"Strahl" means ray and "Strahlung" means radiation. Notably "Strahlung" usually is uncountable and has no plural as it is the entirety of emitted rays, who jointly form the radiation (field); this argument holds both, in German and English. Sometimes the plural is used to describe different types of radiations, though in my experience phrased differently, like in "Strahlungsarten".
As to your example, the more ideomatic expression is "mit etwas Energie erzeugen":
Wir müssen die Sonnenstrahlen / Sonnenstrahlung nutzen, weil man damit umweltfreundliche und nachhaltige Energie erzeugen kann.
"Sonnenstrahlung" is a bit unusual and one would rather use "die Strahlung der Sonne" or "der Sonnenschein"
Or leave out both:
Wir müssen die Sonne in Anspruch nehmen, weil sie umweltfeundliche und nachhaltige Energie liefert.