@Timo, perhaps by TNG time and later, that may be the case. But in "The Galileo Seven" it sounded like an ad-libbed idea:
SCOTT: It's dangerous, but it might work.
SPOCK: Go, Mister Scott.
SCOTT: I can adjust the main reactor to function with a substitute fuel supply.
SPOCK: That's all very well, but we don't have a substitute supply.
SCOTT: Aye, we do. Our phasers. I can adapt them and use their energy. It'll take time, but it's possible.