Harbor freight solar panels. 15 and 25 watt versions

[Supervstech]

They have a sticker on the back showing voltage and wattage...

Also a warning not to put them in series.

Why? I know little about solar panel wiring, and wonder what makes them suitable only for parallel...

My guess is to limit use to simple battery charging with their pwm controller...

 

[Delta-V]

My guess is for safety, fire risk, lawsuit, etc... reasons. They're probably not designed/ rated to be expanded.