When is 100 watts not really 100 watts? The answer may be right over your head. A so-called “100 watt” rated compact fluorescent light bulb actually uses a power of around 23 watts. The “100 watt” classification of the bulb comes from the fact that the 23 watt modern compact fluorescent bulb produces the same output radiant energy per second as an old-style 100 watt incandescent light bulb—around 2 watts of power in the form of light. So today, a 23 watt bulb is still called a 100 watt bulb even though the real input power is quite different! This strange historical anachronism could have been avoided if light bulbs had been rated by the light intensity (or light power) they produced, rather than by the electrical power they consumed.
How much electrical power does it take to power six 100 W-rated fluorescent light bulbs? Express your answer in horsepower.
- 0.031 hp
- 0.13 hp
- 0.18 hp
- 0.8 hp
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100 W-rated flurescents use 23 W of power, so six of them would take 138 W. A horsepower is 746 W, so 138/746 = 0.18 hp. 
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When you think of how “strong” something is, you are often thinking about how much power it produces. Many appliances and cars provide power in units of 
An old-fashioned incandescent 100 watt light bulb produces a power output of about 2 watts of visible light. The other 98 watts of electrical power used by the bulb becomes heat or infrared light (which we cannot see). Modern compact fluorescent light bulbs use less than one-fourth the electrical power, only 23 watts, to produce the same power output of visible light. Fluorescent bulbs therefore lose only around 21 watts of input energy as heat, while incandescent bulbs lose around 98 watts of input energy as heat. Power consumption and heat loss are the reasons why many people now use fluorescent bulbs.