Whereas Synology or other NAS manufacturers can tell me these numbers exactly and people have reviewed the hardware and tested it.
I can buy a NAS, whereby I pay money to enjoy someone else's previous work of figuring it out. I pay for this over and over again as my needs change and/or upgrades happen.
Or
I can build a NAS, whereby I spend time to figure it out myself. The gained knowledge that I retain in my notes and my tiny little pea brain gets to be used over and over again as needs change, and/or upgrades happen. And -- sometimes -- I even get paid to use this knowledge.
(I tend to choose the latter. YMMV.)
For example my ancient tplink TL-WR842N router eats 15W standby or no, while my main box, fans, backlight, gpu, hdds and stuff -- about 80W idle.
Looking at Synology site the only power I see there is the psu rating, which is 90W for DS425. So you can expect real power consumption of about 30-40W. Which is typical for just about any NUC or a budget ATX motherboard with a low-tier AMD-something + a bunch of HDDs.