Water, phone, and, to a lesser degree, electricity, are all reasonable for the average person to understand. A shower obviously uses more water than a tap, and when it's off it uses none. A phone costs me minutes while it's off the hook and doesn't when it isn't. And I know about how long a minute is. Turning on my computer, lights, or microwave costs electricity (although I may not have a great idea of how much, I can make decent guesses to the ratios).

But do even the most tech-savvy have a reasonable intuition for how much bandwidth a computer uses while idle with a given set of applications running? How much a game of StarCraft uses? How much streaming video costs? (The last one is a trick question - it can change dynamically depending on your downstream speed.) It's just unreasonable to ask people to make good decisions about these things.