Right now my Solar Panel controller sends excess electricity back to the grid when all my batteries are charged. I would like to use my excess electricity to power a mining rig. What is the most inexpensive way to start mining? I have old I7 powered PCs loaded up with Debian Linux, and can get GPUs if necessary. I would assume I can start with CPU mining and add GPU's as I go? I know that CPU mining is not viable if you are paying for electricity, but in this case I am not. What can I do with what I've got? Any guides would be helpful.