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Using Mismatched Solar Panel Sizes

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Using Mismatched Solar Panel Sizes

This blog post will teach you how using mixed and mismatched sizes of solar panels in the same array will affect the output of the entire array.

Before we talk about mixing solar panel sizes, lets have a refresher for some, or a crash course for others on how wiring solar panels in parallel vs series affects their voltage and amperage.

  • Wiring solar panels in series adds their voltages while their amperages stay the same.
  • Wiring solar panels in parallel adds their amperages while their voltages stay the same.

How Does Wiring Solar Panels In Parallel Affect it’s Volts & Amps?

Here we see four – 100w solar panels wired in parallel, which means all of the positive wires are connected and all of the negative wires are connected.  Since Wiring solar panels in parallel adds their amperages while their voltages stay the same, we would add 5+5+5+5 amps to get a total of 20 amps at 20 volts heading into the charge controller.  We installed 400 watts of solar panels and by using watts law of V x A = Watts we can see that 20 V * 20A equals 400W for 100% array efficiency.  This means, of the 400 watts of panels installed, we can expect to see all 400w of power heading into the charge controller under ideal conditions.

How Does Wiring Solar Panels In Series Affect it’s Volts & Amps?

Here we see 4, 100w solar panels wired in series, which means that the positives and negatives of neighboring panels are wired together with the positives and negatives of the end-panels are going to the charge controller.  Since Wiring solar panels in series adds their voltages while their amperages stay the same, we would add 20+20+20+20 to get a total array voltage of 80volts and 5 amps heading to the charge controller.  We installed 400 watts of solar panels and by using watts law of V x A = Watts we can see that 80 V * 5A equals 400W for 100% array efficiency.  This means, of the 400 watts of panels installed, we can expect to see all 400w of power heading into the charge controller under ideal conditions.

How Does Wiring Solar Panels In Series-Parallel Affect it’s Volts & Amps?

Here, we see 4 100w solar panels wired in series parallel.  In this array, pairs of panels are wired in series with the two resulting series strings wired in parallel.

Since solar panels wired in series adds their voltages while their amperages stay the same which means that for each of the two series strings, we would add 20v + 20v which gives us a total of 40v and 5a for each of the two series string.

Since series strings wired in parallel adds their amperages while their voltages stay the same, we would add 5a + 5a for a total of 10A at 40V heading to the charge controller.

We installed 400 watts of solar panels and by using watts law of V x A = Watts we can see that 40 V * 10A equals 400W for 100% array efficiency.  This means, of the 400 watts of panels installed, we can expect to see all 400w of power heading into the charge controller under ideal conditions.

Now that we have out solar panel array basics covered; lets’ talk about wiring different sizes of solar panels in the array.

How does Wiring Different Sized Solar Panels Together Affect the Array Voltage & Amperage?

When mismatched solar panel sizes or mismatched solar panel series-strings are wired in parallel or series, the affect is VERY similar to the guidelines posted earlier, except for one change.

  • Solar Panels wired in series gets their voltages added while their amps stay at the lowest amperage of the panels in series.
  • Solar Panels (or series strings) wired in parallel get their amperages added together while their voltages stay at the lowest voltage of the panels (or series strings) wired in parallel.

What Happens when Different Solar Panel Sizes are Wired in Parallel?

For this example, we have two – 200w solar panels and 2 x 100 w solar panels.  The two 100w solar panels are operating at 20V and 5 amps and the 200w panels are operating at 25V and 8 amps.

If we were to wire all of these panels in parallel, solar panels in parallel adds their amperages while their voltages stay the same. This means we would add 8A + 8A + 5A + 5A for a total of 26 amps heading to the charge controller.  Now, although the volts stay the same in a parallel wired array, since we have different panel voltages, we must use the lowest common denominator, which is 20V.  So we have 20 volts at 26A amps heading to the charge controller.

We installed 600 watts of solar panels and by using watts law of V x A = Watts we can see that 20 V * 26A equals 520W for only 86% array efficiency.  This means, of the 600 watts of panels installed, we can expect to see only 520w of power heading into the charge controller under ideal conditions.

What Happens when Different Solar Panel Sizes are Wired in Series?

For this example, we have two – 200w solar panels and 2 x 100 w solar panels.  The two 100w solar panels are operating at 20V and 5 amps and the 200w panels are operating at 25V and 8 amps.

