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 After having this revelation, I scraped the entire website and found that Wolfgang's data is no longer current.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:
So with 20W DC draw, a PSU at 85% vs 90% efficiency would be a difference of 1.29W from the wall. Probably not worth spending more money on that 5% difference, but for the same price might as well get the best you can.