Games eat a lot of power by rendering this way. So, definitely a concern, and I'd want to see benchmarks before switching from Safari on my macbook.

That said, GPUs are pretty clever about this. A big chunk of power consumption comes from IO. i.e., moving data from the GPU to the off-chip DRAM. Mobile GPUs optimize for static scenes where nothing is changing by keeping hashes of small blocks (say 32x32 pixels) of the frambuffer in on-chip memory. If the hash of a block hasn't changed, they don't bother re-writing that block into the framebuffer. But the GPU is still ends up running the shaders every frame.