Thoughts for Solar charging...
Interesting idea you have. Not sure I even want be next to a computer on any river trip but to each and their own..
I built a solar charging system a couple years ago that works great, but now I know it's limitation I would have done it differently.
First thing is to get a good battery: Optima's are good, but even better is get a AGM (absorbed glass matte) battery. They dont ever need water added to them, can be bounced and subjected to vibrations, and have little memory meaning you can drain them with little effect to future charging capacity.
I got 2 15W solar panels from northern tool and equipment (80$ each on sale), a 7 amp controller that wont let the battery be over charge or discharged at night. I bolted the panels to 1/2 plywood into a Pelican gun case, I used boards so the case would flex and not break the rigid panels if someone stepped on the case or even worse I found myself upside down getting worked.
I put the battery (36 amp/hour) in a small ammo can. I put bolts on the battery terminals, and hard wired the the controller, and 12V cigarette lighter to the battery. All I do is open the case, and alligator clip the panels to the controller wires, easy.
I also have a 12V battery charger for AA and AAA batteries. I did this as on a large trip it is sad how many batteries from head lamps go into the land fill, so now I can recharge all my batteries and not add heavy metals to out taxed environment. I also have a 150W inverter that I can run regular electronics with.
I built this setup to ensure that a satellite phone could always be charged and to power a small amp for guitars, etc... I also found that most small speakers for computer or I-pods are actually DC (car) as opposed to AC, all you need is to buy an adapter jack for them.
Using and inverter is extremely inefficient, you loose approx 30% of your power making DC into AC. Even worse is that many electronics are DC to begin with, so you end up buring 30% of your capacity inverting DC power to AC, where the AC device will turn it back into DC.... That makes for using double the power to run something. Most laptop actually use DC to charge them, they convert your household to DC in the little box on the cord.
The first thing you should do is buy a 12V cigarette charger for your laptop, they are expensive but adds a lot of power capacity to your battery.
Also, buy more solar than you think you need, if you maintain a battery solar works well. However I discharge my battery below 11V my solar panels dont have the amperage to recharge the battery. It takes a lot of amps to recharge a drained battery, but not many to 'maintain' it.
If I was to do it all over again, I would get 2 Brunton 26W flexible panels, they are pricy, but are WAY better than the rigid panels I have. You can put them in a small drybag, the are light, and you can bungee them anywhere on a boat. I wish I would have done this, I could put them on my bimini top. The rigid case and panels weigh about 40lbs, where as the brunton would weight less than 2lbs. My rigid case isnt that water tight either, its to big to have a bomber seal ecspecially after being used for 100s of days. It is a pain to open and charge on the water, so I rarely do unless it is absolutely flat water. This means that i dont take advantage of the sun during the brightest part of the day.
You can look on google and find the 26W bruntons for a little over $300 each as opposed to the $600 MSRP.... I would also buy a digital voltmeter so you can see battery status, smount coming in, and how much is being used. This will allow you to keep your battery in good shape, and prevent a deep discharge where 2 small panels will not recharge it.
Once I had power, I found that there were a lot of possibilities.... I can run auto head lights (small) monted on the bimini or front of the boat for night floating, run an electric inflator, or music. Many people have reservations about any electronic on the river...
I have found that having a navigation light is nice when you need to make miles and daytime winds make it a bi***. It is also nice to plug in a guitar to small amp if you have musicans (not me), and I enjoy a little Hank III at low volume when i am just drifting down the river comtemplating life.
You also need to get a desicant or wrap your computer in a towel in the dry box, if you put it in when it is hot, there will be considerable condensation as the box cools down. Think of relative humidity at night as oppsed to the baking sun.... condensation will ruin electronics in a hurry!
or you could just get a small recorder and record your thoughts on the river.
Sport Climbing is Neither