Ok everyone seems to assume a laptop is running on full power all the time
That is not true at all.
A laptop will only use full power when everything is set on full and you use the full processor cycles (eg stacking or long calculations).
The power supply only charges the internal batteries and the amperage fluctuates between 0.2 and say 4 Amps when charging the batteries while using the lappy (measure this if you don't believe me
).
The rest of the time there will be just a trickle charge.
When there is no battery in the lappy the amperage will flutuate a lot more.
Just my 2 cents here about lappy's.