first: GetPowerCost doesn't work.
there is no SetPowerCost function from where to call GetPowerCost, so it's kinda useless.
the torpedoes won't drain any power from your battery except for the tube itself, which is powered by the torpedo system.
second: you are always draining power from your battery, and the warpcore is always making up for that drain and recharging the battery, as long as you drain less power than the warpcore can provide.
.... then you have the tractor system which drains power directly from the battery, but only when they're firing.
...and you have the cloaking system which drains auxiliary power directly, but only when it's active.
if you take a look at the warpcore outputs in MPE, you'll see that the battery conduit is always bigger than the warpcore conduit.
that's because the warpcore is added on top of the battery conduit.
so, the only way to make your torpedoes drain power in a way that it would present you with a management problem, would be to make your torpedo system drain enough power to exceed that of the warpcore and enter the drain margin of the main battery.
but, you would also need the advanced power mod to make those allocations specifically to the torpedoes, because if you cut power to weapons you would also be powering down beams and cannons proportionally to their total power drain.
....I've recently learned that trying to change just one thing in the game almost always ends up creating ten new problems to change

(you might be able to achieve what you want through FTech or NanoFX.... but anyways, you'll need to edit more than just the torpedo)