I’ve made a rough estimate of the relative carbon footprint of driving a Tesla Roadster and riding a bicycle. You might think that a bike has no carbon footprint, since it doesn’t have an engine. Really, though, it’s powered by a person and the person is in turn powered by burning hydrocarbons. These hydrocarbons come in the form of food, which has a very high carbon footprint per unit energy compared to other sources. Furthermore, batteries and electric motors are way more efficient than human bodies at converting input energy into kinetic energy. The bottom line is that they’re about the same.
Some caveats about my calculations. They only consider the cost of a trip, not the production and end-of-life costs of the bike and car, which would doubtless greatly favor the bike. They’re at best crude approximations and depend on assumptions (speed of biking, diet and size of biker, fuel mix of electricity) that vary greatly from person-to-person and region to region. I got a number of facts from websites, and I didn’t try to verify that the websites are accurate so there might be large errors there, too.
Let’s start with the car. 250 Wh/mile seems to be roughly the efficiency that I get when I drive, although different sources for this number vary considerably: the Tesla website (Tesla Motors - well-to-wheel) claims 110 Wh/km = 177 Wh/mi, but that seems too good to me if you’re driving at speed; Tom Saxton said that he measured over 330 Wh/mile at the plug for about 900 miles of driving (only a little of which was drag racing). While I’m more inclined to believe Tom than the numbers from the Tesla website, I’ll just stick with my guesstimate from the display in my car. The Roadster’s charging efficiency is about 86%, so it takes about 291 Wh from the plug to put the 250 Wh in my battery.
Here in the Pacific Northwest, our local energy company is Puget Sound Energy (PSE), which reports a fuel mix of 36% coal, 41% hyrdo, 20% natural gas, 1% nuclear, and 2% “other” (mostly wind) (Power Supply Fuel Mix). This is actually overrepresenting PSE’s carbon production, because they have much more wind power than that mix represents, but they sell the credits for it to other utilities. So, I’ll stick with what they report.
Wikipedia (Carbon footprint - Wikipedia, the free encyclopedia) says that that the EPA estimate of various forms of electricity generation in grams of CO2/kWh is 950 for coal, 600 for natural gas thermal, 11 for hydro, 6 for nuclear and 5.5 for wind (the nuclear and wind numbers are from Vattenfall, since EPA didn’t have estimates for them). I’m not sure if these numbers include transmission losses, but I’ll assume that they do; even if not, it would only be a modest percentage loss and this is a very rough estimate, so it probably wouldn’t change the conclusion. Prorating by the PSE mix gives about 467 g CO2/kWh electricity at the plug. Multiplying by the Roadster efficiency of 291 Wh at the plug/mile gives about 140 g CO2/mile driven.
Now let’s move on to the bike. I tend to ride at about 20 mph on the level, but I’m in better shape than many people, so let’s assume that someone who rides every day for their commute goes 15. Calories Burned During Exercise (selected because it came up first on a web search, not because I have any reason to believe it’s accurate) says that a 190 lb person (just under the US male average of 191 lb) riding 14-15.9 miles/hr burns 863 calories/hour. Dividing that by the 15 miles/hour speed gives 58 cal/mile.
Food sources (again selected by being high on a web search) says that the average (British) person releases 2.2 tons (I’m assuming they mean metric tons, since it’s a British website) of CO2 equivalent per year for food. Assuming a 2500 cal/day diet, that’s 2.4 g CO2 equivalent/cal. Multiplying by the 58 cal/mile on a bike gives about 140 g CO2/mile ridden, the same as the Roadster (actually a hair more if you don't round off, but I put no faith at all in those low order digits).
I listed two digits of accuracy for the final results, but the truth is that they depend so heavily on the assumptions that even if the facts I pulled from the web are perfect, the bottom line will vary over a large range. Maybe you’re a thin, slow-biking vegetarian who lives in a place that gets all of its electricity from coal and knows a bikes-only shortcut to work. On the other hand, you could be a fat person living in Chelan, WA where essentially all of the power is hydroelectric and your trip to work is on 35mph roads. The point that I’m trying to make is that driving a Roadster is roughly comparable in CO2 production to riding a bike given a somewhat reasonable set of assumptions. That is, I'm not telling you to get off of your bike to save the atmosphere, just pointing out that the EV versus bike tradeoff is pretty even.
To me, at least, it’s a pretty surprising result. After all, you’re lugging around a whole car (+ 900 pounds of battery) with you when you drive. What happened to make the numbers come out so far from intuition? First is that PSE is way more efficient in getting energy in exchange for releasing carbon than farmers are. We saw above that food releases 2.4g of CO2 equivalent per calorie. A (dietary) calorie is about 1.2 Wh, so that’s 2000 g/kWh, or about 4x worse than PSE. Second, the human+bike system is way less efficient at turning food energy into miles traveled than the Roadster when you normalize for the mass. Say the (191lb average size) person + bike (+ clothes, water, etc.) is 100kg. It takes 59 cal = 71 Wh to go a mile for an efficiency of 0.71 Wh/kg-mile (sorry for the ugly mixed metric/English units). The Roadster + driver + stuff in the trunk is roughly 1350 kg and uses 250 Wh/mile for an efficiency of 0.19 Wh/kg-mile, almost 4x better as well. These two factors of roughly 4 (together a factor of nearly 16) make up for the factor of 14 difference in mass that favors the bike.
