Your working on a new long range spaceplane or something aren't you? Oh well.
You could use RTG's You could put them in a service bay to make it less draggy I guess, but they are still kinda heavy. The non-moving solar panels aren't that draggy and have the best power to mass ratio. The covered ones are also not very draggy but are relatively heavy. I don't know a lot about spaceplanes, but if you are trying to make a long range spaceplane, I would not recommend fuel cells.
I have no idea what you are doing, but because you are addicted to spaceplanes I would basically say the non moving solar panels or a few RTG's and some batteries. Just what I say. I'm not a power source expert. I do know a lot about impacting parts with stuff though. (I spent 10 minutes just trying to find out the best way to destroy a mk3 cockpit.)
Have fun with whatever your working on!
*whatever you feel when impacted with the random thing that @SpaceplaneAddict is working on.
max_creative's post in Ion Engines. was marked as the answer