backchannel: why would a 120w panel make only 15w at noon?

Why is a 120w panel tilted toward the sun only making 15w at local noon? 

Here is 100% of the information you need to answer the  question with reasonable certainty:



answer sheet

Short answer:  because only 15w is needed.   

Longer answer:  We know the controller is restricting power harvest since it’s running the panel at Vpanel == 21.6v, well above Vmp for a panel of that size. If the system were struggling to make the 15w Vpanel would be be Vmp, ~17-19v.  We also wouldn’t be seeing strongly-defined shadows in the pic. 

My guess is the controller is in float or late absorption with no meaningful loads attached.  

further reading