You can do it pretty cheap and easy. Harbor Freight sells a solar power battery keeper for cars. It is not too large and it is already 12v. Just cut the cigarette lighter plug off and splice in a socket. Then you can plug in any car adapter for cell phones or other low power device.

The main problem is going to be that it is only 1.5 watts. That is not much at all. It probably won't even keep a phone charged that is in use.

http://www.harborfreight.com/15-watt...ger-44768.html

Here is a 5 watt model but it is significantly larger:

http://www.harborfreight.com/5-watt-...ger-41144.html

Neither one of them is going to be able to power up a laptop.