Since we have been talking about energy, thermodynamics and the efficiency of machines, you might also be interested in how we use energy in the United States and how much we use compared to other countries. The two links, Energy flow diagram and pdf graph are particularly revealing. One good place to look is the Energy Information Agency, a branch of the Department of Energy. They provide data about our energy usage and make forecasts about energy supplies and usage. The annual energy review contains much useful information about the U.S.'s energy sources and usage. (The energy considered here is called commercial energy because it it bought and sold on commercial markets. If you cut a cord of firewood from your woodlot to burn in your fireplace, that energy would not be included in these statistics. Be careful. Commercial Energy and the energy used by the commercial sector of our economy are different. The commercial sector refers to businesses and the like, as opposed to residential and industrial sectors.) Subsections under this heading include
Another site worth looking at is the International Energy Agency.
You might want to look at the California Energy Commission's web site.
The Department of Energy maintains a site on Energy Efficiency and Renewable Energy. The section on consumer information is interesting. The web page on different types of lights is good. For the relation of Lumens to Energy efficiency see the Hyper-Physics page on light. The Hyper-Physics site is pretty good.
For some simple population statistics see the POP CLOCKS.
The CIA World Fact Book is also good and slightly more detailed.