well sollar energy as wind would be the way to be able to roi but how many time you think withoyt pay eletricity would take 1 year? more less with the current price of 230 dollars.... would be better to sell and buy it ?
Don't forget that with solar and wind (or any other electricity source you buy and generate your own) you will have to ROI on the investment for that device too, on top of the miner. Essentially, it only extends your ROI and makes your income higher after the break-even point, where the difficulty probably would have already risen.