there are three different kinds of solar panels and they all differ in the efficiency that they will produce electricity, you will have to take into consideration the conditions that the car will be operating. there are three kinds of solar panels, there is the crystalline, polycrystalline and the amorphous solar panels, their primary difference is the cooling process and their efficiencies are a direct cause to the cooling process.
*the crystalline forms uniform bonding caused by the slow cooling process and thus has the highest efficiency of all, under experimental conditions they have reached 24% efficiency.
*the polycrystalline solar cell cooled faster than the crystalline and thus tends to not form uniform shapes such that when photons hit the surface they will actually get deflected to some angles and some of the electrons will be lost, this brings the efficiency down to a mere 18%
*the high cost for these two solar cells is mainly due to the fact that the cells are very thin and generally have to be assembled (makes you wonder how do they get the intrinsic layer in a solar cell which is only a few atoms thick and separate the positive and negative layer
) and they both take long to produce because they have to be cooled slowly. but they genreally have a warranty for 25 years and will operate with out the need of having anything or anyone checking up on the system regularly (if you get a suntracker it will seek the angle to operate at its higher efficiency)
the third type of solar cell and probably the one that i think will have the most future ability is the amorphous solar cell, the advantage of this cell is the fast cooling process, that it does not have to be generally hand aseembled, can be mass produced, and the number 1 reason you can bend this one its not brittle at all and i have seen them in a roll, but under experimental conditions they have only reached 10% effieciency, however....
an interesting feature of this solar cell is that if you put one on top of the other it will increase the efficiency, but only to a maximum of only 15%
i advice that if you do get a solar panel you get an amorphous solar cell, which you can actually put around the project to generate electricity from the sun (and they are very cheaper you could easily get several for the price of one crystalline, the only difference is that a crystalline generally has a 25 year warranty and amorphous does not because it will lose efficiency over time faster), but you will probably need a higher voltage, and solar cells have to be placed in i believe series in order to achieve this, or in other words the negative has to be connected to the positive end for all the cells, and each cell produces 0.5 volts, no matter its size, the size only defines the current. if you connect the solar cells in parallel, however, you will get a higher current but a lower voltage, due to the fact that the solar cells you will be connecting positive to positive and negative to negative and will just be making a system that is like a cell covering a larger area.