Learn Swedish: Det är solcellen som genererar elen på taket. - It is the solar cell that generates the electricity on the roof.

This is a cleft sentence, which is used to emphasize a specific part of a statement. By phrasing it as Det är solcellen som... (It is the solar cell that...), the speaker puts a strong, exclusive focus on the solar cell specifically, rather than just stating a neutral fact.

How could we add the lesson's focus phrase, inte minst (not least), to this sentence to add even more emphasis?

You could place it right before the emphasized noun to single it out among other factors: Det är inte minst solcellen som genererar elen på taket. This implies there might be other power sources contributing, but the solar cell is a highly significant one.

Why is el (electricity) in the definite form (elen) here?

Even though electricity is an uncountable noun, Swedish often uses the definite form when referring to a specific, contextual instance of it. Here, elen refers specifically to the electricity being generated on that particular roof, rather than electricity as a general, abstract concept.

How is the gender of the compound word solcellen determined?

Like almost all Swedish compound nouns, its gender and declension are determined by the final word in the compound. It combines sol (sun) and en cell (a cell), so it takes the en-word definite ending to become solcellen (the solar cell).