I've learned that plenty of effort has been exerted in an attempt to calculate an accurate  Solar Constant,   The light and heat energy that strikes the earth's surface.

The energy that hits a particular square of land depends upon the sun's angle, the weather, the density of the atmosphere above that spot, etc.

The figure scientists use most is 1.37 kW / m�, or 433 btu per ft� per hour.

y = x²/4p parabolic equation for solar reflectors --- p=focal point length