Solar power for a mining rig is not a good idea. Solar power is not reliable for something meant to be draining power 24/7 for the simple fact that even in the most sunny regions there's always night time. Solar energy is better to be consumed on the spot in order to avoid inefficiencies. But that means you'd also need a stable power source. Solar panels would be a good long term investment to save money on energy you purchase, but for something like a mining rig it would just add up more dept and make the whole thing more inefficient and unreliable.
if you have an s-3 that has paid itself off you can run it during sunlight hours.
if you have a new s-7 that has not paid itself off you are correct.