The Radeon Pro 560 on my MacBook Pro can draw "up to 75W". I don't know anything about how it manages power, but I do recall that Apple recommended in a WWDC session on writing energy-efficient apps that one should be careful to not trigger the discrete GPU when the integrated Intel GPU is sufficient. So that leads to another question: on my machine, which GPU would Webrender be using? I suppose I could find out experimentally, but I thought I'd ask in case someone here knows.

For WebRender, we targeted the integrated GPUs. You still need some logic to ensure you don't accidentally kick the discrete one into gear, but the integrated ones definitely have enough performance for almost anything WR will throw at it.

Could you elaborate the edge cases which fall beyond "almost anything WR will throw at it" ie what are the use cases where you would end up waking the discrete GPU?

I would personally imagine anything more complex than Quake Live would probably trigger the discrete GPU.

Is there any requirements for a minimum version of opengl for WebRender that you know of?

That's encouraging news. Thank you!

Since the 15" MBP charger can deliver up to 87W, if you have full power draw on the MBP will you actually not be able to fully supply power over AC? I assume that the CPU also takes up around 20-30W.

The bigger issue is that mac cooling is usually very small. You'll run into thermal throttling long before you can dissipate a 75w TDP.

The MBPs pull power from both AC and the battery if needed.

A modern high-end GPU can draw upwards of 200w.

Edit: Not sure why I'm being downvoted for stating a simple fact. The top-of-the-line AMD and NVidia cards have TDPs of 200-250w.

Your comment is both true and utterly irrelevant to the person you replied to who asked about comparison between CPU and GPU efficiency.

The important information to answer the question is how much power the GPU will use, not how much it can use.

But how much it can use gives you an upper limit on how much it will use. To boot, that also gives you an idea of how much power one can expect to draw under full load at any given time over any given time frame.

High end desktop CPUs draw around 90W and 200W GPUs can do one or two orders of magnitude more computation in specialised workloads than CPUs.

TDP for the i9-7980 is 165W, and the 1080ti is at 250W