The correct answer is **14 m ^{2}**.

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/m^{2}.

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/m^{2} = 77 MW/km^{2}, 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 4.44x10^{5} 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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