Given the current and voltage of a consumer electronic device, how to calculate its power draw?
Your DC formula is correct: P = V * I. The formula applies in AC too, provided the load is resistive. Things get a little more complicated if the 'power factor' is less than one as is the case with motors, for example. The formula then becomes P = V * I * PF.

Your laptop power supply should have a power factor close to one so we can stick with the basic formula for this. 


The input current quoted will be worst case and may be heavily influenced by the in-rush current on first powering up.
On a switched mode power supply the maximum current will be drawn at lowest supply voltage. If you check the ratings you will probably see that it's rated for 80 - 240 V AC. As the voltage goes up less current will be required to provide the rated power. Conversely highest current will be drawn at low voltages.


If you can borrow a plug-in watt-meter (wall adaptor type) you can read the actual watts being drawn by your load and compare with your calculated values. Most will also give you the amps and power-factor too which can be quite instructive.