Here is a great pointer to how to do Solar Calculations:
A solar panel will generate electricity when placed in the sun. Current will flow from a panel connected to an electrical circuit. How many amps of electricity the panel will produce depends on the power of the solar panel, the amount of sunshine falling on the panel and the characteristics of the circuit to which the panel is connected. Calculate the amps produced by the panel making measurements using a digital multimeter.
Look at the back of the solar panel or consult the installation manual and find the maximum rated power of the panel in watts. Look also for the maximum power voltage, Vmp, which is in volts.
Calculate the current produced by the solar panel when it is generating its maximum power. Calculate the current in amps by dividing power in watts by the voltage in volts. For example, if the solar panel is rated at 175 watts and the maximum power voltage, Vmp, is given as 23.6 volts, then calculate the current as 175 watts divided by 23.6 volts, which is equal to 7.42 amps. This is current produced by the solar panel at full power.
Take a digital multimeter and switch the dial to direct-current volts. With the solar panel connected to an electrical circuit, measure the voltage between the positive and negative terminals of the solar panel. Make a note of this value. Disconnect the solar panel from the circuit and switch the digital multimeter to measuring resistance. Measure the resistance of the electrical circuit in ohms and write down this value. Reconnect the solar panel to the circuit.