If you are trying to extract maximum power, you need to have some type of adjustable system on your output. The power curve of a solar panel depends on many things, one being light. As the light increases / decreases, the maximum power point varies. Normally, people use a switching supply and some type of micro to do dithering. In other words, they slightly increase the current draw and see if they get more power, if so, then the system continues to move in that direction until it reaches the maximum. In this way, you always get the max power. The equation governing a solar cell is:
P = V ( Isc + A(1-e^(BV)))
Where P is the power, V is your load voltage, Isc is the short circuit current. A & B you have to solve experimentally for the panel by taking a dozen or so measurements each with a different load and then best-fitting to find the points.