I'm just a lowly newb, but I have a bit of a theory on this. At least if I get told to "siddown" I'll know if it's right or not.
AFAIK there's a certain priority system with regards to power usage. Power is drawn from the nuke reactor first and foremost. I figure this because lots of things will malfunction if I shut down the nucke reactor even if I've got 99 points of energy coming into the solar panels and full power to the Solar Reactor.
As I see it, the Solar Reactor appears to be for emergency purposes only, unless you are in a ship that gobbles energy for breakfast, like a Carrier and are in need of more than 100 power units. As a result, probably the only time you will have any need for power to be drawn from the Solar Reactor is in case of emergencies when your Main Reactor becomes damaged or even inoperable.
Even if only has it's operational capacity reduced, it may not be generating enough power for all systems, so reserve power is collected from the Solar Reactor. After all, you wouldn't want to have your shields or PTA suddenly go offline in the middle of a pitched battle. So don't forget to pay some attention to the level of your solar reactor, lest you be left without enough power to continue the fight or beat a hasty retreat.
Anyhow, that's my theory. I think I'll go search for what the Ion Disrupter Array does. I can't for the life of my figure out what it's meant to do.