i’ve carried a little inverter in my laptop bag for a number of years now — a little 75W job that plugs right into a power outlet in the car. It’s really handy on trips and in situations where you might otherwise completely run out of juice. Apparently Black & Decker makes a few as well, ranging from the 100W version pictured above up to 750W monsters.
Of course, larger versions are available as well. The trick is that once you start drawing enough current to power those larger units, you’ve got to pull some pretty low-gauge wire directly to the battery. Somehow they always forget to mention that in advertisements. But these smaller units work fine for a laptop and require no such messy install.
The B&D unit has a couple of cool features: a low-battery shutdown to prevent you from accidentally stranding yourself, and a 45-degree adjustable accessory plug. It sells for around $25 from a wide variety of vendors online.