C8 is a good state if you can get it. Intel is really good at this. They don't even bother energizing the L3 caches immediately when exiting deep package C-states. But there are lots of conditions that will inhibit C8, notably an Ethernet link on a NIC capable of PTP. This is why wireless is better.

Although Intel processors are efficient, modern AMD processors have much higher idle power usage, due to their chiplet design. They typically use at least 20W more power.

I suggest not putting a ridiculous GPU in your backup server.

You can use nvidia-smi to set a target maximum power draw and performance mode to bring idle power levels down. Also make sure your computer is using the server/headless mode driver to keep idle power consumption down.

If one can afford a GPU with a MSRP of $1999 and was scalped for $2999 during the initial craze, you are probably not struggling to pay your electric bill.

Only put the RAM you need in the box, use peripherals with working ASPM, attach them to the northbridge PCI ports instead of the CPU's root ports, use wireless instead of wired networking, and don't attach a display.

1W is a unicorn as just plugging in a power supply with no PC parts hooked up will register 1W. <10W is more realistic. Select a PC that can run off a laptop charger. Check manufacturer spec sheets for the idle power consumption. Don't install any PCIe cards or hard drives. Use powertop --autotune.