I'm not sure if you are asking about energy ROI as compared to bitcoin mining ROI or as they relate together, so I'll break-down on both thoughts.
1) Energy ROI: If you want to produce your own energy with solar, wind, etc. then the most important variable is the price you pay for the system.
The next important variable is the cost of electricity where you live, usually priced in $ per kWh (kWatt-hour).
You will have an ROI on your energy investment when your solar panels will have produced enough kW that you saved on your utility bill what you invested in the system.
Example: Let's say you pay $10K for a set of solar panels and those solar panels produce 10 kWatts of energy for you at peak time (around noon) and over the course of the day may give you a total of 50 kWh of power (we are assuming that all of the energy is consumed and that you aren't using a storage battery, just to make the analysis simpler). Let's say the price of 1 kWh in your area is $0.15. You generate 50 kWh a day, which amounts to saving 50 x $0.15 = $7.5 every day. It will take you $10,000 / $7.5 = 1,333.33 days (3.65 years ~ 3 years and 8 months) to have your ROI on energy.
2) Now, if you are using all the energy produced by the solar panels to mine bitcoin, then you can estimate how many bitcoins your mining hardware can produce in a day and decide if you keep the bitcoins to sell at a potentially larger (or lower) price later, or sell them right away.
One thing is certain, you can have a faster ROI on your energy investment if you mine bitcoins with the energy you produce.
However, if you have to pay for new mining hardware, then you have to include the cost of the hardware with the cost of the solar panels and you will probably still have a similar ROI overall. BUT, you will be left with free-energy from solar panels and shiny mining hardware for future use at the end, so an investment in both may be a great idea if you can afford it.
I hope this makes sense.
Also don't forget to factor in that a lot of government entities directly subsidize solar power installations or other renewable sourced energy. (It doesn't make sense to me mathematically, but I am not a government).
So on a 12kW system that I just got installed, I receive 30% back as a government tax credit. I also receive a specific subsidy based on the number of kWh my solar power produces - regardless of whether or not I consume the kWh. Versus normal electric even with just the net metering I wind up saving 0.06USD/kWh.
Thank you so much guys for your replies. I hadn't the time to do it earlier but it is much appreciated.
