All the options for pouring a glass of cold water consume energy and water (beyond what you actually drink). However, the least wasteful choice is filling your glass from a water pitcher kept in the fridge.

Refrigerating water leads to energy waste when the fridge door is opened and warm air from the room rushes in. The longer you stand there with the door open contemplating condiments, last night’s leftovers or the meaning of life, the more energy that is required to return the fridge to its previous temperature.

Each time you open the fridge door (even briefly), about 1.4 watts of electricity is used. If the door stays open for longer than a few seconds, the temperature can rise 10 to 20 degrees, and energy consumption can increase by 10 watts or more per opening. Assuming that you open the fridge just to grab the pitcher of water, and then again to return it, your glass of cold water will consume about 2.8 watts of energy.

However, keeping a pitcher of cold water in the fridge also provides an energy benefit. Refrigerators (and freezers) stocked with cold foods and drinks are better able to maintain their internal temperature when a door is opened than empty bachelor fridges with two beers and leftover pizza inside.

Dropping cubes of ice into lukewarm water requires opening the freezer door just once. But that single opening consumes more energy than three openings of the fridge—about 4.3 watts of electricity.

(And did you know, water is necessary to create electricity, too? It’s often used to cool the power plant equipment. So all of the above options use water that you’re not drinking.)

Running the tap to get to cold water is the worst option. Beyond wasting water, it also wastes the energy used to treat, pump, and deliver drinking water. Every gallon of water that flows from your tap requires about 1.5 watts of power. So even if you have a low-flow faucet, running the tap for a minute will consume more energy than you’ll consume by opening the fridge door twice—and you’ll have wasted at least two gallons of water!

Finally, we don’t recommend refrigerators with in-door water and ice, for all sorts of reasons.