My LibreElec system is a J3355B and an SSD in a 10+ yr old gateway desktop chassis with a ~10yr old Antec green PSU or something like that.
I just swapped out the PSU for a pico PSU because I wanted to get rid of the fan noise. Incidentally pico PSU's use less power, so I thought I'd share my results for anyone looking at putting together a box.
Overall the switch saved a little over 8 watts on average compared to the old PSU, where I live that's a little over $11/yr (my system is on 24/7).
My pico PSU (85W non-WI) & AC/DC converter cost me a little under $40 shipped form eBay. Again, I did this because I hated the fan noise in my living room, it doesn't make sense for power savings.
However if you're building new and don't have a PSU lying around, it might make sense for you.
You can get a bargain bin PSU starting at about $20, if you care about 80 Plus, then you start at about $35.
In these cases the picoPSU with it's totally silent operation and low power usage might be worth it!
Old PSU | picoPSU
Idle 23W | 14W
400 Mbps 4k HEVC 10 bit 28W | 21W peak, then 15W
1080p HEVC 18W | 10W