Example - modern system with CPU power management enabled vs. disabled, power measured at wall.
Enabled - 17W idle, ~90W load in Prime 95
Disabled - 29W idle, ~90W load under Prime 95
Now here's the problem - unless you already know the power management settings in this particular system, the room for error when guessing the needed idle power value is significant.
Let's assume CPU idle power usage is 2W. (deep sleep enabled)
- In the first case power delta is 73W. Compensating for losses 73x0.9x0.8=>53W. Add 2W and we get 55W
- In the second case power delta is 61. Compensating brigs us to 61x0.9x0.8=>44W. Add 2W and we get 46W
Now let's assume CPU idle is 10W (no sleep, nominal frequency even when idle). Doing the math again leaves us with
63W and
54W.
The correct value, when we guess right, is
~55W. But what happens if we guess wrong? The difference between wrong estimates can be 8-17W, or 15-30%. That's quite a bit of room for error and debates.
So using only this methodology and selectively choosing idle power usage that fits one's agenda, two opposing fans could argue that the CPU inside the system described above is either a
45W TDP CPU (optimistic) or a
65W TDP (pesimistic). Neither would be right, but the thread would be full of noise and accusations, even if the two fans used the
exact same methodology.
And why is that? Because both ignored missing idle power data to paint desired picture.