Input power rating on power supplies - how does max amp draw apply to 220V
If a power supply says the input power is 100-240V, ~1.5A, I'm assuming that the "1.5A" draw would be at 100V, not at 240V, right? I.e. would the total max watts be 150W regardless of the input voltage? 


Yes, the PSU input power performance will be given by 

$$ P_{OUT} = P_{IN} \times efficiency $$


  ... the output rating on this supply is 19V, 3.42A.


$$ P_{OUT} = VI = 19 \times 3.42 \approx 70~W $$

If the efficiency is 85%, for example, the input power would be
 $$ P_{IN} = \frac {P_{OUT}}{efficiency} = \frac {70}{0.85} = 82~W $$

At 100 V current would be &lt; 1 A.

The 1.5 A is worst case surge current. If the efficiency was only 50%, for example you would have 70 W dissipated in the power supply and it would be very hot to the touch. The main current should be much less than 1.5 A in normal operation.