What does the snow currently do when it comes off of this roof? How does the behavior of snow coming off the existing solar panels differ from what happened before the panels were added to the other roof?
It sounds like you're worrying that raising the height of the roof, by adding solar panels, will cause the snow to travel further horizontally when it leaves the roof, compared to its current behavior. That would make sense if the panels were hanging off the roof, essentially extending the eve out another foot or two, but I would expect snow load to risk damage to a panel hanging out over empty space like that. Based on my understanding of roofs, snow, and gravity, I wouldn't expect panels mounted back from the edge of the roof to make a significant difference in how the snow falls once it reaches the gutter. But I would strongly expect snow on one side of the house to behave roughly the same as snow on the other, with some timing differences due to sun exposure, so the existing panels seem like a perfect test case for how far off the panels the snow actually travels before falling.
Assuming that the snow actually does fall farther from the roof with panels on it than it would without the panels, the simple and obvious solution would be putting a carport over the car, with a single-pitch roof directing the snow back onto your side of the fence. This would add value for the neighbor year-round, would be safer to install and maintain than anything mounted 3 stories up, and might be comparable in cost depending on what alternatives you're comparing it to.
It sounds like you're worrying that raising the height of the roof, by adding solar panels, will cause the snow to travel further horizontally when it leaves the roof, compared to its current behavior.
The height impact is minimal. Instead it's about friction: panels are very smooth compared to shingles, and notoriously avalanche-prone. The faster snow is moving when it leaves the roof, the farther from the roof it travels horizontally.
aha, that makes excellent sense! How much increased snow travel are you expecting based on your observations of the existing panels, and would affixing something high-friction like a fishing net over the panels reverse this?
Three years ago we put solar panels on the only part of our roof that got sunlight, which unfortunately mostly faces WNW. Since then, however, our neighbors have removed a large tree just east of us, and now the part of our roof that faces ESE gets lots of light. So I'm back to thinking about solar.
This roof is a good candidate for solar, with one problem: the panels might avalanche snow onto the neighbor's car:
The standard approach here is to put snow guards on the panels or the roof with the goal of holding back, slowing down, or breaking up the snow. In this case, however, I don't want the snow to stay on the roof and I don't mind it falling next to the house. I just need it not to fall out from the house, into the neighbor's driveway.
Is this a solved problem? Alternatively, is there something you can attach to the roof to redirect the snow straight down? Something like:
Alternatively, people who've tried snow guards, how well do they work? Are they going to keep the snow from squashing the neighbor's car, and the neighbor?