This isn't hundreds of separate sites that have to be hacked individually. This is fewer than 10 clouds with no security to speak of and the ability to push evil firmware to millions of inverters worldwide, where in a few years at the current rate of manufacturing growth, it will be 10s, and then 100s of millions of inverters.

Yeah, the potato cannon filled with aluminum chaff or medium caliber semi-automatic rifle can take down a substation. But this is millions of homes and businesses, which can all have an evil firmware that triggers within seconds of each other. (There will inevitably be some internal clocks that are off by days/months/years, so it's not like it will happen without warning, but noticing the warning might be difficult.)

And the growth in sales is exponential!