for reference, the SLI config consumes about 175~200 watts on battery, 350~400 watts when plugged in. nVidia's Battery Boost 2.0 goes a long way. With two chips running you can get 2~3 hours of battery life on the MSI laptop I was on about, with one chip disabled you nearly double that and can still hover around 30FPS in games like Tomb Raider.
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Some more info/benchmarks.
We're entering a very exciting age of PC gaming. Laptops are going to become almost as effective as desktops soon enough with smaller dies and more power efficient designs coming into play. Battery technology could use some improvement, but it isn't
that bad.