Reader Evan writes: I am going on a trip to Hawaii next week, and I will be taking my laptop and some movies with me. Would it help save battery life if I were to rip the movies and play them back from the hard drive or a USB thumbdrive instead of playing them back as DVDs?

Notebooks use a lot of power, but by isolating which components use it, you can add substantial run time to a machine without sacrificing utility.

To answer this question I performed three experiments on a Dell Inspiron notebook, playing back a movie file as a DVD, as an AVI ripped to the hard drive, and as the same AVI ripped to a USB thumbdrive and played back from there. All other settings on the machine were identical.

The results were interesting. Playing back a DVD, I got battery life of 2 hours, 36 minutes. As expected, when playing back from the hard drive, battery life climbed to 3 hours, 5 minutes. I expected even higher numbers from the USB thumbdrive playback but was in for a surprise: Run time plummeted way down to 2 hours, 33 minutes, the worst of the bunch. Quite interesting.

The hard drive vs. DVD isn't a big shock. A spinning optical drive uses about 5 watts of power, while an active hard drive uses just 2 to 3 watts. So naturally battery life will rise when you're not using the optical drive (especially since Windows likes to access your hard drive periodically anyway, whether you're using it or not).

The USB thumbdrive results were surprising, though, as USB devices have a reputation for not using a lot of power. I did a little research I found that power consumption can be erratic for USB devices, which likely explains the discrepancy (there is also a random component to any battery test, but not 30 minutes of randomness). One thumbdrive may only draw half a watt of power, but another may draw several watts. The USB interface itself draws power, too. Alas, there are no good studies of power consumption among different brands of thumbdrives, and I only tested using a single thumbdrive. The results could have been different had I used a different brand drive, but you'd have to experiment to find the right one.

Putting USB aside for now, one thing is clear: You can get substantially better battery life if you rip your DVDs to your hard drive before you take that trip. Not only that, you'll have to pack less (and DVDs are fragile), and you'll find switching between movies much more convenient, too.