Measuring AC power usage
And how can I calculate watts in AC circuits? it's same as DC (VxI)?


Strictly speaking yes it absolutely is. The instantaneous product of voltage and current is power and will have an average value that is true power. Here are a few examples of different types of loads: -



When the magneta and blue curves multiply you get the red curve (power). The mean of the power waveform is the watts you are billed on.


  I searched about it in the internet and it says we need to know the
  power factor.


Proper watt meters instantanously multiply voltage and current waveforms to get true average power.


  what is power factor..?


If the true watts are 1000 W and the product of RMS voltage and RMS current is 1000 VA then power factor is unity. If the VA is only 500 then power factor is 0.5. 

See my top right diagram - it shows the average power at 50 % of what it is when voltage and current are in phase (top left). If you take the arc cos of 0.5 you get 60 degrees; in other words power factor can tell you how displaced the voltage and current waveforms are. 

From this follows the much abused formula that...

Power = V.I.cos (phi)

This implies that if you know the RMS values for voltage and current and, using (say) an oscilloscope, you estimate the phase angle displacement, you can "calculate" watts. Where this gets abused is that more often than not one or both voltage and current waveform shapes are NOT pure sine waves and this can lead to significant errors in power calculation.

No such error is introduced when voltage and current instantaneous waveforms are multiplied together.