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.
Very true, but a miner that mines for free is never obsolete so theoretically you could keep mining forever. Technically you'll always make your ROI eventually, although as you have said because of difficulty rises and other factors like price that could take some time. I really only think green/free energy is the way to go for people that really want to get into mining but have really high power costs or people who don't care as much about the money as they do the fun of mining and other cool and interesting projects.
The thing is that your miner will mine less and less every month and your income will gradually reduce. Eventually to a point where you can already call your miner obsolete. At this point you are "wasting" your renewable energy on something that is not profitable. For example, I could sell the electricity for 15c/kWh, but by using it for mining if I make less than 15c/kWh of Bitcoins, then I am losing money (wasting energy that could be sold).
If my expenses are double for the same income (understand: I have to invest in e.g. solar plus the miner) then the ROI will be longer, for sure. However, since your income is gradually decreasing (linearly or maybe even exponentially!) you may never even make a ROI if you keep your miner running.
I think investing in solar/wind is a good opportunity, I just don't see why it would have to be combined with mining, apart if it is a hobby project then for sure it is fun. For me these 2 will be separate investments. The idea of having "free" energy from renewable seems very appealing, but I am not sure if I couldn't rent a place with cheap electricity that would make more economical sense and give me a fast ROI at the end of which I'd sell the miner and invest in a new technology.