ALL commodity returns (including the mining of digital assets) will fall to the marginal cost of production. Which is necessarily less than the overall per unit cost of production because of capital costs. In other words, it's impossible to remain profitable if you're producing commodities.
If the capital cost was already defrayed for another purpose (and mine was), the CAPEX to invest in mining Monero approachs zero. That is the beauty of CPU-only algorithms.
Good point. However, in practice, almost everyone for whom this would be a benefit is paying retail electric rates. This also ignores the bot-nets that drive the difficulty much higher because the operators are passing off the capex AND electricity costs.
True enough, on all points. But almost everyone in Western countries ought to invest in a little solar anyway, purely to have some electricity during those periods when poor policy choices make the grid unstable and/or unaffordable. If you also use your surplus solar for mining, some beautiful synergies are to be had.