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For some electric devices, like a fan or air conditioner, I read about their power consumption in watts on their specification guide.

Does it tell about the power at normal or full speed? or Does the speed even affect the power consumption? Can I find the consumption at different speeds?

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You will have to measure it. I have a device called a Kill-O-Watt that does a fine job of it. I've been dissapointed with multiple speed fans, medium and low speeds seem to consume almost as much power (80-90 percent) as high, but airflow is much less than that. I doubt AC scales as poorly. – Omega Centauri Jul 12 '11 at 17:00
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As a practical matter, if (hand wave) +/-25% isn't good enough for you, then measuring the specific device you're interested in with something like the Kill-A-Watt is your only recourse. – Jay Kominek Jul 12 '11 at 21:33

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The policies for measuring and disclosing these energy ratings stem from the FTC and you can read about them on the electronic code of federal regulations (eCFR). Specifically, Title 16 Section 305 should contain the information you're looking for.

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The Wattage written is "The Wattage", actually the regulator works by changing the voltage applied to the fan's coils. This is the case with at least the potentiometer regulators(the conventional one), so even if the fan is slowed down, fan coil will consume less power but the resistors will consume the rest. So essentially, you gain nothing even by reducing the speed!

And as Georg said, the power consumption is much greater at initial startup to overcome the static friction.

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Actually the startup spike has less to do with friction, than it does with getting the gases in the compression cycle into a steady state. Also the inertia of moving parts must be overcome. But I have measured some 3speed consumer fans, and low and medium speed were roughly 90% and 80% of full speed power consumption, although the amount of airflow and noise is much less, so for cheap consumer fans, the resistance hypothesis might be correct. – Omega Centauri Aug 12 '11 at 16:19
@Omega:compression cycle!!!! didn't get that thing?? Aren't we talking about electric fans??? – Vineet Menon Aug 13 '11 at 6:06
An AC works by pushing a working fliud, between high pressure and a low pressure chambers. When the fluid is moved from one chamber to the other, it adiabatically warms/cools, so that the high pressure chamber is hotter than the low pressure chamber. If the AC has been off for a while, the pressures have presumably equalized. There is greater potential energy represented by the pressure difference which must be relaced during the startup transient. – Omega Centauri Aug 15 '11 at 21:02
@Omega: I guess my reply was about fans, electric fans. a nice example would be ceiling fans. – Vineet Menon Aug 16 '11 at 8:28
regulating speed by a series resistance is not at all typical of how air conditioner fans (which are typically powered by AC induction motors) achieve several different speeds. In an air conditioner, the vast majority of the power is consumed by the compressor motor, which is usually a sealed AC induction motor. Induction motors have many really good characteristics unfortunately they are inherently fixed-speed devices, which makes it difficult to match them to anything but a flat-out load (some recent AC units use inverters to produce variable frequency AC) – Daniel Chisholm Nov 10 '11 at 14:59
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