Warning: Betteridge's Law Applies
tl;dr: Compared to a hybrid like the Toyota Prius, the incremental emissions for a Tesla using typical US electricity sources are higher. They just don't occur at the car's tailpipe.
After recently asking whether a person with my habits could ever save money driving a Tesla instead of a Prius, I also wondered about per-mile emissions. Others have investigated the extra costs of EV manufacture and disposal (many elements of which apply to a hybrid as well), while my research is only about the incremental CO₂ emissions created by whatever energy inputs the vehicle needs.
Here's the spreadsheet itself, which includes links to sources for each number.
The top result is that the source of electricity makes a huge difference: China (major electricity source: coal) has 9x the emissions per kWh than France (major electricity source: nuclear). Canada has under 50% the emissions of the US, and I was happy to see that my local electrical utility is 10% under the US national average.
Using EPA fuel economy and the 2015 US mix, I calculate 47.7 lb CO₂/100 miles in the Tesla, and 40.0 in the Prius.
Of course, if you're comparing the Tesla to an "average" US car, it's a different story: The 2014 fleet economy for passenger cars is 31.5MPG, leading to emissions of 71.11 lb CO₂/100 miles.
If you're living in a country that has more nuclear or "renewable" energy, like France, you'll also come out ahead: with only 0.26 lb CO₂/kWh in 2008, a Tesla would put only 21% as much CO₂ into the atmosphere as a Prius running on gasoline.
I also took a stab at how Clinton's clean energy plan would change the numbers. My first guess is that it woud lower the lb CO₂/kWh by around 1/3, which still doesn't make us as clean as Canada's 2008 mix.
Finally, I checked how a Prius would do running on E85, assuming that carbon-neutral ethanol production were actually possible. This brings the prius down to 8.34 lb CO₂/100mi, within spitting distance of the cleanest electricity of countries I surveyed, 8.21 lb CO₂/100mi running a Tesla off the French grid.
Warning: Betteridge's Law Applies
I drive 8000 miles a year in a car with 45MPG actual fuel efficiency (2013 Prius). We paid somewhere around $23000 for it. If I drive this car for 15 years, I'll buy around 2700 gallons of gas.
Compare this to the (discontinued) Prius Plugin Hybrid with MSRP of about $30000. Imagine that I could have done all my driving in electric mode, and that its efficiency is ∞MPGe. I'd break even on the $7000 higher initial price if the average gas price is $2.56. Sounds plausible that I could save money that way, right?
But of course I couldn't go everywhere in "all-electric" / "charge depletion" mode, probably only about 2000 miles/year out of my 8000 miles/year would fall into this category (in-city driving 200 days a year at 10 miles/day). So now I'm only saving only about 670 gallons of gas *OVER THE COURSE OF OWNING THE CAR FOR 15 YEARS*. This is only break-even if gas is $10/gallon.
But of course, the plug-in hybrid is not ∞MPGe, it's 110MPGe. MPGe is actually "miles per 33.7kWh". 33.7kWh of electricity costs $3.17 in my local market at summer rates, or $1.88 at winter rates, an average of $2.31 (only 4 months are "summer"). So that's $623 in electricity to operate the thing in electric mode for 30,000 miles. This pushes the break-even point higher, to around $11/gal gasoline. (Note: a previous version of this page used incorrect math to arrive at a figure of $19 as the break-even point)
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