Jumping Jehusafads Batman! Look what a little math can do for you.
I have a Dell Inspiron 1150, I use daily. Well, My charger always gets VERY hot, so i decided to check out the power labels on it. 65w Charger.
Input: 120v, 1.5a
Output: 19.5v 3.34a
OK, cool... but what does it all mean?
120*1.5 = 180 watts
19.5*3.34 = 65.13
HOLY CRAP ON A STICK! I was losing 115 watts to heat! WOW! Thats alot. Almost all of the Opus 120!
OK, so here's a situation.
65.13/180 = 36.2% effieciency.
Multiply with a 90% efficient inverter: 32% effiencey.
Holy Crap! 32% efficient. On my "65 watt" charger that means it's really sucking up:
65/.32 = 203 watts! HOLY CRAP!
That's like 138 watts of wasted heat! Do you want that heat building up in your car o na hot summer day? Might even cause a fire!
So, that's why you should go with a DC-DC adapter. Much more efficient. :-)