Until a weather event blocks out most of the sunlight. An extreme scenario would be what happened to the dinosaurs, however smaller scale versions or that, such as large volcano eruptions, seem entirely possible and could heavily restrict the amount of sunlight you have access to for long periods of time.

Portugal lies in Southern Europe, we get plenty of sun, and we make heavy use of solar, but that still isn’t enough sometimes, and I’m pretty sure we sometimes get our energy from Spain, who themselves use nuclear.