An S9 costs $150 and will need 1,400W (unless undervolting --- down to even 700W, which isn't hard to do), and it will only produce about $1 per day in bitcoin when running 24/7. Run it just 6 hours, and that is $0.25 per day in revenue, or $7.50 per month, or $90 per year (at current exchange rates and difficulty level).

The rooftop solar project shown as an example in the article is 7.5 KW. Let's say half that goes to household use (including recharging batteries) and that might leave enough excess power to run at most two S9s.

So the point I am making is that the revenue from bitcoin mining is such a relatively small amount that it should really have almost no part of the argument as to whether or not to invest in adding rooftop solar.

If you are going to build rooftop solar anyway and will have extra power, and S9s (or other hardware) are at "throwaway" prices, then sure -- plugging one or two in for a few hours of the day might not be the worst way to acquire some bitcoin (e.g., similar to buying bitcoin using daily cost averaging). But it should not be the main reason as to why you would add rooftop solar.