How to know how much electric power a laptop is consuming?
On a laptop charger adapter it is written '65 Watt'. When I am using that laptop plugged into an AC socket, does it mean that it consumes 65 watt per second?
I think it actually depends on usage. But is the 65 watt the maximum power consumption for this laptop, or can it consume more than that?
However, if it consumes more than 65 watt then the extra power must be taken from battery; but the battery also gets charged simultaneously and it remains fully charged. So doesn't it mean that the laptop uses less than 65 watt, because the maximum capacity of the charger is 65 watt?
Just to clear up something first of all; it's a minor terminology issue, but:
does it then consumes 65 watt per second
A watt is measure of energy per unit time, specifically:
The unit is defined as joule per second
However, this is a noted area of confusion, so don't feel bad about it at all!
Typically a laptop will consume less power than its adapter will allow for. So in your case it will normally consume less than 65W. In normal operation it should not consume more power than the adapter sold with it can supply.
(You can test this by plugging the laptop into an energy usage meter/monitor (such as a kill-a-watt) and taking a reading while it is active.)
You can get into situations you describe in the last part of your question if the charger is incorrect for the laptop and cannot supply sufficient power1. This situation is more common with mobile phone / tablet chargers. However, I am not certain of the behaviour if the laptop continued to draw more power than the (incorrect) adapter could supply. I suspect it wouldn't charge, may discharge the battery or may not, and the operating system may shut down after a warning.
See also Using a higher wattage charger for my netbook
1: The last time I saw this was with a damaged charger, and I recall getting a warning notification to the effect that the battery would not charge. Disclaimer: dodgy recall, years ago, possibly incorrect etc