As several municipalities considering or even implementing bans on disposable grocery bags, I have noticed some claims emerging that reusable bags are actually worse for the environment than disposable ones. The basic argument is that reusable bags not only take more energy to manufacture (they are heavier, so this is doubtless true), but moreover the need to wash reusable bags negates any energy benefit from re-using them. A similar case was made for using recycled paper napkins rather than cloth in restaurants, and the numbers do favor recycled paper. At least, they do for restaurants, which replace their napkins when they get the slightest stain; at home, using that author’s numbers, cloth is likely to be the more “green” choice.
However, I didn’t find any good analyses for reusable and disposable grocery bags (while I did find some contradictory claims), so I decided to see what I could work out myself. I did a bit of searching one weekend to try to figure out how much energy it took to manufacture canvas and plastic bags, estimated how much energy it took to wash the canvas bags, and made a quick calculation of how many plastic bags I avoid using in a year by carrying around my canvas ones. The details of the references, assumptions, calculations, and rough uncertainties are here, and I am posting a summary below.
For manufacture, the canvas bags are heavier, and therefore it takes a lot more energy to manufacture one canvas bag than it does one disposable plastic one. I weighed the bags that I’ve used, and found that my Trader Joe’s 2003-vintage canvas bags weigh about 180 g each, and the disposable plastic bags I get when I forget the reusable ones weigh on average 6 g each. My canvas bags should have taken about 25 MJ to make [PDF] (growing the cotton, weaving the fabric, and assembling the bag), whereas one plastic bag should take about 0.5 MJ to make.
The energy used in transportation per bag should be strictly proportional to the difference in weight. I assumed that it took 5 MJ to transport my 180 g canvas bag [PDF], and 0.2 MJ to transport a 6 g plastic bag.
Only the canvas bags need to be washed. The energy used will depend upon the size of the load, the temperature at which the water is washed, the amount of water used for the load, and whether or not the bags are run through the dryer (mine are). I tend to wash fairly large loads, about 4.5 kg at a time. I would use hot water for the wash, since I wash the bags with our napkins and dish towels, and cold for the rinse. I estimate that each wash takes between 0.6 MJ per bag for our front-loading machine (for a top-loading machine, this would be about 0.9 MJ per bag). Drying the bags takes another 1 MJ per bag (I should get a clothes line! Um, and a backyard. Oh, and a sunny, dry climate). I wash the bags about once a month at most, so the yearly energy budget for washing the bags is 19.2 MJ. Washing the canvas bags nearly doubles their energy footprint.
Summing the numbers, in the first year of purchasing a canvas bag, I estimate that I use 49.2 MJ of energy for that bag, versus 0.7 MJ per disposable plastic bag. The canvas bags carry more, and the plastic ones are always double-bagged, so I estimate that each canvas bag replaces three plastic ones on each trip. I go shopping once a week, so one canvas bag replaces 156 plastic ones in a year. Those 156 plastic bags would take 109 MJ of energy — more than twice the energy used by a new canvas bag. After the first year, I would only need to wash the canvas bag, taking 19.2 MJ of energy, so my reusable bags are 5.7 times more energy-efficient.
I might be off on my numbers by a factor of a couple, so perhaps in the first year the canvas bags are about equally energy-intensive as the plastic ones. However, in the long run, it appears that canvas bags are much more efficient than plastic ones. This would be especially true if I were to line-dry them, as someone truly eco-conscious would… But I mainly use them because I hate stuffing my closet with plastic bags. It just turns out that it’s also the more energy-efficient choice.