Here is the load curve for the solar cell you saw earlier.  While this load curve is very nonlinear, you can still get an approximate TEC for the solar cell.  That TEC uses a linear approximation, and only works for part of the load curve.  Click here to see the linear approximation.  On the next page is a complete description for what you have to do for this problem.
 

• Determine an approximate Thevenin equivalent model for this device using a straight line approximation near zero current.
• Using your approximate model compute the resistance for maximum load power, and compute the estimated maximum load power.
• From the graph, determine the maximum power this source can actually deliver.