2013 Tesla Model S in Queens, NY, service center, awaiting delivery to buyer David Noland, Feb 2013Enlarge Photo
Tesla Motors has an interactive calculator on its website that allows you to calculate the effective carbon emissions of your Model S, depending on your particular state's powerplant mix (coal, gas, nuclear, hydro, etc.). The numbers range from 26 gm/mi in Idaho (mostly hydro) to 310 gm/mi in West Virginia (mostly coal).
According to Weiss, the national average for Tesla's claimed Model S CO2 emissions works out to 163 grams per mile (g/mi). Tesla says the corresponding figure for gas cars is 400 g/mi.
Although not truly zero-emission, electric cars in general (and the Model S in particular) are still better than most gas cars. Or so goes the mainstream scientific thinking.
Weiss begs to differ.
*Tesla's numbers are too optimistic
According to the Tesla website, it assumes a Model S electricity usage of 283 Watt-hours per mile for its CO2 calculations. That's the power required to drive at a steady 55 mph.
Weiss disputes that number as unrealistically low. He cites, among other sources, the EPA's number of 321 Wh/mi, as well as 48 reports on the Tesla owners' forum that averaged to 367 Wh/mi.
He concludes that the real-world power consumption of the 85-kWh Model S is actually more like 375 Wh/mi. That's 33 percent higher than Tesla claims.
Accordingly, CO2 emissions would also be 33 percent higher.
I can't argue with Weiss on this one. In 3,000 miles of driving my 60-kWh Model S, I've averaged 343 Wh/mi. Since my 60-kWh car is about seven percent more efficient than the heavier 85-kWh model, that would correspond to a real-world consumption of 367 Wh/mi for the longer-range car.
Because my driving--as well as that of the 48 Tesla owners Weiss cites--has occurred mostly in winter, I would expect average energy usage to decline as the weather warms. (I've already seen my efficiency improve in May.) I'd guesstimate a real-world year-round number for the 85-kWh Model S of 340 Wh/mi.
But I won't quibble with Weiss's figure of 375.
So a 33-percent bump raises Tesla's claimed Model S effective carbon emissions of 163 gm/mi to 216 gm/mi, or about the same as the Toyota Prius V.
*Charging losses boost carbon emissions by 18 percent
Not every kilowatt-hour of energy that comes out of the wall plug ends up in the Model S battery. Citing EPA figures and reports from owners, Weiss estimates the Model S's real-world charging efficiency at about 85 percent.
Again, Weiss has a good point. I've measured charging losses of 10-15 percent in my own car. Tesla quotes a "peak charging efficiency" of 92 percent on its website. An average charging efficiency of 85 percent seems plausible.
That means a Model S typically draws 17 percent more power from the plug than it uses to power the car.
So now our Model S carbon emissions are up to 254 g/mi, slightly less than those of a 2013 Honda Civic.
*Vampire losses further raise emissions by 55 percent
Whoa! This is truly a shocking claim. It implies that vampire losses--the power used by the Model S when it's off, just sitting there in your garage--amount to nearly as much as Tesla claims the car uses while driving.
Weiss, citing a number of sources, (including my own report on Model S vampire losses on this site), settles on a number for vampire losses of 5.1 kWh per day. He then combines that figure with an estimate of 7,728 miles driven per year to conclude that vampire-related Model S CO2 emissions amount to 140 g/mi.
This brings his new total up to to 394 g/mile, about the same as a BMW 5-Series.
I'd call Weiss's number for vampire drain a bit high, but not implausible. I measured at-the-wall vampire losses averaging 4.5 kWh per day on my car.
One reason for Weiss's high-ball estimate may be his apparent misunderstanding of the Model S battery thermal management system. He claims that vampire losses in the 30-to-50-degree range are nearly triple those occurring at temperaturess of 50 to 80 degrees, due to the extra juice required to keep the battery warm.
This is simply wrong. I have noticed no such variations.
And a Tesla rep confirmed to me that the Model S battery is not temperature-controlled when the car sits idle, so there is no battery heating/cooling power draw. (Elon Musk has publicly confirmed this.) The brief pre-heat/cool prior to the once-a-day "topping off" charge cycle would have only a minimal impact on vampire losses.
I also take issue with Weiss's estimate of the Model S average yearly driving distance of only 7,728 miles. (His derivation of the number is too lengthy to analyze here.)