Protect small DC motor from too much solar power
For this very specific situation - slight oversupply of power from an inherently current-limited supply - you can use a Zener diode.
Pick a 12V one. If you pick one with a power rating above 1.5W, it should be capable of surviving the entire power output of the solar panel, although it would only need to do that if the motor fails.
(For some reason I can't find a &quot;motor&quot; symbol, so have a lamp in the schematic instead)



simulate this circuit &ndash; Schematic created using CircuitLab
Note that solar panels usually can't put out both their maximum voltage and maximum current ratings at the same time. So in the normal case you'll probably see a lower voltage across the motor terminals than the peak 15V.