This is a purely geometric-based solution of light emitting from a source and spreading out in all directions and contacting another sphere some distance away. This does not account for any fancy "scientific" stuff that I am no expert at, such as the energy getting absorbed by the Earth’s atmosphere, or other factors that may be at work in this matter.

Here are some other sources I found that address some of the scientific issues of this matter, as well as other mathematical approaches to estimating this value:

https://www.quora.com/What-percent-of-heat-from-the-sun-reaches-the-earth

https://web.extension.illinois.edu/world/energy.cfm

• Step 1

  Surface area of a sphere: A = 4πr^2

  r is the distance of the Earth from the Sun = 92,960,000 miles (92.96 million miles)

  Surface area of the "sphere" formed by this distance:

  Area = 1.09×10 ^ 17 square miles
  

• Step 2:

  Cross-sectional area of a sphere: A = πr^2

  r = radius of earth
  r = 3,958.8 miles
  A = 4.92×10 ^ 7 square miles
  

• Step 3:

  Divide Area of Earth by Area of Sphere:

  (4.92×10 ^ 7) / ( 1.09×10 ^ 17 )
  
  = 4.5137615e-10   =  4.5292×10^-10
  

  For reference, "1 Billionth" is 1e-9, so 4.51e-10 is even less than this…

  Basically, it’s "Less than 1 billionth"

  It could also be stated as 4.5 "10 billionths"

  = 0.00000000045137615  =   0.000000045137615 %