Help using transistor in active mode as a current limiter
To get the I/V-Curve of the solar panel (and therefore the Maximum Power Point), I would suggest you use a circuit with feedback to change the current sourced from the solar panel. This is possible with a variable current source like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Vref determines the current sinked from the solar panel. The current is calculated as follows:

\$I=\frac{V_{ref}}{R_2}\$

Therefore, from the knowledge of the Vref and the measurement of the solar panel voltage, you can easily determine the maximum power of your panels 

For reference: The circuit is from "A collection of Amp Application by James Wong (Analog Devices):
http://www.analog.com/media/en/technical-documentation/application-notes/28080533AN106.pdf