3 ms·

If you're defining efficiency as energy required to travel a distance, then an electric bike that doesn't require effort from the rider is going to be way more

If you're defining efficiency as energy required to travel a distance, then an electric bike that doesn't require effort from the rider is going to be way more efficient than walking or cycling. That was one of the exercises I had when studying - calculate the energy intensity of various modes of transport. It turns out that if you calculate the whole energy requirements to get the extra food into someone's mouth that they'll want if they are walking or biking, then it's not much different to the amount of energy that would be used by just driving a car. Making food is energy-intensive, and the conversion efficiency into mechanical output by a working person is very low. In contrast, an electric motor and a battery can both be made extremely efficient.

tomjakubowski 3y ago
Would you show your work? I am curious and a little bit skeptical. A gallon of gasoline has about 31,000 kcal. In the US, a typical sedan gets about 30 miles per gallon. Walking a mile at 150lb bodyweight burns about 100 kcal. (This is non linear though: walking longer distances will burn less per mile on average) If we assume linearity, to walk an extra 30 miles in a day you'd need 3000 more kcal of food. What is the energy cost of growing and transporting 3000 kcal of food? Does it exceed 31,000 kcal?
- mnw21cam 3y ago
  This was 10-15 years ago, so I don't have my notes lying around. However, I do remember that it was based on the calorific efficiency of bread bought at a supermarket. Other foods may vary - for instance if you grow potatoes in your back garden then it's probably going to be a lot more efficient. I can well believe that there is a 10:1 ratio between energy in and mechanical energy out of a human. However my point wasn't that cars and humans are about the same. That's probably only true in the "same order of magnitude" sense. My point was that humans are way less efficient at propelling a bicycle than an electric motor is.

tomjakubowski 3y ago

Would you show your work? I am curious and a little bit skeptical. A gallon of gasoline has about 31,000 kcal. In the US, a typical sedan gets about 30 miles per gallon. Walking a mile at 150lb bodyweight burns about 100 kcal. (This is non linear though: walking longer distances will burn less per mile on average) If we assume linearity, to walk an extra 30 miles in a day you'd need 3000 more kcal of food. What is the energy cost of growing and transporting 3000 kcal of food? Does it exceed 31,000 kcal?

mnw21cam 3y ago
This was 10-15 years ago, so I don't have my notes lying around. However, I do remember that it was based on the calorific efficiency of bread bought at a supermarket. Other foods may vary - for instance if you grow potatoes in your back garden then it's probably going to be a lot more efficient. I can well believe that there is a 10:1 ratio between energy in and mechanical energy out of a human. However my point wasn't that cars and humans are about the same. That's probably only true in the "same order of magnitude" sense. My point was that humans are way less efficient at propelling a bicycle than an electric motor is.