Have you considered the cost of buying the equipment required to generate the solar electricity. Just like mining equipment must be paid back by bitcoin mining profits, the cost of solar equipment is going to need to be paid back by the cost of kw/h saved by using solar as an alternative. It may take a very long time for your investment to come close to breaking even, cost wise.
I still believe that if you are in the correct location, cost does not matter to much.
I personally am in mining for the long haul and have no expectations on stopping. I want to see the currency grow to mass market....imagine where we will be with the currencies in 20 years. Imagine how effecient hardware will be in this time.
If you are in a place like the usa, all spare energy that you generate can flow back into the power grid which will help pay back the system. In all honesty, what does it matter if it takes 20 years to pay it back if your generating enough to power your full facility with room to expand. Hydro/wind/solar equipment last a verrryy long time. There is more use to these units even if the currencies vanish(free power for a home/few homes?)