Is it possible to mix and match solar panels within the same PV system? Can different models of solar panels be wired together within the same series string, or is it alright to have a string of one type of panel in parallel with a string of another type of panel?
In theory, the answer is yes if the panels match closely enough in voltage and current. In practice, we don’t recommend doing this.
There are several reasons why it is difficult to mix panel types:
- The output power of panels changes over time. Existing panels that have been installed have power outputs that have degraded over time, and most likely will not meet the specifications on their nameplate. By trying to match new panels to the ratings on the nameplate specification, even a panel that seems to be an exact match, most likely would not have a matching output to the degraded output of the existing panels.
- Panel voltage is additive when panels are wired in series. The only way to wire in additional panels and guarantee that the voltage of all strings would remain equal is to add the exact same number of new panels to each string. The drawback to doing this however is that within each string all panels will produce the same current as the current of the lowest panel. So if you buy a panel that has a higher current it won’t produce the amount of current that is listed on its nameplate (it would perform at the lowest performing panel’s current), and you’d essentially be paying for a panel that will produce less power than its rating.
- Panel current is additive when panels, or strings of panels are wired in parallel. Say you were to try to just add the new panels all in one or more strings composed of only the new panel types. While each of these strings would be able to produce the maximum amount of current they are rated for, the voltage difference between the old strings and new strings becomes a problem. If the panels in the new strings had a higher voltage rating than the panels in the old strings, the new string, and each panel within it, will only be limited to operating at the same voltage as the old strings. So again, you’re essentially paying for a panel that will produce less power than its rating.
- If you are using an MPPT charge controller, mixing panels of different voltages and/or currents within an array will have an adverse effect on the entire system. Different module outputs make it impossible for the controller to determine the optimal operating voltage and current because they will be different between the different types of panels.
So how do you handle a situation where you have an existing system and want to add additional capacity?
The answer to this is very simple actually. The one known constant within any off grid or grid tie with battery backup system is the operating voltage of the battery bank. So the best approach is to combine the output of the new panels with that of the old panels at the battery bank. The only way to accomplish this is to use one or more additional charge controllers (and combiner boxes where appropriate) with the new panels.
With an additional charge controller for the new panels, the entire array of new panels is a 2nd “sub-array” of your overall system, and the original set of panels now called the 1st sub-array. Each sub-array is connected to it’s own charge controller, and each charge controller has a charging output set to charge the battery bank at the proper voltage. With the matching outputs of the charge controllers, you can now combine 2 or more power sources of equal voltage together in parallel and the current from them adds together. This allows you to add additional panels to an existing battery based system and still maximize the output of both sub-arrays.
The above is fine if you have a battery bank, but what if you have only a grid tied inverter and no battery bank? Unfortunately in this situation your best approach is to simply purchase an additional grid tied inverter and connect it up to a breaker in your main AC power panel. By doing this you again take advantage of the fact that for 2 or more power sources of equal voltage joined together in parallel the current from them will add together. The only difference is that instead of this addition being done at the battery bank with charge controllers it is being done right inside your main AC power panel.