Seems like you are implying there's a non-real solution to that curve's maximum as well, which makes sense. Obviously directly these aren't useful, but I wonder if there are any situations where imaginary solns might also be helpful to know?
(In electronics if you find complex numbers they're usually being used as shorthand for complex exponentials via a Laplace transform — in this case, an imaginary or complex solution would mean that the PV panel has some kind of weird resonant behavior — again, unlikely to be the case for any panel you actually have.)