Solar really isn't practical outside of two major areas:
1. Remote locations
2. "Always on" grid-connected systems
Here is the problem... The energy contained within solar radiation is approximately 1000W/m^2 at the surface of the earth. Of course, weather, altitude, time of day, etc, all affect that. But 1000W/m^2 is a pretty good rule of thumb, and is the value used when a module is 'rated' to a certain wattage.
Silicon crystal photovoltaic modules are around 15% efficient. Which doesn't sound good, except that the theoretical maximum efficiency for a silicon crystal cell is 30%. There are nearly no power sources that are already at half their theoretical maximum efficiency.
That means around 150W/m^2 of capturable power. A Macbook Pro needs 85W at full tilt. That means you would need about .6 square meters of module just to fully power the Macbook Pro, at noon, in direct sunlight. That is about a 2' x 3' module. That is a lot to carry around. Module prices are about 3.80$US per watt, wholesale. That means the wholesale cost of an 85W module is almost 350$, BEFORE inverters, control circuitry, etc.
Now, if you are in a remote location where you just can't get power, then you'd just have to suck it up, and pay for the solar modules. But if your concern is offsetting the power used by your computer, it makes more sense to have solar on your home, and charge from it.
If you carry a solar module around, and use it a few hours a day, then it will take a very long time for the module to generate as much energy as it took to build. This isn't good, environmentally speaking. It needs to pay itself back, not just in cost, but in energy, to be good for the environment.
A grid-tied solar system on a home generates power all the time. If it is making more than your house is using, then that power goes out into the grid, and powers your neighbor's homes. The power company credits you for this generated power. In this case, the module is going to pay back its energy cost much faster.
This is why solar cars haven't ever caught on. They actually aren't a good use of solar. It is better to have solar on the home, and use it to charge electric cars. That way when you park in a parking garage, you aren't wasting valuable solar modules.
So anyway... I guess what I'm saying is, it will be very expensive to really power a Macbook (even without charging, you still need at least 60W to run the thing), and if your goal is environmentalism, as opposed to continuous running in remote locations (that you are willing to pay a great premium for), you should get a solar system for your home, and then buy a few extra batteries. You'd actually pay less in the long run, and generate more power.
-Sam
