I'm running an A1SRi-2758F motherboard with an Octacore Intel Avoton CPU. The CPU is advertised as 20W TDP (Thermal Design Power.) Is my assumption correct that when the CPU generates 20W TDP, it the real power consumption at that point in time will be greater than 20W? That's because not all the power will be dissipated in the form of heat, I think. Is there a rule of thumb how much wattage the CPU will need when generating 20W TDP? Something 25W, so 80% of power going into the CPU will be dissipated as heat? Or would it be more like 40W? I'm looking for a way to estimate the real power consumption of a CPU when only the TDP is shown in the data sheet.
Also, TDP and power usage are different things. Although TDP and power usage are related, TDP is a measure of heat, not power. It can be used as a ballpark estimate for how much power a card can use, but it shouldn't be used for exact calculation. Even a reference R9 290 (300W TDP) will use well over that amount of power. And of course, as elvenson mentioned above me, non-reference cards use a LOT more power in many cases. Gigabyte's GTX 970 G1 uses around 270W, while the reference card uses roughly 180W, but PCPP still says 165W for both of them. The PSU calculator also doesn't compensate for the fact that most PSUs can't deliver their rated wattage at load temperatures. Of course, this can be remedied partially by buying a quality PSU, but it isn't good for even the best PSU to be put at full load for long periods of time.
Use this link to calculate: http://www.extreme.outervision.com/psucalculatorlite.jsp