UPDATE JULY 2016: the Goal Zero / Yeti products look perfect for field needs.
A MacBook Pro Retina lasts 3 hours or less while doing photography work—a far cry from Apple’s misleading 9 hour claim (the 2015 model may be about 5% better).
So powering a laptop becomes a challenge out in them thar hills. I charge it as well as camera batteries and flashlights and so on using a power inverter @AMAZON in the SUV (a pure sine wave power inverter @AMAZON is best). I’ll probably replace the 400W power inverter unit I have @AMAZON since it’s a square-wave unit and gets quite hot and has gotten whiny and making unhappy noises*.
The Duracell DRINVP175 175-Watt Pocket Inverter with 2.1-Amp USB Port @AMAZON isn’t bad and I may buy another one since I gave that one to my dad who needed it on a trip.
Anyway, few vehicles are happy about an 85W power brick trying to pull power for very long to recharge a battery that is mostly drained. It’s not an issue if it’s just steady state usage power, but recharging a mostly drained battery draws some serious amperage at 12V for a while. So that means idling the engine at times just to charge the darn thing—highly inefficiently: 12V DC to 120V AC to MacSafe power brick back to DC.
The Wagan EL2600 Elite Pro 200W Pure Sine Inverter @AMAZON looks like a good unit (400W sounds great but 20 amps at 12V is 320 watts and that’s already blowing the fuse, so 200W is plenty).
But all vehicle power inverters have an efficiency problem: they convert a vehicle’s 12V DC to 120V AC which the Apple power brick (or other AC charger) convers back to DC power — very inefficient. And so, over at diglloyd.com, a photographer’s perspective on powering an Apple Macbook Pro laptop in the field where there are no AC outlets: