It is a safe inference that Merlin requires a constant source of
electric power.

The source of power would be internal or external.

If external, the source of power would have to be concealed to the same
extent as Merlin--no transmission lines connecting to Merlin.
Hydroelectric fails that test. Nuclear energy works, but why build an
external reactor?

So internal. A nuclear reactor integral to Merlin itself. Fuel rods do
not have an infinite lifetime. Output falling below a certain point, and
Merlin can't operate.