For Exercises 16–20, refer to the following. Energy can be thought of as power applied over time. Power can be measured in kilowatts and energy in kilowatt-hours (kWh), where 1 kilowatt-hour is 1000 watts of power exerted for 1 hour. For example, a 40-watt bulb burning for 24 hours consumes 960 watt-hours ≈ 1 kWh. Other forms of energy, such as food or gasoline, can be converted to an equivalent in kWh.

Question 18.48

18. About of U.S. land could be devoted to wind energy (“without raising too many hackles,” according to Englishman David J. C. Mackay, Sustainable Energy—Without the Hot Air, UIT Cambridge, 2009, p. 234). The windmills would generate on average . (The hardware required would be 100 times as much as is currently installed.)

  1. How much is that land area in square miles?
  2. How much electricity would be produced per year?
  3. Suppose instead that all that area were covered with solar cells producing electricity at an annual average of . How much electricity would be produced per year?
  4. How much energy per day would that amount to per person in the United States, in kWh/d? (The United States has about 315 million people; over all forms of energy usage throughout society—manufacturing, transport, heat, electricity and its production—Americans average the equivalent of 250 kWh/d per person.)

For Exercises 19–20, refer to the following. How can an all-electric car get 99 mpg? The figure below shows the EPS fuel economy label for an all-electric car. The fuel economy is measured in miles per gallon gasoline equivalent (MPGe). A gallon of gasoline equivalent means the amount of alternative fuel (electricity, natural gas, or hydrogen) that contains the same energy as in a gallon of gasoline.

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The fuel economy label cites how many kilowatt- hours (kWh) of energy the vehicle uses per 100 miles— namely, 34 kWh. That number in turn is converted to MPGe via the equation

Thus, in the label shown, the number of kWh/100 mi is 34, so the .