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Mixing solar panels of different sizes


DeepBass9
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Is it possible to mix panels of different sizes in an array?

For example, I have 6x300w panels in 2 strings of 3. Say I want to increase the array size from 1800 w to 3000 w by adding 6x 200w panels, also in 2 strings of 3. Same voltage panel and same manufacturer.is there any reason why that wouldn,t work?

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If the strings end up having the same voltage it will technically work and not blow up but its generally avoided. One reason i can think of why this is not recommended is that if you run the exact same panels with all the same characteristics facing the same way and angle, the MPPT will track easily and accurately, this is not so if you mix panels. Different panels will have different "power curves" even if the OCV or rated voltage is the same, basically its not optimal and it will likely just perform poorly. 

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20 minutes ago, viper_za said:

Any reason why you are not going for another 2 strings of 300w panels?

if its cost then its understandable, but if its because the charge controller has a max pv power rating that you want to stay below then don't worry about that. 

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15 hours ago, DeepBass9 said:

Is it possible to mix panels of different sizes in an array?

For example, I have 6x300w panels in 2 strings of 3. Say I want to increase the array size from 1800 w to 3000 w by adding 6x 200w panels, also in 2 strings of 3. Same voltage panel and same manufacturer.is there any reason why that wouldn,t work?

Why not run the new strings off another mppt controller?

I have always been advised to match panels as closely as possible! 

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17 minutes ago, Mike said:

it is my understanding that the string will take on the value of the lowest performing panel, that is derate any higher than it.

We spoke of this before, with lots of good feedback re. volts being the main driving factor, so you can mix and match.

But everywhere I Googled to put my 200w panels with the 310w ones, no-one said it was a good idea, difference to big. So I have 2 controllers.

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Depends on the wiring. If you put a 200w in series with a 300w it runs at the lower value. If you put them in parallel and their Vmp is close, then there should be no derating. If Vmp differs between the strings, then the mppt will pick the voltage that makes the best power, which will likely be the lower of the two Vmp values, so one of the strings will suffer a little, but overall you should still see a net increase in power.

Sent from my GT-I9195 using Tapatalk

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16 hours ago, DeepBass9 said:

Is it possible to mix panels of different sizes in an array?

For example, I have 6x300w panels in 2 strings of 3. Say I want to increase the array size from 1800 w to 3000 w by adding 6x 200w panels, also in 2 strings of 3. Same voltage panel and same manufacturer.is there any reason why that wouldn,t work?

Usually solar panels consists of a specific number of individual cells all connected in series - the more cells connected in series the higher the power rating (and voltage) of the panel.  If you are going to use panels manufactured by the same company the cells used in the panels should have very similar characteristics. I am leaning towards the idea of maybe having 3 strings in total with 2 x 300W panels and 2 x 200W panels connected in series, per string. Then you will have 3 equally rated strings in parallel and the MPPT will be happy, but it all depends on if you are willing to re-wire the exiting panels and if the charge controller would be able to handle the higher array voltage - so please answer the following:

  • Will you be able to re-wire the current panels differently?
  • What is the maximum PV input voltage on your charge controller?
  • What is the MPPT range of the charge controller?
  • What are the open circuit voltages of the existing and new panels (or model number or link to the datasheet)
  • What are the MPP voltages of the existing and new panels (or model number or link to the datasheet)

 

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Everyone's answer has zero votes! And not much agreement either....

Lets take the case where I add another string of 200W panels to the 2 300W strings that I have:

So  :

300W + 300W + 300W = 36V + 36V + 36V =  8A at 108V 

300W + 300W + 300W = 36V + 36V + 36V =  8A at 108V

I add

200W + 200W + 200W 36V + 36V + 36V =  5A at 108V

Seeing as the voltage is the same (will be more or less in reality), then I have 21A at 108V = 2268 VA

At the battery charger that will be about 45A at 50V. My MPPT is 60A so no problem right? OR not?

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DeepBass, why not just get more 300w panels?

And if you have the 200w panels, can think 2 people who may be interested in said 200w panels.

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3 minutes ago, The Terrible Triplett said:

DeepBass, why not just get more 300w panels?

And if you have the 200w panels, can think 2 people who may be interested in said 200w panels.

For this reason : 

6 hours ago, DeepBass9 said:

I'm just curious if it is a limitation. I have a 60A Mppt so that limits me to 3kW. Another string of 300W panels is 2.7kW and a 4th string will be 3.6kW. Another 6x 200W panels is 3kW exactly.

 

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I read that then I though, who ever gets exactly on the dot what the panels can deliver?

Read here and elsewhere people oversize. If your 60amp controller cuts out / limits above that fantastic, if not, your on the dot 3kW can give generate more than 3kW on that perfect cool sunny day, potentially damaging the MPPT. 

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Just now, DeepBass9 said:

OK, I'll try 3.6KW, and send you the bill if the MPPT blows! :P

BWAHAHAHAHA ... if you had a real MPPT, it will simply limit or temporarily switch off. Cannot help you got the wrong thing. :P

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I have a 3kw grid tied inverter with 13 x 255W panels = 3315kW. The Voc of the string is lower than the Vmax of the inverter. Most days I get arround 2500W max but it has gone up to 3000W. I've had no issues. As long as the string volts is never higher than the Vmax of the MPPT/Inverter don't think it should be a problem. 

