Yesterday was Earth Day, so Facebook was naturally full of people bragging about how they walked to the store instead of driving, in order to save the Earth. I feel obligated to point out that this very plausibly isn’t true. I’m not the first person to run these numbers, but I was curious and wanted to investigate for myself. My (rather rough) calculations are all here.
As a baseline, we want to calculate the kWh cost of driving a car 1 mile. I’m using a baseline of 33.41 kWh / gallon of gasoline:
|Car||MPG||kWh/ 1 mile|
If you’re bragging on Facebook about your environmental impact, you’re probably driving a Prius, so we’ll roll with that. Feel free to substitute your own car.
To get the calories burned per mile walking, I used numbers I found here. The numbers here vary pretty widely with body weight and walking speed, but I’ll use 180 pounds at 3.0 mph for 95 calories per hour.
To get the energy costs per pound of food produced, I used the numbers I found here. Click through for their sources. kWh / 1 mile is calculated as
kWh/1 mile = 180/(calories/lb) * ( kWh/lb)
Just to be clear: this isn’t the calories in food. This is the energy usage required to produce and transport the food to your mouth, which is essentially all fossil fuels. Numbers vary widely per food source, as expected.
|Food||Calories / Lb||kWh / Lb||kWh / 1 mile|
So what’s the conclusion? It’s mixed.
- If you drive a Prius, you’re OK walking, as long as you replace the burned calories with Doritos (cheese + corn) and (corn syrup’d) Coca-Cola
- If you drive a Prius, and you replace the burned calories with a chicken and apple salad (I couldn’t find numbers for lettuce, but they are undoubtedly even worse), you are destroying the planet
- If you drive an F-150, you’re probably going to replace your burned calories with a steak, so you’re actually saving the environment by driving.
These numbers are of course rough, and do not include:
- The energy cost of producing a car. This becomes very complicated very quickly, becaus you likely would have done less damage by just buying a used car instead of a new Prius
- This assumes you actually eat all the food you ordered, and didn’t leave carrots rotting in the back of your fridge (your fridge, by the way, uses energy). Americans are notoriously terrible at doing this.
- This calculates only energy usage — it does not attempt to quantify the environmental impact of turning Brazilian rainforests into organic Kale farms, to grow your fourth-meal salad.
- This assumes a single rider per car-mile. If you are carpooling on your drive to KFC, you can cut all the car energy usage numbers by half (or more, for families)
Anyway, I’m sure there are many other reasons these numbers are rough, I just wanted to point out that the conventional wisdom is pretty awful on environmental topics.