the 2660 actually has a maximum of 95W TDP, the actual draw is hard to spike to 90, and those platforms tend to be efficient.
Each blade should be in the 200W realm, slightly above under pressure with all wheels spinning (assuming they get kitted with local storage and that it’s a relatively power hungry spindle 7200 rpm you’re willing to cool).
3.5KW actually SHOULD cover two of those enclosures, but if they run at capacity for a long time, or a fault in the PSUs were to kick in the redundant one while the other still drew, you could, however unlikely, get a spike the UPS wouldn’t be able to deal with. So 3.5KW is a beggar kind of coverage, and seems slim margin.
8KW however seems largely excessive, it would support a double failure with double full draw for minutes in case it happened in a protracted blackout.
While theoretically possible, is probably slightly less likely than you having to fret about a meteorite strike on your renderfarm (and therefore setting up an anti meteorite laser capable anti air network around the facility).
When dealing with UPS everybody selling errs on the side of caution, both because they want to skin you for all the money they can, and because as you are fundamentally buying insurance, they want to cover their arse and make sure it’s full insurance, even in unlikely events.
Last but not least, bigger UPS also have better amperage to the batteries, that means the unit will stay up longer. So it’s also possible you guys asked them a minimum time (or they considered one) for the UPS to hold up while you shut down, and the amperage of the 8K unit is needed.
I’d have a chat with them and ask by what parameters they reckon they need to cover well over twice the actual draw, which is a bit of a rule of thumb, but might be overzealous in your case.
Over twice the load with the mains firing on all cylinders for a protracted time seems excessive even when accounting for some multiple draws (redundant units kicking in while anothe burns out). Something in the 5-6KW range would more than cover it, but again, they might have made honest considerations in offering an 8KW unit, or be leaving space for upgrading the CPUs to a higher draw.
3.5K would barely manage but be on the short end of things, and that’d defeat the point of an UPS.
