Why does GPU consume so much power?

Note: This is intended as a short comment on the answer posted by @user1850479 (Why does GPU consume so much power?); the only reason it exists as an "answer" is that posting a comment requires 50 reputation, which I do not have. If anyone wants to help out by copying its contents to a (true) comment in the appropriate place, it's perfectly fine by me and this post can thus be deleted. No attribution nor acknowledgement is necessary. License: CC0 Thanks. :)

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It's also worth noting that CPUs executing vector instructions (SSE, AVX variants) can draw more power than they otherwise would just executing standard instructions. This can be observed by running 3D programs and forcing them to use software rasterizers -- in essence, forcing the CPU to take on a GPU-like workload -- such as WARP or SwiftShader, which usually make heavy use of vector instructions. Intel's processors even have AVX-specific power throttling functionality that kicks in to keep the processor within its rated TDP.

A GPU is a many, smaller processors operating simultaneously in parallel.

Each processor contains a lot of transistors. Many processors in parallel means even more transistors.

Every time each transistor switches, it must charge or discharge a parasitic capacitance inside the transistor. So the more often a transistor switches, the more often this capacitance is charged/discharged which means the more power is consumed. GPUs run fast so they switch very often.

So the fact that GPUs contain many transistors switching at the same time (because it's not just one processor operating at any one instant, its many processors operating at any one instant) and switching very often means very high currents to charge and discharge all those parasitic capacitors simultaneously.

A normal processor is more complex and has more transistors than than any single processor inside a GPU since it is has higher functionality, but it is also doing only one or two things at a time so only a small fraction of the transistors are switching at any point in time.

This question asks why a faster clock in a given CPU requires more power, and has a number of really good answers.

Take those answers, and add to them the fact that the power consumption is a consequence of not just how fast the logic involved is switching, but how much of it there is -- and a GPU has lots and lots of logic, all of it switching madly when it's hard at work.

A GPU is basically a LOT of simplified CPUs in parallel. Each of them is not as capable and flexible as a real CPU, but there are thousands of them to give this massive parallel computing performance.

But this also means that it takes many billions of transistors to build a modern GPU. And for logic chips we use FETs, so with every clock cycle all the billions of gate capacitances have to be charged and discharged. This is where the large amount of power is going.

GPUs consume a lot of power because they have a large number of transistors switching at high frequency.

Relative to a high end GPU, CPUs usually have many fewer transistors switching at any time and so do not require as much power. This is not always true, you can get low power GPUs and enormous server CPUs with very high power requirements.