What are the most important components for low idle power draw?

[jtabc]

I recently built a few computers this summer for various purposes and I was surprised at how big the difference in idle power draw between the two of them are (when running Windows 10 and the same power plan settings).

PC 1:

• HP Elitedesk 800 G4 SFF (motherboard, PSU, and case)
• Intel i7 8700
• 2x 16GB DDR4 (running at 2133 MHz)
• 1 Samsung PM991a NVMe SSD (256 GB)
• No GPU, no other PCIe cards
• The 1 Gigibit ethernet is plugged into the LAN (into a 1 gigabit switch)

PC 2:

• MSI Pro B650M Motherboard
• EVGA 600W GE Gold PSU
• AMD 7950X CPU (45W Eco mode preset, -0.105V undervolt)
• 2x 32GB DDR5 (running at 4800 MHz)
• 1 Teamgroup MP33 NVMe SSD (1 TB)
• No GPU, no other PCIe cards
• The 2.5 Gigibit ethernet is plugged into the LAN (only into a 1 gigabit switch, though)

PC 1 draws 6W to 7W from the wall when idle, while PC 2 draws 43W to 49W when idle. I'm trying to understand what the contributors to this are. I have some suspicions, but I'm not sure what their order is, in terms of importance:

1. The HP Elitedesk has a platinum PSU in it, and it is rated for a lower wattage than the EVGA PSU. The PSU should be more efficient for both of these reasons, although I wouldn't expect these two alone to explain a 40W difference.
2. The AMD 7950X processor is simply not as efficient at idle as the i7 8700. I don't know if this is true of all AMD processors in general or just this particular one.
3. The motherboard in the HP Elitedesk might be more efficient than the MSI motherboard since they are designed differently from the get-go (the elitedesk is designed for office use, the MSI motherboard is designed for gaming)

Are my thoughts correct, and if so, which of those that I've listed contribute the most to the difference in idle power draw I'm seeing? Are there other reasons I'm overlooking? Can you estimate, in watts, what each difference contributes?

 

[Tech Junky]

Pcpartpicker.com will give you the bottom line for power on the AMD and maybe point to the culprit. My bare min is somewhere around 80w with a 7900x.

Every little piece adds up though. CPU ram mobo drives etc.

 

[reasonsandreasons]

Chiplet AMD processors have inherently higher idle power draw. Some of this has to do with power costs in chiplet-to-chiplet communication, some of it's due to the I/O die being made on a different/inferior process. I believe the non-chiplet processors (usually the G-series) have better idle performance.

The Elitedesk is probably also using a 12-volt-only power supply solution with better efficiency than a standard ATX unit.