Here’s a picture from What You Need to Know about Energy by the National Academy of Sciences. It shows 100 energy units of coal being used by an incandescent bulb to produce light that has only 2 energy units:

Reprinted with permission from "What you need to know about energy," 2008, by the National Academy of Sciences, Courtesy of the National Academies Press, Washington, D.C.

Incandescent bulbs get hot because only 2/36 (about 5%) of the energy coming into the house to power the bulb comes out as light — the rest of the energy produces heat. If you trace the energy back to the power plant, it turns out a mere 2% of the energy from the coal is doing the desired lighting job! The power plant itself loses 62% of the coal’s energy! Compact fluorescents use about 5% of the coal’s energy — better, but not much.

It’s important to think about how well our devices use energy to do what we want (such as providing light), and why a compact fluorescent is better than an incandescent bulb. But given how big the energy loss is from simply producing electricity in traditional power plants, it’s also important to think about where that energy comes from. Fossil fuel power plants waste a lot of energy, which makes all electrical devices powered by them relatively wasteful. This can help frame the debate about our energy options by getting people thinking about energy sources (like power plants) and the relationship to energy loads (like bulbs).

This also illuminates some of the confusions about the word “efficiency.” Compact fluorescents and LEDs are more efficient than incandescents in that they use less energy to do the same job, but the majority of the inefficiency in the overall system is at the power plant.