Are you trying to measure the maximum power output of the cell , or are you just trying to measure the variation over time .
Solar Cells only deliver maximum power at one point on their V / I graph, and using a simple load like a resistor or current sink
will not give you the absolute maximum power output.
It will work if all you are looking for is relative changes over time.
To extract the maximum power from a Solar cell you need some kind of peak power point tracking.
This explains what the problem with Solar cells is and why peak power tracking is needed.http://en.wikipedia.org/wiki/Maximum_power_point_tracking