thanks. I ended up lowering max CPU % in power options in windows and limitted my cpu to 800 Mhz at all times, fast enough to browse the web. Idle usage dropped by 2-3w.
Decreasing voltage makes far more of a difference than crippling clock speed. The latter doesn't save anything if it now takes 3.5x longer in the load state to do the task (800MHz vs 2700MHz). If you really want the lowest power consumption, and your motherboard supports, it, then it's worth doing a little manual undervolting, put max CPU% back as it was and let SpeedStep do its job of keeping at 800Mhz 95% of the time anyway but allowing more speed when you need it (which as Yuriman says - it will "race to idle" faster). It's not just instantaneous power consumption that counts but power used per task over time. Underclocking doesn't improve that (because it increases the % of load time), but undervolting does.
Likewise, it's not just "cores" but "uncore" (eg, L3 cache, iGPU, etc) plus all other peripherals (SSD or HDD, USB devices, monitor, printer, etc) that draw power anyway regardless of CPU load. So 800MHz will use more than just 1/3rd of the power of 2.4GHz - leaving you worse off if it's now loaded +250% longer just to save 10-20% power.
If you really want a true "Green PC", you basically go through the whole system:-
- Gold / Platinum rated 80+ plus PSU at the lowest wattage you can get, eg, Seasonic G360 is good for a "full ATX", but Celeron's with no dGPU you could even go for a 60-160w Pico-PSU depending on your setup.
- 2.5" HDD's use less power than 3.5". SSD's use the lowest overall (especially Samsung's) and are incomparable for performance.
- LED TFT monitors use less power than the old CCFL LCD's (you could knock 10-15w off with that upgrade alone), ie, an old CCFL LCD TFT that draws 35-40w would be reduced down to 25-30w for LED screen at same screen brightness & size (and do so 100% of the time whether the CPU is loaded or not).
And general common sense, eg, power off active speakers when not using them, aggressive standby timers if you have a habit of leaving the PC idling for more than 30mins or so, etc.