There are a few educational comments I can add:
Most importantly,
Solar photovoltaic and using solid state decorative devices to light interior are Government assisted welfare for the industry.
Hands on experiment time:
You need a good clamp meter that can measure DC amps.
Connect AA battery and D battery in parallel (+ to +), (- to -)
On their own, D battery can provide more power and ideally you have the current share proportional to this. Realistically, the AA may overload and D don't get fully loaded. This is the problem with parallel wiring, but you can remove one and it will continue to work.
Connect them in series. Current sharing is guaranteed by law of physics, but the weakest link lowers the performance of the whole thing. A 600-1000v series system would be very vulnerable to shading.