I needed a PSU that was efficient at 20W and so I consulted Wolfgang's PSU Low Idle Efficiency List, because it was one of the first results on YouTube. However, the first thing you may notice is that the reports it links to are no longer in the same place on Cybenetics' website. If you manually check the PSU model on their website, you actually get different numbers for 20W. After having this revelation, I scraped the entire website and found that Wolfgang's data is no longer current. I don't know why after a second evaluation Cybenetics had different results, but am inviting you to ask with me.
Here are the top 150:
I mean why 20W?
NAS. 1 drive = maybe 8 watts. Motherboard itself = 12w.
Why does it matter?
Power ain't cheap, especially in a NAS configuration where it'll presumably be running 24/7.
Power is cheaaaaaap https://www.globalpetrolprices.com/electricity_prices/ unless you pull like 100 W. Having said that, I'd go for ARM or x86 SoC that pull like 15 W through a wall wart as opposed to a full ATX PSU.
1.3 W is around 1 kWh per month. I checked on local electric company's website, that's around 0.2 USD per year here. Prices may be diffferent wildly worldwide, but we are definitely speaking about a less than 10 USD difference yearly.