Well, many. But, according to the S-G, it can make a big difference:
Recently, I visited an ambitious project to promote energy-saving lighting in China. By phasing out old-fashioned incandescent lightbulbs and introducing a new generation of lighting, China expects to cut national energy consumption by 8 per cent.
This can have a profound global impact. Consider this: lighting accounts for 19 per cent of world energy consumption. Scientists say we can reduce that by a third or more merely by changing lightbulbs.
Sure, it's one thing to use the nifty-looking CFL bulbs in your own house, but one house times...China...makes for a lot of energy saved.
(image from flickr user TheRogue under a Creative Commons license)