How are you measuring that? I've been scouring the web for low-idle-power builds and 3.5 watts is far below the best builds I've found (assuming that 3.5 watts is measured at-the-wall).
I can't measure values below 10W myself, but with 2 SSDs and 8GB RAM it never goes above that value, even under full load.
[0] https://github.com/R3NE07/Futro-S740/blob/main/power_consump...
The nice thing about n100 Is its ability to disable the iGPU in the bios, saving you a little bit of power. But you have to reset the bios on the board to regain any output if you might need it later.