(a) How much land (in km²) would you need to cover with solar panels in order to generate all of our power from solar energy (assume human power consumption is 15 TW). If one takes into account the day–night cycle and occasional cloudiness, solar panels receive (on average) about 200W/m² of sunlight. And this is further reduced because these panels convert sunlight into electricity with an efficiency of about 20 percent.

(b) What fraction of the Earth would you need to cover with solar panels? Does this seem possible?