Solar cells are tested by the manufacturer with artificial light under what is called AM1 conditions. AM stands for air mass. Air mass is the amount of air the photons have to travel through before they reach the surface of the earth at sea level. Air mass 1 is when the sun is directly overhead at sea level. The energy available to the solar cell at AM1 is equivalent to about 1kW/m2.
Match solar cell output
You need to test each and every cell that will be used in your panel. If you are dealing with off-spec cells, the cells must be grouped into categories of high, medium, and low output. If you include a low output cell in a panel with cells that are higher output, the low output cell will bring all the other cells down to its lower rating.