Quote by <BM>
Power = Voltage x Current, you can get double the voltage in series but you half the current so no net gain. No power into the panel, no power out.
Yep, solar controllers will only start charging batteries when a minimum voltage is reached, usually about 16v. Two panels in parallel will have a max voltage of about 20v (but higher amps) whereas two panels in series have a max voltage of 40v (but lower amps) therefore the panels in series reach the voltage threshold of the controller with much lower light levels than a parallel system.