I think it's awesome that you're actually trying this experiment out for real - great job at doing real science! So let's try to understand what's happening. A solar panel generates energy from sunlight, right? So it will generate the most energy whenever it's receiving the most light. So then the question becomes: at what angle will the solar panel receive the most light? Well, there's an easy way to check this at home. Take a sheet of paper and hold it out in front of you. Imagine the sheet of paper is a solar panel, and your head is the sun; in order for the sheet of paper to receive the most light, you need to rotate it so that the paper has the largest area possible (that means it'll receive the most light from your head).

So start looking at the paper edge-on; then the area you see is really tiny (you just see the edge of the paper). But if you start to rotate it, you see more and more area. When do you see the most? It should be when the paper is facing you head-on; that is, when the paper is angle at ninety degrees.

So that's why a solar panel receives the most power from the sun when it's angled at ninety degrees: that's when it has the most area exposed to sunlight, so it captures more energy.

Great question! Solar panels always receive the maximum power when the Sun is directly overhead (90 degrees). This is because the Sun’s rays have to travel through the atmosphere first before reaching the solar panels. The atmosphere absorbs a good part of the solar power. In fact, there is a numerical coefficient that is used in typical calculations of solar power efficiency called the Air Mass (AM). When the Sun is directly overhead AM=1.

However, because the Earth rotates and its tilt varies throughout one year, the Sun will invariably have to pass through more atmosphere during the day and throughout the year. The accepted “average” atmosphere that the Sun will pass through is AM=1.5. This is about an angle of 48 degrees. Scientists use this value when testing their solar cells to take into account atmospheric presence.

If you wish to align solar panels in order to receive maximum power throughout the day, you would want to place the solar panel such that it is perfectly aligned (90 degrees) with the Sun at high noon (when the Sun has to pass through the least atmosphere). This will vary depending on your location and time of year. This is because commercial solar panels are likely designed with internal reflectors or mirrors that bounce unabsorbed light back into the panel so that more can be absorbed. At any angle but 90 degrees this mirror effect will be reduced, thus giving you less power.

90 degrees. The reason is something you will learn in geometry: at any distance from the sun, there is a sphere over which all of the sun's power is distributed. This sphere has an area. If you hold a flat surface to collect some of that power, and hold it perpendicular (i.e. 90-degree angle) to the sun's light, then that flat surface will be almost exactly fitting a tiny portion of that sphere, and thus collect the power of that portion. Tilt the surface so that it is not on that sphere, then the cross-section of that surface and the sphere is diminished, and so is the power.

Here's another way to imagine it: imagine looking at a flat surface from some distance away. It will take up some part of the field of your vision. It will take up the largest field of your vision (i.e. appear to be the largest) if it is perpendicular to the line drawn between it and your eye. Now, if your eye were emitting light like the sun, then the power that this surface would receive would be in proportion to the amount of the field of your vision that it takes up. Thus, a perpendicular surface would get the most power.