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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 horsepower (hp), which is equal to 746 W. The electric motor in a typical washing machine is ½ hp or 373 W, about the same as the power output of a very fit athlete during competition. The motor in an electric saw is about 1.5 hp. A small car engine operates at around 100 hp. A blue whale can develop 500 hp, or 370,000 W. It takes 0.13 hp to power a standard 100 W bulb. How many horses would it take to power your house? 
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An old-fashioned incandescent 100 W light bulb produces a power output of about 2 W of visible light. The other 98 W of electrical power used by the bulb become heat or infrared light (which we cannot see). Typical compact fluorescent light bulbs use less than one-fourth the electrical power, only 23 W, to produce the same power output of visible light. Fluorescent bulbs therefore lose around 21 W of input energy as heat, while incandescent bulbs lose around 98 W of input energy as heat. Power consumption and heat loss are the reasons why many people now use fluorescent bulbs.
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When is 100 W not really 100 W? The answer may be right over your head. What is sold in stores as a “100 W” rated compact fluorescent light bulb actually uses a power of around 23 W. The “100 W” classification of the bulb comes from the fact that the 23 W modern compact fluorescent bulb produces the same output radiant energy per second as an old-style 100 W incandescent light bulb—around 2 W of power in the form of light. So today, a 23 W bulb is still called a 100 W 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.
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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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Fluorescent bulbs rated at 100 W require only 23 W of power, so six of them would take 138 W. One horsepower is 746 W, so 138/746 = 0.18 hp. 
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