Solar energy has been suggested as a source of renewable energy. If, in the desert direct radiation from the sun (save for 320 days) is 975 W per m2 between 10 AM and 3 PM, and the conversion efficiency to electricity is 21%, how many square meters are needed to collect an amount of energy equal to annual US energy consumption of 100 quadrillion BTUs? Is this construction feasible? How many pounds of coal (of heating value of 10,000 BTUs per pound) would be needed to provide the 100 quadrillion BTUs if the efficiency of conversion to electricity was 35%? What fraction of the US coal reserves (estimated at 250 billion short tons) is the calculated quantity?