Watts are a unit of energy per unit time. So not like a fixed budget in dollars, but rather a recurring budget in dollars per hour. Obviously, our energy use varies a great deal over time (just as our monetary spending tends to), so we can smooth that out (amortize it) to get a rough idea of our average rate of energy use.

If you use a 400 watt computer all of the time your personal remaining energy budget is 1600 watts. I use a computer around 10 hours a day during the week, and three or four on weekend days (so around a third of the time). Amortized, that means I would be using on average around 133 watts on computing, if I used such a desktop. But I actually use a Thinkpad T420, and its consumption averages out to around 30 watts when in use (so amortized to 10 watts).