HT does add power usage, but not alot, best performance i would disable HT, and keep all cores active, but disabling cores would prob get u the most power savings. But not everything uses HT, and if its an i3 it would be pointless to disable cores, an i7 should do ht off, or ht enabled and disable one or two cores, two cores would basically make it an i3 and a waste of an i7.
disabling HT basically makes an i7 a i5, and a i3 a Pentium. But disabling HT would be best option for performance and power usage.
A good way to test would be to use something that monitors the wattage going to the pc from the outlet with core or HT disabled.
I3 does become a pentium,but i7 will become a xeon e3-1220(v2)(v1)
Because i7 has 2 more mb of cache.