I haven't explored the solar side of this site very often, but I recently got a windfall of many odd sorts of panels, different voltages and watts and thought it would be nice to tap some of the great minds here on Permies.

Can I mix 36 volt panels with 20 volt panels (in parallel) and feed them all into a controller ? Will the 36v panel lose energy to the lower volt panels? Or will they all just work together sending all those nice solar electrons down to the controller

If so will the controller (mppt) really sort out everything and come up with a solution that makes the most out of all the energy available? Or will I have to take a substantial loss of amps that might have been produced when the controller reduces those voltages down to the correct charging voltage  .

Generally you are looking for a 10% maximum mismatch in voltage, but it is common to have arrays facing different directions on the same controller.

My home system does just that. Uni solar on the west roof and conergy mono on the east, tilted south. This is not ideal, not recommended by the maker, but it works fine.

You could run same type modules in groups to small controllers and then land them on the battery bus or run optimizers to avoid drastic mismatch and harvest more power.

I was looking at my controller today, hooked to older 20v panels, charging 12 v system, newer panels are 36 volt, so I will probably move the 20v panels to other places for pumps and such. and keep the newer 36 v panels on the roof. that will mean matched panels that are meant fore a 24v system, and I was really just curious if  anyone knew if a controller could step the voltage down and amps up that far. Maybe it depends on the controller, mine are not the cheapest , but far from the most expensive.

I have two set of questions that I need answers to, so that I can better help.
How many 36V solar panels do you envision, what is their current/wattage?
How many 20V solar panels do you envision, what is their current/wattage?

20v/36v=55% so you will be losing 45% of your power in parallel.
If they all have the usual 8A current then connecting them in series would be no power lost

I always hook my panels in parallel anyway, but it's good to have that validation. I would think though that the actual system voltage would be somewhere higher than the lowest voltage in the system.

I get how the minimum amps holds back the overall amps in series.

In parallel, the system voltage will always be the lowest voltage.
So if they all have around the same amperage, in series is actually better.

Do have an idea of ho many panels and amperage.

I'm guessing the answer is a great big 'it depends'. If you have panels listed at ~20V max power and panels listed as being high 30's to ~40V then they would probably be for 12V nominal and 24V nominal, respectively. Do you have any pictures or links to manufacturers websites for these panels, and a count of how many of each? Do you have other infrastructure for solar in place (wiring, batteries, inverters)? What MPPT charge controller do you have, or were you talking in future tense and haven't bought one yet? What is your planned usage for the power?

The answers to those and other questions would help put you on a good path towards getting the most from your panels. I would try and separate out which panels work with which nominal voltage (12V, 24V, or whatever) and if you have a few larger panels of similar power output then you might be able to use them easily with a good MPPT charge controller. Without specs it is hard to say. With the panels that are lower wattage, different voltage, or otherwise oddball from the majority, I would look at the much cheaper PWM charge controllers. You might not get the best efficiency possible out of the panels used this way, but if you already have panels just waiting to charge batteries then it can certainly work. I got a cheap $30 one off of Amazon around 5 years ago or more and it still seems to work fine and using the same pair of batteries that I started with. It probably wouldn't make sense to get a bunch of really expensive MPPT charge controllers for each individual panel, but it also wouldn't make sense to leave a bunch of perfectly good panels in the dark indefinitely.

I personally love the idea that a set of panels, along with the charge controller, batteries, and other related hardware, can be set up similarly to a single branch circuit in a house. Flip the breaker and only those loads applied to that branch are affected. Taking a boatload of panels, stringing them all to one point, buying a massive charge controller, connecting them to a single massive bank of batteries means you have numerous points of failure that can shut down all of your energy should a failure occur. I guess it really depends on whether you are grid-tied, or using it in a house where you need to pass inspection vs off-grid in a shed or something.

You have given me enough information/ insight to start to chart a course. The main drawback was just getting plywood down on the roof so I could mount some panels (beyond the five small ones up there now.

Toward the end of today I was covering up the roof and disconnecting them, but I also had moved  several of the other more powerful but mismatched panels up there, so now the roof is quite a mix of stuff, most of it hooked to nothing. So tomorrow early I go ahead and load the 7 newest matched panels still on the ground onto the roof and take down all the others. This will be my primary house supply--1500 watts in all.

The others will get used on secondary systems, and i do like the idea of multiple systems, so maybe I'll have a second system  set up with a dedicated battery bank for a refrigerator- freezer (I'm using a propane one now, spending 10-20$ each month (not counting travel time and car wear and tear). Maybe a third system for the fish tank filter, etc etc.

I have several smaller controllers on order--I usually only buy mppt , but the chinese seem to use those initials loosely, and even though I'm also spending 67$ on a new 80A controller,  maybe I need to consider them all mostly pwm's, and think about a really well engineered  one for the central system that won't waste so many watts.

I actually run one system without a controller at all, and have found two 35 watt (ancient) panels put just enough juice into the 12v pump battery to keep it charged and running. Hooking a controller in the circuit seemed to interfere with the charging, although those were all pwms I tried, 10 plus years ago, maybe the newer pwms are better.

today with a 20v panel and a 36v panel hooked in parallel , the controller read about 26 volts overall.

Your controller will run the input wherever the power is best in some conditions, at battery voltage at others and at open circuit. It might be best to read the module output leads open circuit.

The modules should just flow electrons. Modules are fine to mix as long as you keep within 10% match, widest spread across. If this is done, there will be no major detriment any larger than many non ideal design issues. Just mind your manners with the other details and having lights will be way better than not using the power conversion capability you have onsite. If you must, you could have a couple controllers to break it up. They are cheap, especially at lower current levels.

