Well, panels produce much, MUCH more power than RTGs. I use them for Space Stations and manned stuff. I always have at least 2 RTGs in unmanned probes/landers/vehicles, so that I can deploy panels for more power in the event of a crisis. In short, panels aren't effective for tiny probes going to Eeloo. They are, however, effective for large space stations or large interplanetary vehicles with lights or torque.