What precisely are you trying to measure?
The output power of a solar panel varies depending on a large number of variables.
Primarily the solar radiation intensity , which changes over the course of the day, and whatever the solar panel is connected too.
Fixed value resistors will provide some data , but it wont mean much , as the values have to be changed depending on the level of light intensity.
To accurately measure how much power the solar panel can deliver over a period of time , needs a MPPT (Maximum Peak Power Tracker) which is an electronic device that continuously adjusts the apparent load to match the solar panels peak power point , and a battery to absorb the energy.
You could use a resistor instead of the battery if you dont care what happens to the Solar panels output.
You can make a MPPT charger with an Arduino if you wish.
Heres an article on how to do so.http://www.timnolan.com/?page=arduino-ppt-solar-charger