Testing solar cells
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.
They dont have to all have the same dead-on output, but they should be in the ballpark for what you want the panel to produce. One cell that is of very low output can deprive you of a lot of energy from the other cells.
Cells can be tested in the sun on a very clear day. The ideal time to test cells outdoors is during the summer when the sun is at its highest point around the solstice, and at solar noon. This gets you the closest to AM1 conditions.
However, you can test your cells using the sun at any time of the year. If you do this, take into consideration that the output from the cells will be less than their peak output under ideal conditions. Any light conditions can be used to tell how well the cells perform in comparison to each other, since you dont need to know their peak output for matching. The comparison of each cells output to the others is really the critical issue.
Tools for testing solar cells
To test the cells you will need a multimeter that gives a current (amperage) reading and a voltage reading. All multimeters have these two readings available. Its also useful to make a stand that will hold the cells at the same angleas the sun above the horizon, and that can be pointed in the direction of the sun. You can just hold the cells with your hands, but this can be clumsy.
My testing stand has a piece of copper clad circuit board to lay the cells on. With this arrangement I can connect the multimeter with the back of the cell simply by touching the multimeter probe to the copper on the circuit board. With this method, however, you have to be sure that the contacts on the back surface of the cell connect well with the copper on the board.
To take a reading, touch the negative probe to one of the cell fingers on the face of the cell and touch the other (positive) probe, to the back of the cell (or the copper surface of the circuit board if you are using one). The cell should be facing in the direction of the sun and at the suns angle.
Take both the voltage and current reading for the cell, and write it down. Proceed similarly with the other cells, grouping them as you go along.
Test all of your cells on the same day. If you test the same cell on two different clear days, you may get quite different readings, although conditions one day might appear to be the same as the other day, there can be a significant difference in available sunlight due to the level of aerosols present.
Particulates, moisture, general pollution, and pollen all affect cell output readings. With repeated observations, you will be able to discern the aerosol levels in the atmosphere.