Sunlight is an vast, constant source of energy available in abundance on a daily basis everywhere on the Earth's surface except during winter seasons near the poles and rainy seasons in temperate zones.
Humanity's average energy usage of 15tW ( 1.5x1013W ) corresponds to the capture of all the sun's flux at the surface, ( assuming 1kW/m2 ) over an area of 1.5x1010m2, or (1.225x105m)2, or (1.225x102km)2 or a square 122 kilometers on a side. Assuming 6 hours per day of energy production at an average efficiency of 40% gives a square 387 kilometers, or 240 miles, on a side. (This may seem like a large area, but it is only about one tenth of one percent of the earth's land area. 1 It is also probably significantly smaller than the aggregate area of rooftops of buildings throughout the world.)
The earth's aperture is pi*(6.4x106)2m2, or 128x1012m2, giving a surface flux of 128x1015W.
The maximum solar flux at the Earth's surface is about half that above the atmosphere. Most of the solar energy is in the visible spectrum, with a substantial additional energy in the infared spectrum.