The best way to test a solar panel generation
Basic measurements  are  Voc, Isc and compute R=Voc/Isc as your ideal MPT load from 82% Voc at max solarity.  

Since PV’s are a current source, your loading resistance at some voltage must track this variable equivalent resistance source with a matched impedance load. As this current reduces,  the optimum Vmpt also reduces to 72% Voc at half power and continues down from here. 

Depending how you convert to store this  variable power, will determine the useable energy per day. 

The efficiency drops away from these MPT operating points. 

Voc = open circuit voltage.
Isc= short circuit current.
MPT = maximum power Transfer (operating point)