There's Nvidia's Optimus switching technology, but last time I looked, that was available only in some select laptops. So yeah, there's really no way to do what you want. I'll echo what others have said though: these days, all hardware is much more power-efficient, so even when sitting at idle, you're not looking at that
big of a hit. Take a look here
for some system-wide power consumption numbers with that graphics card.