To get back to the OP the Voc or open circuit voltage should not vary greatly between the existing panels and ones that are added. If a 18v panel is added it will cause the controller to work with the other panels as though they too were 18v ones.
The Zamp Obsidian panels have a Voc of 25.4. A good and relatively small panel to consider is the Grape GS-Star-190W-US that has provides 190 Watts and has a Voc of 24.0 volts and sells for less than $300. It is 58.2 x 26.5 inches in size with a 40mm frame.
What is interesting is how many people will use very large gauge wiring to keep current losses below 3% but then put in a cheap controller that results in far greater losses.
Linear Mode
