La Fábrica de Luz is a former hydroelectric and thermal power station on the banks of the Sil river, converted into a museum of energy and industrial heritage. The building itself is a striking piece of early 20th-century industrial architecture, all brick and cast iron, and it has been sensitively restored rather than scrubbed clean — the rust and grease stains of decades of operation remain visible in places, giving an honest sense of what working in such a place was like.
The museum covers the history of energy production in the Bierzo, from early water mills through the coal-powered electricity generation that sustained the region's industry for most of the 20th century. There is also a section on renewable energy, which feels slightly incongruous but serves as a reminder that the mountains around Ponferrada are now dotted with wind turbines. Entry is free or very cheap; it makes a useful complement to a visit to the castle.