If you mine less than what you could have sold your free power for, you should not mine.
I hear about people having solar panels (paid off), so now they can mine all they want because power is free.
If you just sold the energy, you would most likely get more money than if you mined with it.
That really depends. Here, the price for energy is about 6 to 7 cents per kWh, you just have to pay a ridiculous amount of tax on it, making the total price around 22 cents. If you have solar panels that generate more than you use, you only get the bare price for that extra energy. At that price mining is still very much worth it.