The following is based on Van Buren and has not been confirmed by canon sources.
“A suit of power armor built from designs for the pre-warsicT-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
“Though that 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.”— J.E. Sawyer, No Mutants Allowed