If i dont remember too wrong from when i looked at that switch, you are looking more at in the 10-15x area at idle.
I seriously doubt those 10x-15x figures; all the numbers i've seen were for S5148F-ON fully packed with transceivers, in 400W range.
If we're comparing idle consumption i'd expect it to be in 120-180W range, which is still a lot, or ~4-6x vs S5860-20SQ.
But Dell switch has more than double capacity of FS switch - 2x ports, and all those ports are SFP28 vs SFP+; plus 3x of QSFPs.
So it's more fair to compare it with a pair of FW switches, bringing power imbalance to 2-3x (which is still substantial in the long run).
If somebody has S5148F-ON handy it would be nice to check it's idle power consumption.
I've S4048T-ON (48x10Gbase-T + 6xQSFP with similar Atom inside), which is just under 100W idle;
for a pure fiber S4048-ON (48xSFP+ + 6xQSFP) idle consumption was even less.