Does this help Jeff? Picked it up off a forum.
The maximum power voltage of a nominal 12 volt panel (Vmp) will typically be 18 volts or more and the Open Circuit Voltage (Voc) will be even higher. The output voltage needs to be as high as 14.8 volts for some batteries at some times, and the input voltage has to be high enough to give some room for the controller work.
When you use a Pulse Width Modulation (PWM) controller for which the input and output currents are identical it is not good to make the panel output voltage too high.
But with an MPPT controller (with a decent input range) you can use much higher voltage panels (~36 volts or more) which are primarily intended for Grid Tie systems and you can put multiple panels in series to reduce the amount of current the wires need to carry.