If we were to wire all of these panels in series, solar panels in series adds their voltages while their amperages stay the same. we would add 25v + 25v + 20v + 20v to get a total of 90 volts heading to the charge controller.  Now, although the amps stay the same in series wired arrays, since we have different panel amperages, we must use the lowest common denominator, which is 5 amps.  So we have 90 volts at 5 amps heading to the charge controller.

We installed 600 watts of solar panels and by using watts law of V x A = Watts we can see that 90 V * 5A equals 450W for only 75% array efficiency.  This means, of the 600 watts of panels installed, we can expect to see only 450w of power heading into the charge controller under ideal conditions.

What Happens when Different Solar Panel Sizes are Wired in Series-Parallel?

Now, lets say we were able to find solar panels of different wattages, but their voltages are the same, or at least REALLY similar:  In the earlier example of mismatched solar panels wired in parallel, our 20V at 26A figure MAY not produce enough array voltage to really let our MPPT solar charge controller do it’s job properly, OR we are installing a 24v battery bank and we need to boost the array voltage to a more appropriate level (Panel array voltage MUST be at leave 5V higher than battery bank voltage).

This array shows mismatched panel sizes of 100w and 200w, but we were fortunately able to find panels with similar voltages.

Wiring the similar wattage solar panels in series would yield 40V at 10A for the 200w panels and 40v at 5 amps for the 100w panels.  Wiring those two series strings in parallel would yield 40v at 15A since 10A plus 5A equals 15 amps and the volts stay the same at 40.

We installed 600 watts of solar panels and by using watts law of V x A = Watts we can see that 40 V * 15A equals 600W for 100% array efficiency.  This means, of the 600 watts of panels installed, we can expect to see the full 600w of power heading into the charge controller under ideal conditions.

Great! So, we can just always wire similar panels in series and wire those series strings in parallel, right?  Not so fast…

When Mixing Solar Panel Sizes is Not Advised

Same solar panels as last time, but if the three 200w solar panels were wired in series and the 100w solar panels were wired in series, then those series strings were wired in parallel, by all of the same math we’ve been using for the previous however-many diagrams, we would end up with 60V at 10A for the 200w series string, 40v at 5a for the 100w series string, and 40v at 15A for the total array. 

We installed 800 watts of solar panels and by using watts law of V x A = Watts we can see that 40 V * 15A equals 600W for 75% array efficiency.  This means, of the 800 watts of panels installed, we can expect to see only 600w of power heading into the charge controller under ideal conditions, which means that the two 100w panels are effectively useless in this array and by just using the 3, 200w panels in series, we could expect the same amount of power output with fewer panel

What Happens When Dissimilar Solar Panel Series Strings are Wired in Parallel?

If you are wanting to install, say, seven panels into your array; the only possible ways to wire these without severe power loss are series OR parallel. NOT series-parallel. Here’s why:

These seven 200w panels have operating voltages of 17V and an operating amperage of 11.76A. Let’s say our charge controller has a maximum voltage input of 100V which we must stay below. If we wired the 7 solar panels in series, we would see an array operating voltage of 119V; which would exceed the capabilities of the charge controller (NOTE: When sizing the charge controller; the Open Circuit Voltage AND temperature compensation must be used. More info: https://www.youtube.com/watch?v=MxziHKvTRh8). Wiring in purely parallel is not advised due to reasons we’ve discussed here: https://www.explorist.life/solar-panels-series-vs-parallel/ so to combat these issues, wiring the array in series-parallel is the common fix. Since 7 panels can’t be evenly divided (Those damn prime numbers strike again!), let’s see what happens when 2 series strings of three are wired in parallel with a single panel like the diagram above.

The two series strings get their voltages added while their amps stay the same, resulting in 51V and 11.76A per series string.

The Two series strings and the single panels wired in parallel get their amperages added while their voltages remain at the lowest common voltage from the wires coming into the combiner. This means that the array is operating at 35.28 amps and 17V. Using watts law of Amps x Volts = Watts we can see that 35.28A x 17V = 599.76W. Since we installed 1400W of solar on the roof; wiring as previously described will yield a measly 42% efficiency rate.

Literally omitting the 7th panel and only using 6 panels in a 3s2p configuration would yield twice as much power as having 7 panels wired as previously described.

Another fix – If 7 panels was truly desired in this scenario; a charge controller capable of handling the higher voltage should be purchased so the panels could be wired with all 7 panels wired in series.

Finishing up…

Unequal solar panels CAN be used in the same array, but proper array planning is CRITICAL to avoid array inefficiencies. But… If array inefficiencies are unavoidable, make sure you account for this in your power audit and manage expectations by making sure you understand how these inefficiencies can impact your system performance.

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