In principle you can shut off anything when it's idle and get its power consumption arbitrarily close to zero. Discrete GPUs obviously use more power because t

In principle you can shut off anything when it's idle and get its power consumption arbitrarily close to zero. Discrete GPUs obviously use more power because their use case is when the iGPU isn't fast enough. Hypothetically you can make a discrete GPU with the same performance as an iGPU and it would be no more power hungry. These have occasionally been available, basically monitor cards for CPUs without an iGPU etc. But the ones with a lot of fans and VRMs have them because they do indeed use a lot of power -- which isn't the same as having poor performance per watt, because they're also really fast.