that statement doesn't align with my personal experience due to the ATX power supply used in a PC.
To get a baseline, would you be in a position to measure consumption of your setup and publish it here?
I think it is not a question of starting a competition to see which one consumes less. It is clear that an ARM platform will consume less than a conventional PC. What I mean by that phrase is that the difference in consumption is not considerable considering the other advantages, at least in my case.
And if you want, let's go to the numbers, so as not to leave you without an answer. I just did a measurement of my system, I have disconnected it from the UPS and I have started it with a meter. The total result has been 52W. Let's analyze this. First, my CPU is about ten years old. I use it because I have it and it's free. It is clear that if you wanted to make a computer now, you would buy something else with lower consumption. Still look at the numbers. In my system there are five hard drives, which are also always spinning, I don't put it to sleep (my manias), and four fans. This is what they consume:
DISCS
1 x TOSHIBA N300 12TB _________________ 6,5 W __x1__ 6,5 W
2 x SEAGATE ST5000LM000 ____________ 2,1 W __x2__ 4,2 W
2 x WDC WD40EFRX ___________________ 4,5 W __x2__ 9,0 W
1 x OCZ VERTEX 2 _______________________ 2,0 W __x1__ 2,0 W
FANS
3 x BE QUIET SILENT WINGS 120MM ____ 1,44 W __x3__ 4,3 W
1 x ALPINE 12 CO ________________________ 1,08 W __x1__ 1,1 W
TOTAL 27,1 W
Just half of the total consumption, coincidence, is taken by hard drives and fans. This cannot be removed with any system, it is a fixed consumption. You could remove the fans if you want but it is not recommended. My disks are always between 30th and 35th, each one who thinks if it is worth reducing the life of the hardware. The last album I bought cost me 300€. The truth is that I prefer to pay a few euros more a year in electricity consumption and that the hardware last me a few more years.
And now you will tell me my system consumes 2W at rest. Very well. That means you stop the discs, you do the math when they go bad. If you keep them running, the consumption would be 27W plus the 2W of the ARM system on equal terms. Total 29W. Any modern CPU would lower that total consumption of my intel 3225 by at least 15W. Which leaves us an approximate consumption of 37W compared to your 29W. For a difference of 8W more than you I have an upgradeable, cooler and better maintained equipment. When the RPI5 comes out you will change it entirely. I can upgrade my RAM by adding more, I can even change CPUs, you can't. I can add expansion cards, connect 10 or more hard drives, etc.
Regarding the issue of the power supply, just say that the actual consumption is what you are spending at that time plus the percentage of loss. Calculate a source for the consumption of the hard drives you want to have. Generally with a 300W it is more than enough and it will consume depending on what you have with less loss. To that add that a standard source will also make your hardware last longer, it has better protections than an external source in general.
I insist, I respect all opinions very much, and of course ARM systems have their market. But I still think that it is much better to use a conventional system for OMV in a home, it has much more possibilities in the long term.
PS: I have two RPI3s and I am happy with them, I use them as clients with Kodi. I do not consider using an RPI4 as a server because of everything I said before. 