Solar panels, though often integrated into complex solar power systems, are simple devices. Small solar panels, usually referred to as solar cells, are suitable for small electronics projects, and they can often function as direct replacements for batteries. The key to using a solar cell for an electronics project is understanding the model that describes the electrical characteristics of the cell. This simple model includes the voltage that the cell can generate and the internal resistance of the cell.
Determine the voltage and current needs of your project's circuit. Small solar cells can generate plenty of voltage, but they often have high internal resistance, which means that they cannot supply much current. As an example, if your circuit uses a low-power wireless transmitter chip to report outside temperatures, you might need 3.3 Volts at about 0.020 Amps.
Review the information about the different solar cells in a hobbyist or electronics catalogue. You want to find specifications for the voltage rating (or "open-circuit voltage") and for the short-circuit current of the cells.
Divide the open-circuit voltage by the short-circuit current to find the internal resistance. For example, a cell might have an open-circuit voltage of 5 Volts and a short-circuit current of 0.035 Amps, so the internal resistance = 5 / 0.035 = 143 ohms.
Multiply your circuit's required current by the internal resistance of the solar cell. This is the voltage that will be lost (or "dropped") by the internal resistance. In the example, the voltage dropped by the internal resistance = 143 x 0.02 = 2.86 Volts.
Subtract the voltage dropped by the internal resistance from the rated voltage of the solar cell. If the result of this subtraction is lower than your circuit's required voltage, the solar cell cannot generate enough power for your application. In the example, 5 - 2.86 = 2.14 Volts, so this cell is not sufficient for powering the wireless transmitter.
Repeat this calculation until you find a suitable solar cell.
Connect the positive lead (usually red) of the solar cell to the positive voltage supply of your circuit, and connect the negative lead (usually black) of the solar cell to the ground terminal of your circuit.
Orient the solar cell so it is receiving sunlight as directly as possible. The voltage generated by the cell will not change significantly according to the orientation, but the short-circuit current will. If the voltage on your cell seems adequate but it cannot power your circuit, you may need to find a better orientation.