I am always surprised how people find it difficult to understand cogeneration economics and theory. I like to use graphics, but your food analogy works too. Another good analogy is an automobile. An automobile engine converts fuel into work and the heat from the engine is nice to have on a cold winter day. I believe Carnot would approve.

Some people also relate to the idea of aboriginal conservation methods, specifically with food preperation. Aboriginals used the entire bison or deer for tools, food, and clothing. They left very little to waste. Similar to cogenerators.

In the end, it all comes down to your audience. Personally, I like to speak to the youth as they seem to understand system thinking much better than my elders.