There is a simple way that you can effectively increase and yes, even double the output power of your solar panel array.
You do this without any special tools or having to spend more money on PV (solar) panels.
For those who are electronically inclined we will be wiring the Solar Panels in Series - Parallel.
You need to do this in groupings of 4 like Solar Panels in order for this to work correctly. (ie., 4 15 watt panels in one section are wired as described in the steps below) The Solar Panels need to have the blocking diode removed if they have them. These blocking diodes prevent the back flow of electricity from the batteries back out to the solar panels at night. You will need to purchase replacement Blocking Diodes of the correct range. As you will need to attach them directly inline between the solar array and your batteries to prevent this back flow of electricity.
You will need to mark each solar panel 1 - 4 and line them up on your frame in that order. Mark them on the back side so that you can keep track of which panel is which. If at any time you get confused refer to the diagram in the introduction. If you click on the picture it will enlarge to give you a better view.
Wire the negative connector wires together between panels #1 and #3 then add a third wire to this negative connection that will run out to your Negative terminal at your battery. This connection should look like a Y.
Next you will wire the positive terminal from Solar Panel #1 to the negative terminal on Panel #2
Wire the positive terminal on #2 to the positive terminal on #4 and out to the battery. This connection should look like a Y
Wire the positive terminal on #3 to the negative terminal on #4
Do this for each of any 4 sets of like size solar panels that you have in your array. Each grouping of 4 solar panels now has the capacity to double the output of what they would have been if wired simply in series.
Next connect your Blocking Diode to the positive lead on your solar array that connects to the battery. This Blocking Diode is to prevent the back flow of electricity to Solar Array from the batteries at night. Remember to take into consideration that your PV output has doubled when you purchase your blocking diodes.