How can I evaluate solar panels to power night lights in areas that don't get direct light?

Question

I have a creek crossing in the woods: This photo is in the early spring. During the summer it's a bit like a jungle. We've considered the idea of putting foot lights along the edges of the crossing. They could either go on at dark or use motion sensors to go on when a person or vehicle comes along. Solar would be a nice option since it would save the effort of running a line from the barn (in the background) or the house all the way to the crossing. (I realize that solar powered lights would not last for more than a few hours after dark.)

But I have a question that came to me after reading this answer about using solar panels to provide power for lights in a an area that does not get direct light. The answer in the link provides good information, there's a frustrating issue when you're looking at doing something like this: There aren't clear numbers for what you can do or what kind of power you can generate this way. If the question is, "What do I do to provide power to this spot for lights," and someone specifies the wire gauge to use, there's clear math to figure the voltage drop and so on. But for solar, everything is unknown.

That means researching small solar panels (including, possibly, finding ones that go with the atmosphere of the place), then buying them, setting up a test install, evaluating, and possibly returning them. Repeat as needed to see if other similar products perform better. It's guesswork.

Is there a way to do this more precisely? For instance, are there light meters that can be used to figure out how much light a spot is getting and how much power that would provide for charging a battery? I've thought about the idea of getting a small test solar panel and putting a Raspberry Pi on a battery so it can monitor the output of the panel. Of course, a regular Pi can only work but so long on a battery, so it might be necessary to go with a Pi Zero or something.

While this is one example, I have other areas in the woods (paths, garden spots...) where a small light would be helpful, so I'm looking for something I can re-use in different areas to evaluate whether solar would provide enough power. I don't mind doing math (of course a lot of that can be put in a simple spreadsheet for reusability).

The goal is to find a method, without spending a lot of money (and I already have extra rPis, so that isn't a problem), that can be used to evaluate if an area gets enough light to power small LED lights, like footlights.

Answer 1

Maybe explore Power over Ethernet (POE) as an option. If the run is under 100 metres (328 feet) from the barn then you can get up to 15 watts per run of cable. All you need is a POE injector per run OR a POE switch.

Equip each run of wire with a POE splitter at the bridge, and you can have 5V or 12V at the far end, then hard-wire that to a low-powered LED string on either side of the bridge. Use waterproof boxes to protect all the joints, of course.

It would be more of a "being seen" light than an illumination or security light. Another advantage of POE runs is that you could run CCTV camera/s in the area and monitor from the house.

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As for logging the light levels, it may be more cost effective to simply go overboard on the solar panel size, and expect to replace them every 3-10 years. Cheaper home-grade solar panels will work adequately even in the shaded areas.

In your photo, I'd put a solar panel laying almost flat on the stonework on either end+side of the bridge, for four in total. Not completely flat so that rain and condensation can drain off. There are "in-road" solar panels that you walk or drive over, but they'll be pricey and directly over a creek is likely to be a harsh environment especially in the winter. I would plan on the roadway being overtopped by the stream at some point.

If you're a coder, then consider a Raspberry Pi has many kinds of sensor available, and can also be powered by POE. This could be located at the bridge in a waterproof housing.