The average solar flux incident on a horizontal surface in the United States (averaged over 24 hours and 365 days) is about 180 W=m2 [57.06 Btu=(h ft2)]. How much land 20 Heating and Cooling of Buildings: Design for Efficiency would be needed to supply the total energy demand of the United States, approximately 801018 J (76 quad) per year, by means of photovoltaics (efficiency around 10%) and biomass (efficiency around 1%)? Compare with the land area of the United States, 9.16106 km2 (3.54106 mi2).