For an overview of power armor models, see Power armor.
The following is based on Van Buren and is not canon.
“A suit of power armor built from designs for the pre-warT-51b prototypes. Power armor has a built-in fusion generator capable of supporting the massive energy needed for its strength enhancers and other mechanized components”— In-game description, Van Buren tech demo
Joshua Sawyer stated that, "the design was intended for the 'real' F3 power armor (T-51b), we wanted to have an alternate texture for the armor Armstrong describes, T-45d, which ran off small energy cells. It would only have appeared in the tutorial as a 'back in the day' detail."[Non-game 1]