Why does solar panel output vary across the day? I have seen engineering projects where they build a solar panel to track the sun such as this one
However, I don't understand their rationale fully. They argue that at one point in the day, the sun is shining with no component onto the panels and at midday the light is perpendicular which produces the most energy.
The part I don't get is that at say, 45 degrees, why does this decrease the energy output compared to 90 degrees?
Aren't there still the same number of photons that reach the solar panel if I point my flashlight (or the sun) at 90 or 45? My flashlight beam is still contained within the solar panel area in both cases (as is the sun's light) so there are the same number of photons. Then shouldn't I get the same number of electrons released, meaning the same energy?
Why does tilting my light decrease the efficiency of my solar panel?
 A: This is easy to see if you use a larger flashlight. Say, at some distance, the circle of light produced by it barely fits on the panel. It would look something like this.

Now turn the panel off at an angle. It would look more like this.

We immediately see that the panel can no longer capture all the light. The same goes for the sun. It emits a certain number of photons per solid angle from its surface radially outward, so the number of captured photons is proportional to how much of that solid angle we can capture. This solid angle has a maximum when the surface is at 90° to the direction of the rays, because they propagate radially outward.
For some intuition, it is also always helpful to take it to some extreme. If you turn the panel all the way sideways, the light only hits the edge of the panel, but none of the surface area, so it would not make sense to expect the same energy output as for a horizontal orientation. As you turn it back to the horizontal, more and more light hits the panel surface, and energy output increases.
A: I think in addition to the reduced effective area of a tilted panel, reflection from a surface generally increases with increasing angle of incidence from the normal to the surface. So when the panel is tilted, more sunlight is reflected from the front surface.
A: The sun's light is not contained in any solar panel smaller than a Dyson sphere.
You cut out a tiny angular fraction of its light cone. And this fraction is bigger when you face the sun 90°.
For the flashlight it is different. Its cone is much narrower and can be fully absorbed by a large panel, even when you don't shine on it at 90°
