Quick back-of-the-envelope calculation seems to agree with the Danes:
244 miles (392km) on 70kWh. For coal (0.95 [1] kg CO2/kWh) this comes down to 70*0.95/392 = 169gr/km. This is less than for a Porsche Boxster (222 gr/km tank-to-wheel [2]), but much more than a Prius (89 gr/km).
Using gas (0.60 kg/kWh) instead of coal reduces Roadster well-to-wheel emissions to 107gr/km. This means you have to include wind/solar/nuclear in the mix to get the CO2 emissions down below that of a Prius.
This is not what I have been led to believe (implicitly?) by Tesla Motors. What am I doing wrong?
1:
CO2 Emissions Report gives 2.117 pounds/kWh for coal in the US in 1999.
2: well-to-tank is approx. 12% of tank-to-wheel so the 222/89 figures for the Boxster and Prius should really be 252/101 to be more fair.
In general, I found the rule of thumb is an EV on all coal will be cleaner than the equivalent normal gas car for sure, but not necessarily vs a hybrid.
So it's wrong when they say an EV is dirtier than a "normal car". I don't consider hybrids such as the Prius as normal cars since you give up performance.
However, coal is still a very dirty source in general, both in terms of GHG and also other pollutants like mercury and sulfur. Which is why it makes sense to move away from coal as we upgrade our grid. It's encouraging there's a trend of lower coal usage in the US.
http://www.eia.doe.gov/cneaf/electricity/epm/epm_sum.html
I think what Tesla Motors does is to compare efficiency. I'm not sure if they ever directly say the Roadster is twice as clean as the Prius, but I'm sure I heard them say it's twice as efficient. They just take the energy in a gallon of gas (34.7kWh/gallon) and convert the Roadster's efficiency number to mpg. This comes up with 100-135mpg plug-to-wheel. So that's where they get the Roadster is twice as efficient as a Prius. They aren't technically lying since the car IS twice as efficient as a Prius if you consider the input/output of energy for the two cars; as an automaker they aren't responsible for and can't control anything else, such as the powerplants where you get electricity.
So what Tesla does isn't a well-to-wheel kind of comparison, but rather a gas pump/plug-to-wheel kind of comparison.
For people in the US, I usually recommend this tool to figure out whether you are better off with a hybrid or an EV/plug-in (IMHO every country should have a similar tool if they want to be serious about plug-ins or EVs):
How clean is the electricity I use? - Power Profiler | Clean Energy | US EPA
From here, I get the US average emissions is 1329lbsCO2/MWh -> 0.60kg/kWh which corresponds to your 107gr/km.
Where I live (in California), I get 724lbsCO2/MWh -> 0.33kg/kWh which means 60gr/km for the Roadster in California.
So those driving a Roadster in California is polluting 35-40% less than a Prius (depending which generation of Prius). Not quite half, but not too bad for such a quick sports car.
There are some states that use a lot more coal and for those states it doesn't make as much sense to have EVs until they get cleaner. Coal heavy states like Wyoming (can try zip 82082), North Dakota (58203), West Virginia (24985) for example has 70+% coal and 1800+lbsCO2/MWh.