There's an outfit on amazon/ebay that sells 18v "Allpower" solar panels with an array of adapters to use with whatever brand laptop you have for reasonable prices. They just unfold, and then plug right in to your laptop. On a modern laptop, you could probably get away with running/charging a laptop on a 21w array for $90, but they make up to a 28w array for $130. Modern Haswell/Broadwell laptops run at about 15w with the display at full brightness. There's a 14w array too for $60, but if you're seriously considering buying a device like this you probably want the ability to run and charge at the same time, and it's unlikely you'll find a place that allows you to put the panel in full sun and comfortably work on the laptop. With 18v, you'll never fully charge the battery (you need 19-19.5v to do that) but it'll satisfactorily charge your laptop to about 93% very reliably.
Of course, if you're stuck in a rainstorm for three or four days and you wear down your laptop battery, you might have trouble getting it charged back up until the sun comes back out. But with modern 15 hour batteries in laptops you should be able to squeak by for a day or two of normal office work.