I always just assumed he wasn’t as powerful as the Emperor. And I see no reason to abandon that theory.
Well he isn’t. Vader is a Master of the Dark Side but even lesser Sith can conjure up Force Lightning with varying degrees of effectiveness.
Show me anywhere in the movies where that’s indicated.
There’s The Emperor and Vader. There are no “lesser Sith.”
Dooku is a lesser Sith and he can conjure Force Lightning.