Sizing of supercapacitors

Sizing of supercapacitors

I would like to have some insight on the sizing of supercapacitors for regenerative energy harvesting.
Going by the formula E=0.5*C*(Vmax^2-Vmin^2), How is Vmin estimated ?

What are all the methods to estimate the regenerative energy left untapped uptill now ?

Also if one is to install a supercapacitor based system, what is the estimated cost per farad ?

Thanks in advance