[Research for this fact sheet was by super-intern Justin Brant.]

Paper versus plastic: We hear that question every time we go to the grocery store and see it raised with increasing frequency in the media. But does it matter? Or do the bag contents matter more than the bag itself. Sightline did the math and found out just how much that was true.

Assuming that a grocery bag holds one day's worth of food for a family of four, the choice about what to put in the bag is about 186 times as important as the bag itself. (For an illustration, see the graph to the left.)

This number was calculated using the concept of embodied energy -- the energy used to produce,  transport, and dispose of a product over its entire lifetime. For food this includes making fertilizers, processing, transportation, storage, and cooking.

Here's my thinking.  Using data from the most on-point study we could find, I calculated the energy used to produce, process, transport, store, and cook four servings of two different diets:  the first, a meat-based diet that included beef, potatoes, tropical fruit, and drinks such as soda; the second a vegetable-based diet composed of produce grown within the country where is was consumed and a soy-based protein source.

The first diet takes 113 MJ (megajoules) of energy to get to the table, while the second takes 24 MJ of energy.  The difference between these two numbers is 89 MJ.

In contrast, it takes about 0.5 MJ (pdf link), give or take, to produce and dispose of one plastic bag.

Thus, the energy saved by four people choosing the vegetable-based diet for one day equals the energy needed to produce 186 plastic bags, or drive the average U.S passenger car over 15 miles.

If anything I’d say the above number is conservative.  I didn’t include processed snack food in the high-energy diet, even though highly processed foods can require lots of energy.  I also didn’t distinguish between conventional and organic produce, which typically uses less energy to grow than  conventional produce.

What about canvas?

From an energy standpoint, using a canvas bag is 14 times better than plastic without factoring in the littering, landfill, recycling, and foreign oil dependence issues with plastic bags.

Canvas bags are also 39 times better than paper, at least from an energy standpoint.  Again, this number is conservative, since it assumes that a canvas bag is used 500 times over its lifetime. 

Geekage notes: The study used to calculate embodied energy in food is from Sweden and can be found here (pdf link).  Bag embodied energy is from an Australian study available here, though other figures are available as well. I’m assuming that the embodied energy for food and bag production in the United States is similar.

Other note: This analysis was originally posted on Sightline's blog.