It is free ….not exactly ! , It has one time installation cost , and then it is usable for 15-20 years, with less maintenance. Yes- This is solar energy. We studied in our school days that silicon and germanium are semiconductors, this means that silicon and germanium both lies in the category in which they act like conductor (the material which allows ,flow of electron in it ) somtimes , on the other hand they can act like insulator ( the material which oppose the flow of electron in it )also.
This property of semiconductor, sometime condution , and sometime oppose,, varies on availability of Light/temperature and other environmental condition. means when Sun Rays Falls on semiconductor , then it has changes its property , it becomes conductor from insulator.
Rays from sun falls on semiconductor and its electron gets energy, then it receives power to cross the boundary of semiconductor and we collect these flow of electron in metal and then batteries, and thus current flows.
Sun rays is free to all, a efficient devices uses it , and make current at very low cost.
If you want to know more about Solar and its power transform , life of semiconductor , then contact to us…… at www.systemindus.com