The sun is not shining at night. Simple fact. So you have to provide information how expensive a kwh is. If you want to run a miner only when the sun is shining you probably never get roi. Your solar energy is also not free, it has a price. You have to buy and install it, sometimes even maintain or insure it. But you also have to ask yourself if the heat produced is waste or if you can use it. If you use ac to cool the room mining there even gets more expensive.