Whether the heat energy you apply to boiling food is wasted depends on what you’re trying to do.
Let’s say that you’re boiling some eggs. Once the water reaches boiling (at 100 °C) further heat flow into it serves only to convert liquid water to gaseous water vapor, without further raising the temperature. Of course, since the water would cool off if you just turned off the stove, you need to keep some heat flow in just to keep the water boiling. But keeping a lot of heat flowing in just boils the water faster, not hotter, so the eggs wouldn’t cook faster. You would waste energy, and increase the chance that the water would boil away before the eggs are even cooked.
On the other hand, let’s say you’ve made a very runny spaghetti sauce. You may want to boil some of the water off. Then the heat you put into that boiling isn’t wasted.
(published on 03/25/07)