Why "produiraient les panneaux"

Carl, 

have a look at ‘ stylistic inversion after a relative pronoun ‘ in the link below. It is formal and optional. 

 https://www.lawlessfrench.com/grammar/inversion-with-nouns/

Thank you Carl from bringing this up and thank you Maarten for providing an explanation! I am currently reading "L'Élegance du Hérisson" and I stumbled about inversion in relative clauses a lot, scratched my head but never bothered to look it up.

 

I have now had a look in one of my favourite French grammar books ("Advanced French Grammar" by Monique L'Huillier). I would like to quote from the book the rules when this inversion can be used in relative clause because Lawless French does not go into this:

 

a. "If the relative pronoun is a direct object and there is no other complement"
Example (from the beginning post):
"On pourrait utiliser l'électricité que produiraient les panneaux solaires [...]"

The inversion is not possible, if the subject is a pronoun e.g. only "La voiture qu’il a achetée" is possible.

 

b. "If the relative pronoun is an indirect object itself and there is no other complement"
Example: "On nous a donné la liste de tous les avantages dont jouissent les employés rattachés au Service de la Documentation."

As the text explains, this is an example where inversion is even recommended because of the long subject.

I assume the restriction for pronoun subjects applies here as well but is not stated in the text.

 

A very common form of inversion in relative clauses is "qu'est":

Example from the novel: "Apparemment, de temps en temps, les adultes prennent le temps de s’asseoir et de contempler le désastre qu’est leur vie."

More on this topic and more examples can also be found here:
"https://www.madbeppo.com/french-language/que-etc-and-inversion/"