To add to what TheWhiteFalcon says: The current 13-inch MacBook Air with the i5 CPU gets above what Apple states it should (12 hours) in most formal testing. In light-usage informal testing, it's good to go for about 17 hours of continuous, screen-on usage if you're just using Safari and, say, Pages (or another light Apple app.)
If your goal is just to stay away from outlets for a day, there are already dozens of good solutions.
If you're just looking for something with which to write, the Microsoft Surface 2 is probably your best bet in the PC world, at least for double-digit battery life. Though, the HP Stream 11 and Asus X205 are each $200 and claim 8 and 12 hours of life, respectively. There's a car charger for the SUrface 2 and the Stream 11 and Asus X205 each use USB chargers, so there should be a variety of options for recharging those devices based on collected solar energy.
If your goal is just to stay away from outlets for a day, there are already dozens of good solutions.
If you're just looking for something with which to write, the Microsoft Surface 2 is probably your best bet in the PC world, at least for double-digit battery life. Though, the HP Stream 11 and Asus X205 are each $200 and claim 8 and 12 hours of life, respectively. There's a car charger for the SUrface 2 and the Stream 11 and Asus X205 each use USB chargers, so there should be a variety of options for recharging those devices based on collected solar energy.