For Ryzen CPUs (as this is automatically enabled on Pre-Ryzen) if you go into your BIOS and find the CPU Options... there are various Advanced Features disabled; such-as Virtualisation along with Cool'n'Quiet Mode.
Enable Cool'n'Quiet Mode, this allows the CPU to run as low as 25MHz when "Not In Usage"
For Navi (currently this is just RX 5700/XT at Retail) they will automatically scale down to 6MHz Clock and 100MHz Memory Frequencies; which means the card is using ~8w Power (although this actually fluctuates between 8 - 26w, as the memory has a habit of randomly jumping up and down) … which is where it will stay whenever you're on Windows Desktop, Watching Netflix, YouTube, etc.
And as a note., while Auto-Undervolt will not really improve this "Idle" Mode; it does help reduce how much is used while Gaming.
Personally I have my CPU (Ryzen 5 1600) Undervolted to 1.20v at 3.7GHz "Boost" / 3.4GHz "Base" … which does reduce it from 65w (62.3w) Peak down to 58w Peak; no I'm not hitting 3.9 - 4.0GHz, such-as what the CPU is technically capable of but that extra 200MHz only really helps in Benchmarks... rather than Real-World workloads.
And the Auto on my RX 5700 XT (Reference) reduces it from 192w Peak down to 180w Peak; which frankly isn't that far off my RX 480 OC (Stock) while providing negligible performance difference over Stock ("Manual") and still offering approx. 2x performance over my RX 480 OC Clocked at 1420MHz (i.e. RX 580 OC Stock)
If only the RX 5700 XT had been £250 - 300 instead of £350 - 400., then I'd never have even given the upgrade a second thought... that it came at a premium; while sure, still remains good value was definitely a notable premium for a Mainstream Card; meaning it's a bit hard to justify.
Still, on the other hand... given I'm not constantly gaming nowadays and end up working on the Desktop more (Netflix, Office, YouTube, Blender/Maya, ZBrush, Photoshop); there is the argument that over 24 - 36 months (the expected lifespan before I upgrade again) … I will be saving quite a lot in terms of Energy Costs.
(Potentially ~£25 - 35 / Year; so over the 2-3 Year Period, sure it works out to costing the same overall)
Mind where I think this will make the most sense is in the Laptop / Notebook Versions., as they'll still be able to deliver a "Full" Experience while idling at almost no power consumption; and as noted it's the MEMORY that causes the major peaks in Power Consumption... the GPU Frequency itself can rise to 150MHz before it adds more than 1w Power., which is all it needs to run various eSports Games on 'Default' Settings 1080p60 V-Sync. (Fortnite / Overwatch / Diablo 3 / etc.)
