The correct answer is 14 m2.

First, the average power demand of the car: it needs 18,000 W for 5% of the time and 0 W for 95% of the time. Thus,

(18,000 W * 5%) + (0 W * 95%) = 900 W

Second, the (average) 900 W we retrive from the battery is only 80% of what we put in:

(input power) * 80% = 900 W, or (input power) = 1,111 W.

Finally, we can apply the formula (area) = (power)/(flux) and the answer is 1,111 W divided by 77 W/m2.

This is double the area on the top of a compact car (hood, roof, and trunk).

The remaining questions consider the power requirements of society as a whole. The power consumption figures are given in megawatts (MW). Since 77 W/m2 = 77 MW/km2, answers in square kilometers are easy to obtain:

Question 9.

How much solar power collecting area would be required to replace all electrical power supplied in the United States? In 2003 average power demand was 4.44x105 MW. (This figure represents electrical output and is less than the energy consumed in power plants because of the less-than-perfect conversion of any form of energy to electrical energy.)

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© 2001-2002, 2005 by Wm. Robert Johnston.
Last modified 11 January 2005.
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