The "Power Center" consists of a trace inverter, trace charge controller, and a trace disconnect box.  There are eight golf cart batteries in the box underneath the power center.  The white tube coming out of the battery box is for venting the hydrogen gas produced by the batteries.  A special motorized fan located in the tube pumps the gas out when the batteries are gassing.

The excess power produced by the panels is stored in the battery bank.  When the sun doesn't shine power is drawn from the batteries to the inverter which converts it from 24 volt DC to 115 volt AC.  This all takes place automatically.  Below is a graphic representation of the output from my solar array on a sunny day.