Several months ago, when I first posted about my manual two-axis solar tracker, a couple of readers asked whether a tracker really made that big of a difference. I had a theoretical answer based on simple trigonometry and the amount of light that falls on a surface relative to its orientation to the sun. Specifically, the amount of direct sunlight that falls on a square surface should be proportional to cosine(θ) x cosine(γ) where θ and γ are the angles between where the panel is pointed and where the sun is, in the x and y axes. By this theory, a panel that is perfectly aligned in one axis, but off by 30 degrees in the other axis would only get about 86.6% as much direct light. (This formula ignores ambient light, which in reality would clearly be present in addition to direct sunlight.) But I had no empirical data to back up my theory… until today.

On a sunny day like today, I’d re-orient my 100W solar panel 2 to 3 times in the course of the day. I usually point the panel to the east before going to bed so that it’ll catch the morning rays, and I’ll move it to point due-south later in the morning. In the afternoon, I might move it one or two times as well. The general idea is to keep the panel pointed to within about 20 degrees of the sun, since that should give me over 94% of available light at all times.

Today, when I went to re-orient the panel a little after 3pm, I decided to get a couple of actual readings. I first checked the voltage of the whole system (charge controllers hooked up to battery array), and got 13.3 Volts. I then measured the current between the charge controller and battery array, with the solar panel in the noon position, and also in its optimal position at the time, which is about 45 degrees from the noon position. With the panel in the “noon position”, I got 3.85Amps, or 51.2 Watts. I then moved it to the 3pm position, and got 5.55Amps, or 73.82 Watts.

The verdict, I might say, is that yes, the tracker makes a significant difference. If the panel had been fixed pointing due south, by 3pm I would only be getting less than 70% of the power that I could be getting, and that number would rapidly diminish as the sun continued moving away. This would also be the case in the morning, when I would get significantly less power than is available for the first few hours of sunlight. And, as it turns out, the numbers fit my theoretical model fairly closely, since according to my theory, my panel should be outputting 70.07% of its maximum when pointed 45 degrees away, while the actual numbers I got today were 69.37% (also, the angular difference was approximate, though, in theory, the sun should move by 45 degrees between noon and 3pm).

One thing to note, however, is that these results were obtained with my monocrystalline panel, which work best in direct sunlight. Thin-film panels, including amorhpous silicone panels, supposedly get more power from ambient light, so they may be less sensitive to orientation, though this is another hypothesis I’ll need to test with my 45Watt amorphous panels sometime.

Another question I got about the tracker was the effectiveness of the “manual” nature of the tracker. Wouldn’t an automatic tracker that constantly aligned the panel with the sun be more effective? Well, yes. But, to get 90% of available energy, the panel can be off by as much as 25 degrees in one axis (arccosine(0.9)). Or, at any given time, if I point the tracker 25 degrees ahead of where the sun actually is, the sun could move through a 50 degree arc and I would still be getting over 90% the whole time. Since it takes the sun over 3 hours to arc through 50 degrees, even manually moving a tracker every 3 hours will ensure that my panel gets 90-100% of available light at all times. So, an automatic tracker with all its complexity only gets maybe 5% more power than a manual tracker that’s re-oriented every 3 hours.