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).

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I’ve seen in this thread:

4 acronyms in this thread; the most compressed thread commented on today has 10 acronyms.

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I had an optiplex 3060 micro with 8500t, 24gb ram and 2 ssds idling around 3-5watts.

Something like an old laptop will make a power-saving homeserver. But that won’t work if you want to attach lots of storage.

I don’t think an Optiplex is the most energy-efficient choice. They seem somewhat okay, but you’d need to put some effort in and read some tests and reviews to find a really efficient mainboard and PSU. That’s not easy

You can spin down your harddisks. I have some udev rule that executes hdparm -S60 /dev/sdb after boot. That’ll spin down the hdd after 5 minutes of inactivity. It’s alright for low usage scenarios. And it doesn’t spin up that often because the hdd contains my photos, backups and a few movies. And my operating system and files that are accessed often, are on a SSD. Starting and stopping disks like once a day should work for many years. But don’t cycle it every few minutes.

And obviously, you can also shut off your server over night or just wake it on demand, if that fits your use-case.

Wake-on-LAN is probably what you want, if your specific hardware supports it which it probably does. This is a case of figuring out your exact hardware and a little RTFM-ing about how to enable and use WOL.

As for the drives it, in theory, would add more load/unload cycles to them and thus reduce their lifespan. But, in the real world, that almost certainly doesn’t matter, unless you’re turning the system on and off every 5 minutes: modern drives expect to go in and out of power saving modes and most controllers (especially usb enclosures!) do this pretty aggressively, so a couple more load cycles more-or-less are unlikely to actually cause your drive to fail any quicker than it would anyways.

Wake On LAN maybe?

But that requires a full startup sequence before the file is available.

If you only use it so rarely, why not just connect a USB drive to your network router? Most have an option to serve files from that over the network.