Electric Vehicle Efficiency Analysis
Last Updated November 17, 2009
There are two interesting things to consider when evaluating the efficiency of a vehicle: energy efficiency and emissions. An electric vehicle (EV) starts with a huge advantage over an internal combustion engine (ICE) vehicle: ICE vehicles generally run at about 20% efficiency, meaning that 80% of the energy content of their fuel is wasted, versus EVs which put about 80% of their input energy into turning the wheels.
EVs get their charge from an external power source, which could be from a clean, renewable energy source like wind or solar, but for most people will be our current electrical grid. As our grid gets cleaner so do EVs, but even when powered from today's electric grid, EVs reduce both greenhouse gas emissions and the pollution that causes smog.
The exact efficiency numbers depend on the efficiency of the vehicle, but we have personal experience with two highway-capable production electric vehicles which have surprisingly similar efficiency numbers.
According to the EPA energy efficiency sticker shipping with early production models of the Tesla Roadster, the Tesla Roadster is rated at 32 kilowatt hours (kWh) per 100 miles for city driving and 33 for highway. We have measured the actual efficiency we get driving a 2008 Tesla Roadster (produced in early 2009) and our 2002 Toyota RAV4-EV by measuring energy into each vehicle (in kilowatt-hours) using a dedicated electric meter on each charger and dividing by the number of miles driven in each vehicle, averaging over hundreds of miles of mixed city/highway driving. The Roadster comes in around 33 kWh per 100 miles and the RAV4-EV at 32 kWh per 100 miles. We will therefore pick 32.5 kWh per 100 miles as a reasonable estimate of real-world EV efficiency.
If you burn gasoline completely under perfect conditions, it generates energy in the form of heat equivalent to about 36.4 kWh per gallon. So an EV requires the energy equivalent of about 0.89 gallons of gas to go 100 miles, or about 112 miles per gallon equivalent.
EVs have no tail pipe and emit no polluting gasses when driven, but they do increase load on the grid, which in turn causes more emissions at polluting power plants. This is often called the long tailpipe. How much comes out of that tailpipe depends on how the power plant is fueled.
The most interesting question for US consumers considering an EV purchase is how much pollution is created by electric production in the US. A 2007 Department of Energy study found two interesting results.
In 2007, a DOE-sponsored analysis (PDF) found that there is enough excess off-peak capacity in today's electrical grid to power the conversion of "up to 84% of U.S. cars, pickup trucks, and sport utility vehicles (SUVs)" to plug-in hybrids with a 33-mile pure electric range. That's about 200 million vehicles. So, we don't need to build any new power plants to handle many years of EV production.
That same study found that under that scenario overall greenhouse emissions would decrease by up to 27%. Emissions of volatile organic gases and carbon monoxide would drop over 90%. Without improving generation technology, particulate and SOx emissions would increase, but we have a lot of time to solve that problem. It's a lot easier to solve a pollution problem at a few hundred power plants than it is with hundreds of millions of tailpipes.
For more detailed and specific numbers, this EPA study (PDF) gives values for the CO2 emissions from coal, natural gas, the overall average of the US electrical grid, and the average for various regional grids within the US. The following table assumes EV energy consumption 32.5 kWh per 100 miles (as explained above) to determine kilograms of CO2 per 100 miles, and compares that to emissions from gasoline combustion at 8.8 kg CO2 per gallon to determine an equivalent miles-per-gallon for an ICE vehicle to have the same emissions per mile.
All the Way to the Well
For a full well-to-wheel calculation for ICE vehicles, we'd need to consider the energy involved in refining and transporting the fuel to the gas station. Likewise for EVs, accounting for transporting fuel to the plants that power the grid and factoring in a slight loss in power transmission between the plant and the vehicle charger. Both of those calculations have many variables for which it is difficult to find concrete values. For that comparison, we suggest checking out Tesla's Well-to-Wheel Efficiency page.
With their greatly increased energy efficiency and their ability to be powered from green energy sources, EVs offer significant long-term environmental benefits. Replacing ICE vehicles with EVs powered from the current US electric grid has immediate considerable upside on average, and huge benefits in regions with electric grids that take advantage of green energy sources (like the US west coast). As our grid moves toward greener energy (wind, solar, geothermal, even nuclear), EVs get progressively cleaner while ICE vehicles just get dirtier with age.
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