The 26v I mentioned was the voltage output from two panels in parallel, and more or less a direct data driven response to the formula put forward earlier in this thread that the lowest voltage in the circuit was the determinant voltage of the pv supply.

the power point modulated by the controller  was likely around 16-18 v, but I didn't directly record that.

Ok, my response looks at the reading as a reading of the pv input terminals..... which are dragged to the point the controller is dragging the panels. Unless you turn the controller off or disconnect at least one module lead, it cannot be determined what voltage the string is. The control if mppt, will have the characteristic of manipulating the input voltage also. If pwm, then the string could be dragged down to or close to battery voltage in bulk mode. An mppt will behave like a pwm control under certain conditions.

In my understanding they will meet in the middle and not go by the "lowest voltage" in the parallel. They will balance between highest and lowest.

So you're saying if I hook up in parallel, two panels of different voltage without a controller,  and read the open circuit voltage, I won't get some voltage in between the high and low values of the panels?

bob day wrote:So you're saying if I hook up in parallel, two panels of different voltage without a controller,  and read the open circuit voltage, I won't get some voltage in between the high and low values of the panels?

No, i said;

"In my understanding they will meet in the middle and not go by the "lowest voltage" in the parallel. They will balance between highest and lowest."

You and bengi said that the lowest voltage will be the voltage that the parallel groups will ruun at......

Data set above etc, etc.

That is not the case and the only way to read a module without interference is at open circuit or at mpp if it can be had, but not likely, so a read at open circuit will show raw unadultered module voltage max.

This will show the meeting at a middlepoint between the higher voltage and the lower. And it will not make the other circuits fail to produce power or destroy performance, thats all.

S Bengi wrote:In parallel, the system voltage will always be the lowest voltage.
So if they all have around the same amperage, in series is actually better.

Do have an idea of ho many panels and amperage.

In series, mismatch voltage has a greater detriment to the string. In parallel, the strings will balance and the effect of mismatch is not a major detriment. Of course strings of 2 are an order of magnitude different than strings of 12.

that actually makes more sense to me as well,  

the good news is with two identical  panels so far hooked up (i have seven in all) it's looking like I'll have  plenty of extra power--in fact I'm already thinking it's time to double my storage

Solar Panels have two set of voltage; open voltage (no load), and regular closed voltage (with a load e.g battery)
So a 20V panel will actual have a open voltage closer to 26V
And a 36V panel will actually have a open voltage closer to 42V
So when you combine them in parallel you will see a voltage of 26V for the open voltage which might look like the 'mid-way point' between 20V and 36V, but that 26V is just the open voltage and once an actual load is added it will go to the regular/loaded voltage of 20V

Also we are looking at overall power from the panels not just voltage or current.
So if you can, connect them in parallel with a load get the total power (V x I), then connect them in series, that would give better comparisons.
The load has to request more power than what the panels are rated for, because unlike wind/hydro solar panel only give as much as is requested including zero, which is why no dump load is needed.

Also it is best to think of a controller as a dynamic 'transformer', it will always try to change the actual panel voltage to what it thinks the battery needs.
So if the string of panels is 60V it will step it down to '26V' to charge a 24v battery.
Likewise if the string of panels is giving it 20V it will step it up to '26V' to charge the battery.
The voltage and current profile actually depends on if the battery is floating, charging, etc,
Thus measuring the voltage after the controller will not give the actual panel/string voltage, just the 'transformer' voltage.


Another more ideal (but super expensive) way to mix solar panels is after the multiple controller.

I found the site you used to produce the diagrams/formulae and have bookmarked it, but have too much to do to worry about it now. webpage

It may be that these are formulae given to field installers that helps them maximize systems, without needing to do the (likely) much more complex computations, a shorthand  of sorts . I say this because the voltage increase of the lower volt panel  (in my little experiment) was not proportionate to the increased size and voltage and total watts of the larger panel. To preserve all the watts (or most of them), the voltage increase should have gone closer to 30.


It's all just theoretical at this point, my system is already up with newer  matching panels and outproducing the old system, and our discussion has convinced me to match voltages as close as possible in parallel, and amps as close as possible in series as I go forward setting up outlying systems.

Oh, it has also reinforced my idea of many smaller independent controllers and battery banks

speaking of which, I just heard the water filter in my little koi aquarium come on, an independent system I set up with a good controller yesterday. That means the battery voltage is already up to 12.7. Seems like it will be a good day :-)

S Bengi wrote:Solar Panels have two set of voltage; open voltage (no load), and regular closed voltage (with a load e.g battery)
So a 20V panel will actual have a open voltage closer to 26V
And a 36V panel will actually have a open voltage closer to 42V
So when you combine them in parallel you will see a voltage of 26V for the open voltage which might look like the 'mid-way point' between 20V and 36V, but that 26V is just the open voltage and once an actual load is added it will go to the regular/loaded voltage of 20V

Also we are looking at overall power from the panels not just voltage or current.
So if you can, connect them in parallel with a load get the total power (V x I), then connect them in series, that would give better comparisons.
The load has to request more power than what the panels are rated for, because unlike wind/hydro solar panel only give as much as is requested including zero, which is why no dump load is needed.

Also it is best to think of a controller as a dynamic 'transformer', it will always try to change the actual panel voltage to what it thinks the battery needs.
So if the string of panels is 60V it will step it down to '26V' to charge a 24v battery.
Likewise if the string of panels is giving it 20V it will step it up to '26V' to charge the battery.
The voltage and current profile actually depends on if the battery is floating, charging, etc,
Thus measuring the voltage after the controller will not give the actual panel/string voltage, just the 'transformer' voltage.


Another more ideal (but super expensive) way to mix solar panels is after the multiple controller.

Thats better.....