Learn French: Ensuite, ils ont monté les panneaux photovoltaïques sur la pente. - Then, they took the photovoltaic panels up on the slope.

Monter is usually a verb of motion that takes être in the passé composé when it is used intransitively (without a direct object) to mean "to go up." However, in this sentence, it has a direct object: les panneaux photovoltaïques. When you "take something up" or "carry something up," the verb becomes transitive and must switch its auxiliary to avoir.

If the direct object is plural (les panneaux), why doesn't the past participle monté have an "s" at the end?

In the passé composé with avoir, the past participle only agrees with the direct object if that object comes before the verb. Here, les panneaux photovoltaïques comes directly after ont monté, so the participle remains unchanged as monté.

Does monter always mean "to take up" when used with avoir?

Not always. While it means "to take up" or "carry up" in this physical context, it can also mean "to assemble" or "put together" (like furniture, a film, or a project). Depending on the broader context, this sentence could also be understood as "they assembled the photovoltaic panels on the slope."

Why do we use sur la pente instead of à la pente?