A photovoltaic solar cell converts light to electricity. That is, photons -- the tiniest quantity of light -- transfer their energy to electrons, freeing them to move away from their home atoms. The current and voltage a solar cell can supply is not constant even when the illumination is constant. The current and voltage depend upon the resistance of the electrical load connected to the solar cell. Load lines are tools for evaluating solar cell performance for a given electrical load.
Measure the current and voltage of the solar cell for different resistances. You can do this by illuminating the solar cell with a constant intensity and changing the value of resistance. Change the resistor from zero -- an open circuit -- all the way to infinity -- a closed circuit. Measure the current and voltage at every point.
Plot the current vs. voltage on a graph. "I" is the symbol for current and "V" is the symbol for voltage, so you have now measured the I-V curve at the light intensity you supplied.
Plot the I-V curve for other values of light intensity. You can repeat the measurement of Step 1, or simply multiply the current of the first measurement by the ratio of your new light intensity to the light intensity during the measurement. That is, if you measured the I-V values for an illumination of 500 watts per square meter, the current for 1000 watts per square meter will be twice your original measurement for each voltage.
Select a load value of interest. For example, you could have a circuit with an effective load of 25 ohms.
Plot the current vs. voltage line for the load you selected in the previous step. The values are given by the equation: I = V/R. That is a load line.
Observe where the load line crosses the I-V curves plotted in Step 3. The intersections of the I-V curves with the load line show the current and voltage that will be supplied to the circuit as the intensity of light changes.
If you already have an I-V curve supplied with the solar cell, you can skip directly to Step 4.