This is exactly why I’ve to replaced my home server by a low-power x86 NUC instead. No custom build needed to run NixOS and idle power consumption turns out to be slightly lower than the Raspberry Pi 5.
Idle consumption is truly horrid on the Pi 5, even with all the hacks and turning absolutely everything off and hobbling the SoC to 500 Mhz it's imposible to get it under 2W. I'm convinced that the Pi Foundation doesn't think battery powered applications are like, a thing that physically exists.
I’m using an ASUS NUC 14 Essential Kit N355. It’s a bit more expensive than the Pi 5, but also more powerful (8 cores and decent GPU). There is also a more affordable N150 model. And even lower budget are the N150 mini PCs from Chinese manufacturers, but they often mess up things like cooling in a hardware revision (compared to the favorable review that you’d read).
And forgot to mention this before: Intel CPUs with built-in GPUs have very performant and energy efficient hardware video codecs, whereas the Raspberry Pi 5 is limited and lacks software support.
I get 3-5W, mostly 4W on my N100 nuc. WiFi disabled through bios. And I ran powertop and made the suggested changes. 1 stick of 16gib lpDDR5, 1 nvme ssd, 1 4TB SATA ssd. Under full cpu load usage goes up to 8-12W. When also the gpu is busy with encoding the consumption grows to 20-24W. This is with turbo clock enabled. With it disabled power draw stays around 4W, but it is annoyingly slow I enabled turbo again and just content with the odd power peak.
I am not the OP, but I got an $150 (at a time) fanless quad core Celeron box at Aliexpress about 5 years ago, and it just runs with zero problems with openmediavault and dockers. Attached is external HDD over USB 3, it’s still fast enough (and the HDD is the bottleneck, not the USB interface).
Few months ago it was possible to get Intel N100 (i5-6400 performance at much lower power) based mini PC with 8GB RAM and 256GB SSD for 100-120 USD on sale. Unfortunately, 'rampocalypse' happened.
