If you're without power in the daytime, you might consider turning off the backlight on your portable system. If you have sufficient light (i.e. the Sun) shining down on your system, then you have all the illumination you need, which will save precious watts to power your system a bit longer.

Of course, systems like the DS and Game Boy Advance almost need a backlight because they're just so dark without one, but direct sunlight is actually pretty playable.