I built a system ground up with a focus on power-usage a few years ago. You can go far down the rabbit hole when you start googling, but the key take-aways for me are:

• Eliminate features you don’t really need. E.g. do you realy need a RAID 10 configuration with 4 disks? Or can you get by with 2, or maybe even 1 (which might mean you experience downtime while waiting for a new HDD, but you do keep backups right?).
• Standard 300-400w ATX power supplies are inefficient under low loads (meaning much AC goes to waste converting it to DC). Use a PicoPSU (or a motherboard that runs on DC). Search for a high-quality power adapter (e.g. Leicke) that’s rated for low loads. Just make sure your PSU can handle booting, when your system will draw much more.
• Install drivers for motherboard features you don’t need (like soundcards or wifi) and then switch them off in the OS as this usually sets them in a more power-efficient state rather than disabling them in BIOS.
• SSD’s use less power than HDD’s but get expensive for higher capacities. I use a Nvme SSD as a startup drive and to host my containers, and 2,5" 5400rpm HDD’s for storage. Those disks are slow but get the job done, and despite running 24/7 there hasn’t been any failures.
• I installed a passive cooling-block on the CPU and optimized the airflow in the case. It has 3 (small, as it’s a 1u server case) case fans that are controlled by fancontrol and only kick in when the cpu’s temp is > 60°C, which is not very oten. This also means it’s very quiet ;-).
• Make sure to configure all power settings in the OS of your choice (powertop and tlp for linux).