Jeffrey Yasskin’s blog

Say you’re driving your car at 70 mph on the highway, and you need to stop. As a rough calculation, you’re going 110 km/hr or 30 m/s. Your car weighs on the order of 1000 kilograms, so the energy your brakes absorb is 1/2mv² = 1/2*1000*30*30 = 450 kilojoules. Unless you have regenerative breaks (i.e. are driving a hybrid), this energy is wasted. A desktop computer uses on the order of 300 watts. So the amount of energy your car wastes every single time you stop from 70mph is enough to run your computer for 25 minutes. I’m sure you can find other good comparisons.

Since we drive on city streets more, it might be more fair to consider a stop from 30mph. That’s 13m/s and 85kJ, with which you could run a computer for about 5 minutes, for every time you stop at a traffic light.