In Captain America: The First Avenger Red Skull used the Tesseract (Space stone) as a power source for Dr. Zola' weapons. But the tesseract isn't the Power stone. Then how could it be used as a power source?
2See my comment to the question Why is the Space Stone wrapped in a cube and called Tesseract?. Back then the writers didn't know that the Cosmic Cube was going to be an Infinity Stone (in the comics Cosmic Cubes are not the same as Infinity Stones).– Oliver_CNov 8, 2018 at 19:27
Just because it's not the Power Stone doesn't mean it doesn't have power or energy of its own.
The tesseract is a mystical object with enough power to contain an Infinity Stone, in fact the Space Stone and being able to use a fraction of the power would be quite something.
Indeed, Arnim Zola manages to tap into just a small fraction of the energy..
DR. ARNIM ZOLA The exchange is stable. Amazing. The energy we’ve just collected could power a battleship. Ten battleships. (beat) This will change the war.