I was thinking of a 2d canvas context. Basically, I would expect battery consumption to be lower when WebGl is rendering 2d than it would be if it were rendering 3d. It just seems intuitive that 3d requires more computation. But I really don't know. I've never seen any battery usage comparisons.
the browser may be able to optimize the 2D canvas operations by using the GPU more efficiently, but drawing with the webgl stack is going to suck the same amount of battery whether doing 2D or 3D. After all, 3D is really just correctly positioned 2D triangles.
