The battery life on bootcamp of most Macs isn't all that great. Windows isn't as power optimized as OS X since it has to support a plethora of different hardware configurations.
In my personal experience with Macs running windows: take your OS X battery life and cut it by a ⅓ or even ½, that's what you'll be getting in Windows.
Since you want to use CAD programs, you'll be running windows. Might as well buy a more powerful windows machine with great battery life for less, no?
FWIW, I graduated from university as a mechanical engineer a bit more than a year ago, and my computer was doing a lot more Word, Excel, Matlab and Maple than CAD. And when I did want to do CAD, the school's computer blew my laptop out of the water performance wise.