I use a cheap power meter for initial testing and a tasmota plug for having a monitoring solution as well as a second measure and a possibility ro remote hard reset in case of a freeze or something.

Turns out that power supply and motherboard are the most important to save power - besides low C-states (powertop). I had best results with Fujitsu D3x17 / d3644 and Gigabyte C246 wu2.

Today these are unicorns not worth hunting for. Like I said: No modern server grade board is that good while being cheap. You could take a look at

  Kontron K3851-R ATX

If i remember correctly. Kontron bought Fujitsus Mainboard segment a while ago.