I am just full of questions today and getting a lot of great answers. I am looking into using a TL431 shunt regulator as the regulator for my solar panel. The panel puts out around 25 VDC at around .6 amps max and I need to regulate that down to 13.6 and 14.5. I figure I could use the PIC in my project to select the proper voltage at the appropriate time. I found this calculator on line. They don't allow you to play with R2. That is the one that is calculated. Also they don't suggest a value for R. What value should I use? What wattage?