I have this 2400 mAh solar charger.
It works with N900, after the usual fix by soldering Data+ and Data- pins together. There are photos about the inside and in discussion "Will not work with newer devices...", which explains it.
Haven't much used that yet, except tested it works after the fix, but now in the summer time I may need it and keep it with me in longer trips.
If I can remember correctly the output of the regular N900 charger is around 5W. The chargers you saw probably charge the batteries first -> batteries charge the N900.
By using a 1,2W charger directly with the sunlight I got pretty good results. A drained battery is charged to about 20% until the power is not enough to charge the N900 anymore
i bought this one when i headed out to finland. its from a solar. their website is www.a-solar.nl the specific product is power charger am-101. it can power your n900 but has 12 connectors for other devices sudch as samsung lg sony ericson sonypsp nintendo ds cameras and even the dreaded iphone or ipod. it wasnt to expensive eitherl and contains a 3.7v 2000mAh battery. it can be charged through solar energy or through usb.
The chargers you saw probably charge the batteries first -> batteries charge the N900.
The solar panel charges an internal battery slowly, then you transfer power from the internal battery to your N900 battery. I'm pretty sure that is how they all work. Otherwise it simply wouldn't work for electrical reasons, or you would need a BIG solar panel.
I would just buy a 2nd battery or a car charger. The only reason I would consider solar is if I was going somewhere that didn't have electricity. Otherwise its just not efficient. You'd be better off with something like this:
That seems exactly same as this Dynamo Hand-Crank USB Cell Phone Emergency Charger. Seems like just adding the printing on the case gives +us$20. I have it. The same trick had to be made, soldering D+ and D- pins together inside the case, otherwise it wouldn't work with N900.
