I dunno, a gamecube with an lcd screen is cheaper than this dreamcast, plus more powerful, and potentially drains less energy. Not sure about that, but they did actually make batteries for gamecube, guess roughly the size of a labtop battery, last about an hour and a half, which is what high power labtops last.
Not sure if you could power a ps2 or xbox plus a screen(with speakers) though, they definetely drain more power than any labtop. Well, maybe not the ps2s if they've upgraded the design, but I think the nv2a on the xbox could push it over the edge. Actually, now that I think about, gamecube's time is pretty low too, unless those high drain labtops actually last an hour, but those labtops have to power a larger screen, plus a harddrive....which probably uses less power than a constantly spinning disk drive....but they have ram which uses more power per megabyte and a lot more of it.