if you manually turn off the integrated graphics in the energy saver tab then the system is force to use discrete graphics all the time. for mild usage it will just hog up battery juice.
naturally, nvidia optimus will use discrete graphics when system process idles to extend battery life, and switches to discrete graphic processing when a demanding load requires it. like video games or photoshop or auto cad or the likes. that's what the discrete video chip is there for.
although nvidia driver will down clock the GPU while video processing is at idle, but i wouldn't advise running it all the time as it's just hogs energy.