"Tab"? I don't know the term.

Its not a duplicate. I read those, as well as the faq. None of them say how to evaluate power needs of a drive array. See comment to ArtOfCode abovr

@ArtOfCode - But I'm not really after a product suggestion. I want to know, if I have 20 HDDs churning, do I need to allow 20 x absolute peak wattage, or does some kind of averaging take place that its unlikely they will all be exactly at peak at the same time, what safety margin is sensible, or how large HDD array power needs are calculated. That's why I originally asked at electronics SE. (They moved it here, but it seems that was wrong?) I don't want any specific product suggestion, because that's something I'll consider in my own time and depends on a lot else irrelevant to the Q'