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?
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.