Now, I’m going to drive my Roadster to the gym and ride a stationary bike, completely killing any carbon advantage I might have had by driving electric…
Some caveats about my calculations. They only consider the cost of a trip, not the production and end-of-life costs of the bike and car, which would doubtless greatly favor the bike. They’re at best crude approximations and depend on assumptions (speed of biking, diet and size of biker, fuel mix of electricity) that vary greatly from person-to-person and region to region. I got a number of facts from websites, and I didn’t try to verify that the websites are accurate so there might be large errors there, too.
Let’s start with the car. 250 Wh/mile seems to be roughly the efficiency that I get when I drive, although different sources for this number vary considerably: the Tesla website (Tesla Motors - well-to-wheel) claims 110 Wh/km = 177 Wh/mi, but that seems too good to me if you’re driving at speed; Tom Saxton said that he measured over 330 Wh/mile at the plug for about 900 miles of driving (only a little of which was drag racing). While I’m more inclined to believe Tom than the numbers from the Tesla website, I’ll just stick with my guesstimate from the display in my car. The Roadster’s charging efficiency is about 86%, so it takes about 291 Wh from the plug to put the 250 Wh in my battery.
Here in the Pacific Northwest, our local energy company is Puget Sound Energy (PSE), which reports a fuel mix of 36% coal, 41% hyrdo, 20% natural gas, 1% nuclear, and 2% “other” (mostly wind) (Power Supply Fuel Mix). This is actually overrepresenting PSE’s carbon production, because they have much more wind power than that mix represents, but they sell the credits for it to other utilities. So, I’ll stick with what they report.
Wikipedia (Carbon footprint - Wikipedia, the free encyclopedia) says that that the EPA estimate of various forms of electricity generation in grams of CO2/kWh is 950 for coal, 600 for natural gas thermal, 11 for hydro, 6 for nuclear and 5.5 for wind (the nuclear and wind numbers are from Vattenfall, since EPA didn’t have estimates for them). I’m not sure if these numbers include transmission losses, but I’ll assume that they do; even if not, it would only be a modest percentage loss and this is a very rough estimate, so it probably wouldn’t change the conclusion. Prorating by the PSE mix gives about 467 g CO2/kWh electricity at the plug. Multiplying by the Roadster efficiency of 291 Wh at the plug/mile gives about 140 g CO2/mile driven.
Now let’s move on to the bike. I tend to ride at about 20 mph on the level, but I’m in better shape than many people, so let’s assume that someone who rides every day for their commute goes 15. Calories Burned During Exercise (selected because it came up first on a web search, not because I have any reason to believe it’s accurate) says that a 190 lb person (just under the US male average of 191 lb) riding 14-15.9 miles/hr burns 863 calories/hour. Dividing that by the 15 miles/hour speed gives 58 cal/mile.
Food sources (again selected by being high on a web search) says that the average (British) person releases 2.2 tons (I’m assuming they mean metric tons, since it’s a British website) of CO2 equivalent per year for food. Assuming a 2500 cal/day diet, that’s 2.4 g CO2 equivalent/cal. Multiplying by the 58 cal/mile on a bike gives about 140 g CO2/mile ridden, the same as the Roadster (actually a hair more if you don't round off, but I put no faith at all in those low order digits).
I listed two digits of accuracy for the final results, but the truth is that they depend so heavily on the assumptions that even if the facts I pulled from the web are perfect, the bottom line will vary over a large range. Maybe you’re a thin, slow-biking vegetarian who lives in a place that gets all of its electricity from coal and knows a bikes-only shortcut to work. On the other hand, you could be a fat person living in Chelan, WA where essentially all of the power is hydroelectric and your trip to work is on 35mph roads. The point that I’m trying to make is that driving a Roadster is roughly comparable in CO2 production to riding a bike given a somewhat reasonable set of assumptions. That is, I'm not telling you to get off of your bike to save the atmosphere, just pointing out that the EV versus bike tradeoff is pretty even.
To me, at least, it’s a pretty surprising result. After all, you’re lugging around a whole car (+ 900 pounds of battery) with you when you drive. What happened to make the numbers come out so far from intuition? First is that PSE is way more efficient in getting energy in exchange for releasing carbon than farmers are. We saw above that food releases 2.4g of CO2 equivalent per calorie. A (dietary) calorie is about 1.2 Wh, so that’s 2000 g/kWh, or about 4x worse than PSE. Second, the human+bike system is way less efficient at turning food energy into miles traveled than the Roadster when you normalize for the mass. Say the (191lb average size) person + bike (+ clothes, water, etc.) is 100kg. It takes 59 cal = 71 Wh to go a mile for an efficiency of 0.71 Wh/kg-mile (sorry for the ugly mixed metric/English units). The Roadster + driver + stuff in the trunk is roughly 1350 kg and uses 250 Wh/mile for an efficiency of 0.19 Wh/kg-mile, almost 4x better as well. These two factors of roughly 4 (together a factor of nearly 16) make up for the factor of 14 difference in mass that favors the bike.
Now, I’m going to drive my Roadster to the gym and ride a stationary bike, completely killing any carbon advantage I might have had by driving electric…
