Electrofelon, I know what the book says, but the words "continuous load" aren't as obvious. 690.8 seems to apply to the DC side, for instance.
What I am trying to say is that even if the book says solar is a continuous load, it really isn't. Take a look at this solar performance screen shot on three cold February days:
View attachment 2565431
The first day was clear blue sky, and the system produced the classic half sine wave. But its maximum power peaked for only part of one hour.
The second day, some clouds reduced the peak and cut a couple of notches in the sine wave.
The third day, we had snow and that 110kW system barely harvested enough power to brew my coffee.
That is simply NOT a "continuous load."
As I said, we follow the rule but I don't like it and don't think it accurately reflects what really happens. The notorious intermittence of solar and wind harvesters is what drives the 24/7/365 power folks nuts. They need to have a lot of sine curves like day 1, OR zero-days like day 3.. There is no way to throttle up their massive steam turbines to react to the cloud blips in day 2, and line impedances, automatic tap changers, and the like may not even allow them to see those blips.