Is there any case law or precedent or? regarding a neighbor's trees whose shadow covers another's roof for a good part of the day and thus block solar panels from working as efficiently as they might. The trees were there first if that makes a difference.
It isn't precisely clear which jurisdiction you are located in (recall that this website handles matters from everywhere in the world).
But, generally speaking, in the United States, you have no right to limit someone's existing tree on their property merely because it casts a shadow on your solar panels. The installer should have known better.
A minority of U.S. states, including California, consider new construction that blocks the view of existing structures a form of "nuisance" that can be abated if it unreasonably interferes with the enjoyment of the existing property. But, that protects existing structures, rather than new ones.
In Japan, there are building code requirements designed to insure that key portions of every home get natural sunlight daily. Again, this only applies to the construction of new buildings.
I know of no law that gives someone who newly installs a solar panel a right to remove or trim a neighbor's tree simply by virtue of doing so. And, without knowing whose law is involved it would be impossible to determine with any reliability.
The property with the solar panel could seek to buy the right to an unobstructed view from the property with the tree, in what would probably be called a "view easement", but that would only happen if the terms were such that both consented and it was written up in a legal document to that effect.