Generally, you don’t run out of roof space before meeting a 2 story buildings energy needs. On taller buildings installation costs would be significant, coupled with lower effects and it’s more viable to put them just about anywhere else.
The garage door is a rather complex place to put them as their are a huge range of styles and you would need to either fit the opening and replace the mechanism, or design a door for each existing type sold. On top of that wiring up solar to your house is the tricky part not putting something on a roof.
Around here (Boston) roofs tend to have a lot of obstructions: dormers, skylights, chimneys, vent pipes. They're also typically very steep (1:1) so North-facing isn't an option. Then you lose some space due to shading from trees and other houses. When we got solar there was only room for panels covering 1/4 of our electricity usage.
That’s an extreme situation as Boston is rather far to the north and has poor weather for solar. Covering a skylight with solar is a net energy win, as would taking down the tree(s). Trees are also going to block the front of your building even more than the southern facing roof.
Getting shade from other buildings on your roof is also unusual for 2 story buildings.
> Covering a skylight with solar is a net energy win, as would taking down the tree(s).
Solar isn't the only thing people are optimizing for -- skylights and trees are pleasant. The trees also aren't on our property, and even if you removed all the parts over our property our east-facing roof area would still be completely shaded.
> Trees are also going to block the front of your building even more than the southern facing roof.
It depends on the situation. Our roof is East-West with a big Gable dormer halfway along the West side. The East side is fully shaded by a tree, as is the back of the West side, but the front of the West side and the South facing side of the dormer get good sun (and now have solar). Our house faces the street, and there aren't trees in front of our house, so front-facing solar would work well if that was a thing people did.
> Getting shade from other buildings on your roof is also unusual for 2 story buildings.
I agree it's unusual in general, but houses here are close enough together that some roof shading is the most common situation.
It’s 42.3601° N closer Toronto at 43.6532° N than New York City at 40.7128° N.
By comparison Miami is 25.7617° N and still above the Tropic of Cancer 23°26ʹ which is the furthest north you can get where the sun is ever directly overhead.
PS: Europe is kind of deceptive as the equator is so far down below the bulk of Africa. But so many maps stop at the top of Africa it’s easy to think of it as some sort of mid point. Further, due to weather patterns it’s warmer than you would expect.
There are clearly plenty of places where it wouldn't make sense. On the other hand, I can easily imagine that there are a million houses in the Great Plains and desert west where trees aren't a big problem.
> The garage door is a rather complex place to put them as their are a huge range of styles and you would need to either fit the opening and replace the mechanism, or design a door for each existing type sold. On top of that wiring up solar to your house is the tricky part not putting something on a roof.
I think to be practical you'd need a manufacturing line that could build variable sized doors. Since I'm a software engineer and not a manufacturing engineer I don't know if that's economically feasible or not.
The garage door is a rather complex place to put them as their are a huge range of styles and you would need to either fit the opening and replace the mechanism, or design a door for each existing type sold. On top of that wiring up solar to your house is the tricky part not putting something on a roof.