Question: Can you tell me if I add another panel, does it have to be the same 120 wattage or could I add say 50 watt panel and not have any problems

Hi,

From my understanding you can connect mismatch and partially shaded panels together if you match up the voltage rating of the panels as close as possible, however it will have an effect on the overall performance of the circuit. Thin film panels and crystalline panels react differently to shading, because when crystalline panels have a partially blocked cell, it effects the entire line of cells within the panel creating a resistor. and lowering the output of the panel. Thin film panels are not affected as bad by shading due to the routing of the current within the panel. That being said, crystalline panels will be effected differently by wiring them in series vs parallel. Also, the mismatch can also lead to problems if the by-pass diodes are not rated to handle the current of the entire parallel connected array. Here is a link to more information on mismatch panels:

http://pvcdrom.pveducation.org/MODULE/Array.htm

With all that being said, to be on the safe side you should put mismatched panels (or partially shaded panels) on there own charge controller/circuit. That would fix the concern and would make your system more reliable by including some redundancy. Redundancy is good to have in a solar system because if one charge controller/circuit is broken, you still have another that is working until you can fix the broken one. So, i recommend buying an additional charge controller and putting the mismatch panels or partially shaded panels on there own circuit. I can sell you a 10 amp charge controller for the 50watt panel for $55 and you will not have to worry mismatch problems.

Thank you for your question and please feel free to contact me if you have any further question.
 

Link to video describing difference between thin film panels and crystalline panels when partially shaded: http://www.youtube.com/watch?v=7ZNgP_U64a8

http://en.wikipedia.org/wiki/Photovoltaic_module

Shade tolerance is important. It is better in glitter photovoltaic cell than in conventional PV panels, because portions of our units not in shade will keep sending out electricity where a partially shaded conventional panel may turn off entirely.^[10]

In a larger PV array, individual PV modules are connected in both series and parallel. A series-connected set of solar cells or modules is called a "string". The combination of series and parallel connections may lead to several problems in PV arrays. One potential problem arises from an open-circuit in one of the series strings. The current from the parallel connected string (often called a "block") will then have a lower current than the remaining blocks in the module. This is electrically identical to the case of one shaded solar cell in series with several good cells, and the power from the entire block of solar cells is lost. The figure below shows this effect.

Potential mismatch effects in larger PV arrays. Although all modules may be identical and the array does not experience any shading, mismatch and hot spot effects may still occur.

Parallel connections in combination with mismatch effects may also lead to problems if the by-pass diodes are not rated to handle the current of the entire parallel connected array. For example, in parallel strings with series connected modules, the by-pass diodes of the series connected modules become connected in parallel, as shown in the figure below. A mismatch in the series connected modules will cause current to flow in a by-pass diode, thereby heating this diode. However, heating the by-pass diode reduces the saturation current and effective resistance is reduced such that an additional string of modules is partially shaded. The current may now flow through the by-pass diodes associated with each module, but must also pass through the one string of by-pass diodes. These by-pass diodes then become even hotter, further reducing their resistance and increasing the current flow. If the diodes are not rated to handle the current from the parallel combination of modules, they will burn out and allows damage to the PV modules to occur.

Bypass diodes in paralleled modules. There are typically two bypass diodes in each 36 cell module.

In addition to the use of by-pass diodes to prevent mismatch losses, an additional diode, called a blocking diode, may be used to minimize mismatch losses. A blocking diode, shown in the figure below, is typically used to prevent the module from loading the battery at night by preventing current flow from the battery through the PV array. With parallel connected modules, each string to be connected in parallel should have its own blocking diode. This not only reduces the required current carrying capability of the blocking diode, but also prevents current flowing from one parallel string into a lower-current string and therefore helps to minimize mismatch losses arising in parallel connected arrays.

Impact of blocking diodes in parallel connected modules.