When you go to the supermarket, you may be the kind of person who looks at the ingredients written on the packaging. Often you can also find the number of calories and the amount of fat in the product. But what you won't find is the amount of energy required to bring that product to the store: the cost to fly bananas to Europe; the energy needed to process and can vegetables; and the energy required to keep fish in the supermarket's frozen food section.

The Swiss publication énergie environnement, Number 14 (in French), refers to this hidden energy -- energy that is used but that we never see -- as énergie grise, which literally translates to gray energy.

The distance that an item has to be shipped is one aspect of hidden energy. The further away the food is grown, the more energy is required to transport it. Asparagus flown to Europe from California uses 12 times more energy than asparagus grown in France. Transporting by airplane uses roughly three times as much energy as transporting by ship.

Another aspect is the time of year: Is it currently the normal growing season for those vegetables you are buying, or have they been raised in greenhouses that are lit and heated? Even local greenhouses use lots of energy: Tomatoes grown in a Dutch greenhouse require three times more energy than tomatoes grown outdoors in Spain.

And énergie grise doesn't just apply to fresh produce: Water bottled in Switzerland uses 40 times more energy than water from the faucet in a Swiss kitchen.

So... how gray are those green beans?