> Well except you have to model the position of the sun for every hour and day of the year That sounds exactly like some things xkcd has done in the "what if?"

> Well except you have to model the position of the sun for every hour and day of the year That sounds exactly like some things xkcd has done in the "what if?" series. He's done all sorts of variations on similar calculations. And while xkcd is smart, he's not superhumanly so (his fame is due to the unique combination of creative thinking, humour, geekery and smarts). In other words, this is possible and a good engineer should be able to calculate this. Especially when you know that any concave reflective surface is going to focus the light somewhere. In addition you know that due to the movements of the sun during the day and the year, this focal point will in 3D to different positions. So yes, odds are that it's going to hit some spot, somewhere, where it's going to be able to do damage, at some point during the year. Then you get to make a simple risk calculation about expected strength of sunlight that time of the year. And then you will remember to realize that over the course of the lifetime of this building, climate always has a non-zero probability of powerful outliers[0], and the conclusion is: Simply don't build gigantic concave mirrors in residential areas. [0] I'm not 100% sure if this is the case for strength of sunlight, since it's mainly dependent on what angle the sunlight arrives, which is not really up to chance. Still, even in the winter when the sun is quite low, as long as the sky is clear, I can easily light an incense stick with an A4-size fresnel lens. And I guess a whole building-side has a "slightly" bigger margin of how narrow the point of focus needs to be.