Add a Priority setting to transformers (primary, secondary, tertiary) same way as it is for example in the mod Editor Extensions (https://mods.factorio.com/mod/EditorExtensions).
Right now you can't prioritise solar arrays over coal generators or accumulators if they are separated into different power systems by transformers, this suggestion could help with that.

Mmm good idea, but tricky to implement. I won't be doing it any time soon, I'm afraid.

You can at least work around it via the circuit network. Wire a power switch to the transformer (on either end) before it goes to the rest of the network. You may have to stick a power pole between the switch and transformer, I forget, but that lets you cut the connection at will. I had mine set up using the time tools mod combinator to watch brightness, and disconnected non-solar when it was bright enough out.

+1