If you connect your multimeter in series on the power cable of the book pc and let the current run thru the multimeter you can measure how many amps you will be drawing. multiply # of amps it draws at the power source * 115 and you will know how many watts its drawing in AC. What you really need to know is how many amps its using in DC. I'm not really sure of a way to figure out how many amps your drawing in DC without taking readings from the mobo power, hd power, display, floppy, fans, etc etc etc and then adding it all up to know how many amps and then * 12volts to know how many watts your drawing. I think you'd be alright on that power supply without taking any measurements, though.