@submariner- Part of the answer to your questions has to do with how solar panels work, and the rest of it has to do with something called air mass. Solar panels, like any other flat surface, will have the largest surface area in contact with light when its angle matches that of the light source. Think about holding a piece of paper to create a shadow. If you match the plane of the paper to the angle of the light, you will create the largest shadow. This shadow represents the amount of solar radiation intercepted by the piece of paper, same as a solar panel.
Air Mass on the other hand, has to do with the amount of solar radiation absorbed by the atmosphere. If the sun is directly overhead, it passes through less air mass to reach a point on the earth. If it passes through at an angle of say 23.45 degrees, then the zenith angle is greater, meaning that the sun has to pass through much more air mass to reach the same point. The atmosphere at this shallow angle will diffuse more sunlight than if the sun were directly overhead.