricity for driving this car. He could not find any data anywhere. Joe knows that he must explain to his wife why he needs to save energy and money before buying the Tesla, a new, very fast electric sports car.

The Tesla will accelerate faster than his Corvette. There is no doubt that an electric vehicle can have a faster acceleration than a gasoline car. Electric motors and liquid fuel engines are just two different devices converting electric energy or petroleum fuel energy into mechanical energy. Electric motors can generate much higher torque at the wheels at much lower turning speeds.

In comparison, electric motors will have several shortcomings, too. They certainly will emit more pollutants and more carbon dioxide as long as coal is used for producing electric power. Overall energy efficiency of the electric car, from power plant to the road, is still worse than that of a modern automobile propelled by an advanced combustion engine.

The biggest drawback of electric cars is the small number of miles they can drive after a complete recharge. Additionally, the charging of an empty electric battery will take forever, high performance batteries are expensive, and will only have a limited life expectancy.

What about energy costs for driving an electric car compared to a gasoline powered car? Both vehicles will have to store energy. The electric car stores electric energy in its battery, the combustion engine powered car stores energy in the form of gasoline or diesel fuel in its fuel tank.

Now let us take a comparative look at the cost of storing and paying for enough electric energy or liquid fuel energy to drive 100 miles. Let us assume that both cars will have the same power requirements to drive 100 miles. In this respect the match-up between Tesla and Corvette is perfect. But how do we compare prices at the gas station with utility costs at the receptacle?

Energy contained in gasoline can be converted into mechanical energy only at the limited energy conversion efficiency of a typical heat engine. The Corvette engine will have an energy conversion efficiency of about 35%. (Fuel fired engines may eventually reach 50% peak energy conversion efficiency after decades of future advancements). Conversion efficiency of electric power from the receptacle into stored energy first and into mechanical energy later is much higher at about 85%.

The energy content of gasoline is 131 MJ/gallon (megajoule per gallon). For driving 100 miles the Corvette will burn four gallons of gasoline or 524 MJ/100 miles. Only 35% of the energy in gasoline or 183 MJ will be used to propel the Corvette. This is the mechanical energy transmitted to the rear wheels of the Corvette. Virtually the same amount of energy must be transferred to the wheels of the electric car, the Tesla. Both cars are very similar in size and driving characteristics.

However, the Tesla has to take a little more energy from the receptacle because the charging, storing, and discharging of electricity in the battery experiences energy losses. These losses are about 15% of the electric energy taken from the receptacle and will not be available at the wheels. The Tesla owner will, therefore, pay 1.15 times as much to get the same energy to the wheels as the Corvette or 211 MJ per 100 miles. Electric energy is priced in the form of dollar per kilowatt hour or $/kWh. The average price of electricity in the US is $0.09/kWh. The factor for converting energy measured in MJ to energy measured in kWh is 0.2778 kWh/MJ.

To drive 100 miles, the Tesla will consume 58.6 kWh of electricity at a cost of $5.28. The Corvette will consume 4 gallons of gasoline at a cost of $16. Now we know that the Tesla will save about $10 per 100 miles or $10,000 over 100,000 miles.