For the sake of convenience, let's believe you possess a a 100 watt appliance or load that you would like to operate, free of charge through solar power, for around ten hours every night. In order to exactly deter. 1) First you will need to estimate how much watts of electricity you may require for the specified load. Let's say you have a 100 watt load that needs to be operated for approximately 1. 2) Next, we need to determine the approximate dimensions of the solar panel for satisfying the above estimated load requirement. If we assume a roughly ten hour daily optima. 3) Once you have calculated the solar panel as per the above calculations, it's time to calculate the AH rating for the batteries that might be required for operating the spe. 4) Now, to figure out how big your solar charge controllerwould need to be for the above calculated parameters, you might need to take your solar panel current or the Amperage spec.