A Fistful of Pencils – Measuring Solar Radiation on a Building

Lucas Hamilton

Earlier this week during a webinar I conducted on working with solar radiation, I gave an example to help people visual how the energy of solar radiation strikes a building or object. 

In physics and mathematics we would picture this energy as a vector component. I know that is not clear to a majority of non-science or non-mathematics practitioners so I often use an example with a fistful of pencils to help people visualize exactly what this means.  This is a fun little exercise but is not meant to be a scientific determination of the impact of solar radiation on a surface. This is simply a way to visualize the invisible.

Imagine the sunlight or energy coming across space and beating down on the surface of the roof at a normal angle which is a mathematical term for a 90-degree or right angle. To understand the impact of the solar radiation on that roof, take a piece of paper and draw a 1-inch square. Take a fistful of pencils (as many as will fit comfortably in your hand) making sure all the tips are even and bring your fist straight down on the paper striking it within the square.  Then count the number of strikes within that box and if you imagine each one of those strikes as being a unit of energy it gives you some idea of the impact of solar radiation on your roof.

If you want to imagine how that same sunlight is striking your wall, picture the angle that your wall is from that sun – usually about a 45-degree angle give or take.  Take the pencils in your hand and while sliding them to keep them flat to the paper turn your hand to a  45-degree angle and strike them into a 1-inch square box, you can see the number of strikes and what that impact would be significantly less. So if you again imagine that the pencil points are units of energy, you can see that only a fraction of the energy hits your wall compared to the roof.

This can be done with any angle and it gives you a very general idea of the solar energy impact on a surface.  While this does not give you scientific data to help you determine where your peak power would occur, it is one method that can be used to help visualize the best angle for solar panels on a roof. 

There are some online tools that can help calculate the intensity of the solar radiation based on geographic location. One example of such a tool can be found at: http://www.kahl.net/solarch/.

Lucas Hamilton is Manager, Building Science Applications for CertainTeed Corporation

Leave a Comment