Apple touted some pretty decent battery life numbers at the new MacBook / MacBook Pro press event this week: up to five hours on the Pro with discrete graphics off, and four hours with it on (both surely assuming ideal low-power conditions). One of the things that didn’t come up at the presser, however, was that new MacBook Pro batteries actually have just under 20% less energy than their predecessors. While both kinds of MBP batteries are 10.8v, the old ones are rated at 5600mAh / 60Wh, while the new ones are rated at 4700mAh / 50Wh. (MacBook numbers updated below.)

I’d estimate that the integrated NVIDIA chipset and ever more behind-the-scenes power-saving techniques are why Apple is claiming such solid life despite killing a fifth of the machine’s energy supply — but a 20% reduction is still no small number. It also means that as your new MBP’s battery degrades, you’ll have a smaller pool of potential energy to rely on, meaning you could wind up having to replace your battery more often (although that’s a little conjectural, at this point).

But as some are now postulating, one technique Apple may now be employing to save power is making use of the machine’s GPU(s) to accelerate video playback. We already know that the new MacBooks have a different build of OS X than older gen machines (9F2114), but one thing I also noticed is that Quicktime, the engine behind Apple’s video encodes and decodes, was also revved in the new machines, now clocked in at 7.5.5 995.23.3 up from the last machines’ 990.7. With a little luck, perhaps video encodes will finally be hardware accelerated, too. (Will have to test that one later!)

Update: Matt at Gizmodo mentions that the numbers add up, when comparing the extra 20% battery life you get in new models using the integrated GPU vs. that same five hours claimed in old models using discrete graphics. Also, new MacBooks have also decreased battery capacity (as expected), from 55Wh in last-gen models to 45Wh.