Don't MAC have any other soulutions?
Firstly, please, MAC is an acronym for
Media Access Control, Mac is Macintosh, while Apple is the company that produces a line of computers named Macintosh. I never thought I'd be so annoyed by this, but it really does get to me.
For the OP: in my experience, attempts to make direct DC converters always fall short in price and compatibility. A good ~140w converter runs about $35 in the US (got mine at walmart about 3 years ago). All DC options are at least $100, usually more. Converting the power to AC is inefficient, yes, but oh well. It's much cheaper, will run most anything that requires less than 100w AC (phone chargers, battery chargers, printers, you name it), and will run ANY laptop you'll ever own.
If i had loads of money I'm sure I'd buy the direct dc option, but in my case i'm glad i didn't. 3 years ago i wanted to run my crappy old P4 dell laptop in the car on a very long trip, and looked into direct adapters. Luckily I asked my father who just told me to go to walmart and get a cheapo inverter. my 140w inverter ran my 130w laptop (stupid P4 chips never belonged in laptops) perfectly for days on end, and also charged my camera batteries, phone and more.
If you don't need to worry about photo gear or some kind of non-mac laptop etc. and can find something for a decent price, it'll probably be smaller and more convenient, but I never found such a unit.
And no, a reasonably powered inverter will do nothing to harm your alternator.