I have an HP g3 mini and a Dell Optiplex flying around, both similarly specced. The HP has an i5 6500t and 16gb DDR4 RAM, the Dell has 8gb DDR3l, so nothing too different.
However, the Dell draws around 15W while idle, the HP one 5W.
The only difference I could think of (and that is in my power to change) is the PSU. The Dell has one of those SFF PSU for up to 180W while the HP has an external 65W power brick with a barrel jack.
So my question is: Does anyone have experience with one of those Pico PSUs? I guess they should be more efficient? I’m not planning to put anything power hungry into the optiplex.
The answer for your question is ‘no’.
You’re never going to reduce power usage substantially by swapping PSUs, because there’s just not enough efficiency gains to be had even if a Pico PSU was more efficient which they really aren’t.
You say the hardware is ‘nothing too different’ but you mention ddr4 vs 3, which makes me think the Dell is a generation or few older which could easily impact power draw by 10w.