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17 hours ago, DeepBass9 said:

Everyone's answer has zero votes! And not much agreement either....

Lets take the case where I add another string of 200W panels to the 2 300W strings that I have:

So  :

300W + 300W + 300W = 36V + 36V + 36V =  8A at 108V 

300W + 300W + 300W = 36V + 36V + 36V =  8A at 108V

I add

200W + 200W + 200W 36V + 36V + 36V =  5A at 108V

Seeing as the voltage is the same (will be more or less in reality), then I have 21A at 108V = 2268 VA

At the battery charger that will be about 45A at 50V. My MPPT is 60A so no problem right? OR not?

Usually most companies make use of exactly the same cells when constructing panels of different sizes - larger panels consists of more cells all connected in one series string and smaller panels consists of less cells also all connected in one series string. By adding more cells in series the total voltage of the panel increases but the current stays the same. In your post above you claim the voltage to be the same and the current to be different between the different sized panels - are you 100% sure this is the case?

If the panel voltages of the 200W and 300W panels are indeed the same, then you can connect the strings in parallel without any issues, but if the voltages do differ I stand by my suggestion above (3 strings of 300W+300W+200W+200W) if your charge controller would be able to handle the higher voltages.

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Case point of 2 panels, same brand - quite a bit difference in volts.

200w panel - 54 cells
Voltage at max. power : 26.7
Current at max. power : 7.5
Open circuit voltage : 32.9
Short circuit current : 8

300w panel - 72 cells
Voltage at max. power : 36
Current at max. power : 8.4
Open circuit voltage : 45.05
Short circuit current : 8.8

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6 minutes ago, DeepBass9 said:

But you can get 36V 200W panels, the same as the 36V 300W panels that I have

Yes but your only reason for doing this is to stay under the 3000w limit?
Do you see a regular 1800W being made by your current array now?
If you have 6 strings of 300W panels you will have a broader range of higher Watts available during the day then say if you go wit 3x 300 and 3x 200w

10 minutes ago, DeepBass9 said:

Are you sure this will work SuperDIY?  3 strings of 300W+300W+200W+200W

I would not do that, you are going to loose the extra current the 300w panels make over the 200w ones.

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If you don't mix the panels in the right way you will loose a lot of power.  Here's why

Lets take the case where you have two types of panels 300W and 200W that you want to place in the same string. Lets assume the following is true for the two types of panels:

  • V open circuit300W = V open circuit 200W
  • V max power300W = V max power 200W

Then it follows that

  • I max power300W > I max power200W

But since you put them in series, the current that will run though both of them will be the same. The MPPT will search for volt and amp combination that will produce the maximum power for the string. So what would happen is that the I max power string will be somewhere between the I max power300W and the I max power200W In other words not optimal for either of the two types of panels. The 300W panel will never produce 300W and the 200W panel will never produce 200W

Now if you put the 300W and 200W panels in separate strings (parallel). The voltage over both strings will be the same. The 300W strings will then contribute I max power300W and the 200W strings will contribute I max power200W. So both strings will run optimally.

There is one other way it can be done in the case were the following is true:

  • I closed circuit300W = I closed circuit200W
  • I max power300W = I max power200W

Then it follows that:

  • V max power300w > V max power200W

If the above is true, then you should mix the 300W and 200W in the same string. Make sure each string has the same amount of 300W and 200W panels (the strings must be symmetrical)

The problem comes in when the panel's specs do not match up on the Volts or the Amps. Then it becomes quite difficult to ensure maximum power out of the strings.

 

 

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Don't mix panels, get 300W ones.

It is easy to forget that current is drawn from a source, while voltage is "pushed" from the source. This means that the panels cannot "give" the MPPT more watts than what the MPPT wants... the MPPT will use as many wtts as it can get, up to it's own maximum. Why do I say this? Well, if your MPPT maxes out at 3KW of PV then it simply means it cannot use more than 3KW. You can put a 5KW collection of panels on, it will not damage the MPPT... the MPPT will simply only use up to 3KW and the other 2KW will be wasted.

In other words, use 300W panels, go up to 3.6KW or whatever, and let your MPPT use as much of it as it can.

This is the same as using very large capacity batteries instead of smaller ones... as long as the voltage is correct it does not increase the power you suddenly get. The batteres just last longer but your instantaneuous power remains the same.

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Reading all and having had this discussion here before, I have again made up my mind: Don't mix and match panels unless they are as close as damn to each other. 

300w and 200w are not a close match.

If it was a very small X00w system, low budget and you get a 2nd Y00w panel cheap, off course it makes sense as the person will have more than before. 

But on a larger array powering a house ... no.

Three options:
1)) Get another controller for 200w array - but I bet the MPPT in the Voltronics cannot be adjusted to sync the two controllers.
2)) Get all 300w panels and be done.
3)) Or my favorite one: Reduce the load to fit the existing array - then you do not need more panels. :D

EDIT: @DeepBass9 Go ahead and mark this post as the answer - No mixing of panels. ;)

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