Adding Capacity To Your Existing PV System

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.

9 Comments

  1. Dave

    Are there not MPPT charge controlers capable of combinig 2 different array strings? I have the same issue of trying to combine 2 18v 4.72 pvs with 2 30v 8.35 pvs

  2. Jay Hersh

    Hi Dave,

    Not that I know of. If you put an 18V 4.72A in series with the 30V 8.35A panel the voltage will add and you’ll get 48V on the string but only at 4.72A. And if you put the 2 18V in series and the 2 30V in series you’d get only 36V at 8.35A on the string of 2 36Vs.

    Your best bet is to try to get 2 inexpensive charge controllers. For the 18V panels the power per panel is 85W. If you put them in series you’d have 36V at 4.72 and in parallel 18V at 9.4A with total power of 170W either way. If the 18V is Vmp and they are “nominal” 12V panels you could choose an inexpensive 12V PWM type controller that handles at least 10A for parallel or a 24V or 12/24V controller that handles at least 5A if you put them in series.

    I’ll leave it to you to do the math on the the 30V panels, but hopefully you get the idea. Choose a charge controller that meets the voltage and amperage spec for each separate sub-array and connect their output in parallel at the batteries. In your case you’re much better off with a PWM controller since given the losses you’d encounter by trying to combine these panels an MPPT controller won’t make up the difference. You’ll actually get more power at lower cost to go with 2 lower priced PWMs than by trying to combine the panels and use an MPPT.

    Hope this has been helpful.

    – Jay

  3. Stephen

    Expanding the number of panels in an off-grid system usually requires expanding he capacity of the battery bank. It seems that this is where the larger problem comes in. Even if similar batteries are used, the age of the older batteries will make them less efficient and will cause them to drain power from the newer batteries if they are connected together (series or parallel) before being fed into an inverter. In this case the first battery bank is about 18 months old. One solution would be to replace all the batteries each time the size of my system increases, but this is not ideal.

    I have been told that the only way around this is to separate the two systems: each battery bank (new and old) must feed separate loads through separate inverters. This is not an attractive solution because as loads change, one system’s batteries may be drawn down to 50% of capacity while the other may remain at 80%. Since I am completely off-grid, this requires running a generator to meet the load demands of the depleted system while there is still sufficient power in the other.

    How can an expanded PV system, with new and old battery banks, be integrated so that the composite battery bank drives a single, integrated load?

    Thanks for any suggestions

  4. ehsanullah

    Thanks for the information on your wonderful website and it is very useful. I have a quick question: If I design a system of 48volts PV with MPPT charge controller and use 24 volts inverter. Is it fine and will work?

    Regards,
    Ehssan

  5. Anton

    I have panels of different ages which I want to combine into a 24 V system. They are 8 x 75W panels (about 5 years old), 2 x 50W (old – bought 2nd hand) and 1 x 250W which will be new. Fortunately I have 3 charge controllers, so can keep the three groups separate as the article suggests. So the 8 x 75 W panels will be controlled by a Victron MPPT 75/50, the 2 x 50 W panels by a Phocos 10 amp contoller, and the new 250W by a Stecca PR3030 controller. Does this all look right? I presume I can wire the output from the three controllers to a central post and from there to the battery bank?

  6. JP Corcoran

    I have a completely off-grid system using a Trace SW4024 inverter. Until now, I have avoided installing a backup generator. Now, due to increased load and increased usage of the cabin, it is time to add a self-starting generator. I have 16 Deka 105 ah (12 volt) AGM batteries for storage and an array size of 1, 160 watts at a nominal 12 volts.

    Do any of you have a recommendation on a gasoline powered generator (I’d consider a small diesel as well) that has worked well with this inverter? I want to make sure that it will start and stop automatically and run efficiently within the parameters set by the inverter. I’m concerned about mistakenly purchasing too large of a generator or one that has reliability issues. Suggestions? Experiences?

    Many thanks,

    JP

    1. Jocelyn

      Hi JP,

      Since we do not sell generators I cannot give you any specific recommendations, but I can tell you most of the major brands (Generac, Onan, Honda, etc.) offered at home improvement stores have worked with the old Xantrex products in the past. You may also be able to off set your increased usage with more solar too as an alternative option if you have some sunny space available. If the solar option interests you, you can give us a call at 1-800-320-9514 and one of our sales reps can help you out.

      Regards,

      Jocelyn

  7. Billy Schutz

    What if I had two MPPT controllers feeding one battery bank, and the two controllers are being by supplied by arrays of different voltages, say array is 150 volts and the other 100 volts. Both controllers are set for 48 nominal battery voltage?

    1. Jocelyn

      Hi Billy,

      This is absolutely fine, as long as both controllers use the MPPT technology, the PV voltage can vary and you can hook both up to the battery and charge your bank from two PV arrays with a 48 volt output. Feel free to call us at 1-800-320-9514 if you are interested in any of the MPPT charge controllers we carry.

      Regards,

      Jocelyn

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