many small panels or fewer large ones? 12 vs 24?

[GAmountain]

2 questions:

1) If you want to get the most power out of solar panels on cloudy days/shading, is it better to have more small panels rather than fewer big panels? For instance (for a 2kw system for an off-grid house): 10 each of 200w panels or 20 each of 100w panels? I'm thinking of the shading effect, or impact of reduced power on smaller or larger panels.

2) If you want to get the most out of the panels on cloudy days/shading, is it better to have a 12v system than a 24v system? I'm thinking of the requirements for charging voltage if the panels are producing low power. Or will an MPPT compensate for any difference (therefore the choice doesn't matter on this question)?

 

[snoobler]

All circumstances have their trade offs.

The various scenarios may be more or less efficient for various conditions by ±2%.

A well balanced design will probably perform best over all. Weather based shading is typically the entire array or none. If there are fixed things that cast full or partial shadows on panels, those need to be addressed specifically.

 

[Steve_S]

MPPT controllers are far better at handling shade & adverse conditions compared to PWM which really doesn't cut it.

Micro-Inverters are actually the best solution for shady areas, because they work at the Panel level, not the String Level, meaning that the entire string will not be lowered due to one shaded panel.

Panel Power Optimizers are another option for shady areas but this takes you into yet another method of solar generation tech. More info and great overview here: https://www.solaris-shop.com/blog/power-optimizers-everything-you-need-to-know/

Larger Panels cost much less per watt than small panels., there is also less BOS (wires, cabling, connectors, racking parts etc) which is yet another cost saving. There is mixed input with regards to MONO or POLY panels performance in shade and to be honest, I personally don't think there is much of a significant difference. I suppose if you want to "nit pick" there may be some marginal points.

Be aware that 450W to 665W Panels are on the market now, these new generation panels are helping to push the cost of lower wattage panels down. Right now, 300-375W Panels are at a great price point in general and if considering used panels, these are the best bang per buck.

12V/24V/36V panels are more or less moot... It all has to do with your solar controller and how many Watts & Volts it will take. If the SCC takes a Max of 200V, you can use 8x 24V Panels or 16X 12V panels to get there, the SCC will determine their max input based on the Voltage of the battery bank it is attached to. This is why you will see different ratings on SCC's for the various battery bank voltages for solar input.

edit: I was still chugging mugga #1 and my math went sideways, so fixin it. lol.

 

[GAmountain]

So, I know just enough to be dangerous, but here goes... I'm inclined to go with fewer larger panels, but I wondered about the potential downsides. In the case of shading, I've heard that if you shade one cell completely, the whole panel loses output. So if I had one 200w panel, it would all go dead. However, if I had two 100w panels, and shaded one cell, I would lose just that 100w panel and the other panel would function, correct? I agree that taking shading out by managing the installation makes sense, but I wondered if some of the shading concerns might apply to a cloudy or overcast day. I'm likely to get plenty of those in the Georgia mountains. And does a 200w panel require more threshold intensity in order to be useful to the MPPT charger? Or is it just as likely as two 100w panels to provide usable power on an overcast day?

Likewise, would a 24 battery system require a higher threshold of voltage from the panel in order to function (vs. 12v), or does the MPPT work with low voltages and boost them to where even a little power is still useful?

The system I'm envisioning (I'm in the idea stage) is a 2000w array connected to a Titan solar generator (with a second battery). It has two MPPT charge controllers, each rated at 40a. The Titan is a 24v battery system.

 

[Steve_S]

Take my system for example, see the About my System link in my signature.
2kw solar panel using 8 Canadian Solar CSP-260P setup 4s2p. These are "24V" 60 cell panels.
My Midnite SCC takes 200V and 2100W for a 24V battery system, Max Amp Output for charging with my C200 is 79A.
Even on a cloudy day they still generate something but lower amps delivered, that's the nature of the beast. Nice day like today, I was in Float by 1pm. I'm using 11A right now and it's all coming off solar, battery bank is full.

What I paid for those panels & controller back then, I could have 16 330W panels and two SCC's in parallel.

I get some shade in summer for part of the day because of an ancient tree I refuse to cut down (I happen to live within a forest) so the input varies a bit but not significantly.

One way to deal with such issues is to over panel to compensate, this is something fairly usual up here in the north, BUT (it's doozie) not all SCC's can handle it, or handle it well. The Tier-1 products all offer some form of allowance because panel efficiency is affected by temperatures as well as other issues, cooler = more generation. Additionally, when solar trackers are used, there are often periods where high generation occurs, so they are equipped to deal with it. With the cost of used panels these days and new, old stock & even B-Grade panels, the few extra panels won't set you back much at all. Panels are actually the cheapest part of all of it unless your going for the latest hottest stuff like the 600W+.

 

[Jeremiah]

...
Likewise, would a 24 battery system require a higher threshold of voltage from the panel in order to function (vs. 12v), or does the MPPT work with low voltages and boost them to where even a little power is still useful?

no

 

[GAmountain]

Steve, thanks for your response. The Titan takes a max of 2000w of solar, so a plan to go with 2000w is as much as I can use. And I think it is generally over-sized, except for the summer when I would run a small Air Conditioner. Larger wattage panels would also take up less roof space, which is another reason I'm interested in them. Given the same wattage, then, would you expect no appreciable difference between 10 each 200w panels vs. 20 each 100w panels (as a case in point)?

 

[GAmountain]

Thanks, Jeremiah. I'm reading your comment as 'no, the MPPT doesn't boost low voltages.' So if I expect my situation to often have overcast or cloudy days, would I be better off with a 12V system to squeeze more energy out of it, rather than a 24V system?

 

[gnubie]

Solar panels still ise up to their Voc even in heavy clouding. All else being equal, ie we have a typical cloudy day etc etc etc, an array built around '12V' class panels produces the same watts as an array built around '24V' class panels.

If you built either array and put them into a 12V battery the charging current would be 'the same'. As per usual it's important for array voltage to be over battery voltage by a good margin but that's another story.

 

[GAmountain]

Gnubie, I think my question was more about 12V vs. 24V battery systems than nominal panel ratings. But your comment about performance in heavy clouding leads me to the more newbie question, as I try to understand the electrical response or panels to sunlight and how the MPPT controller controls the panel response. So, with all components and other variables being equal, as the intensity of sunlight changes, what changes in the panel response? Does the amperage change and the voltage stays the same? Or do volts and amps both change as intensity changes? Is that the power curve that I've seen (related to how the MPPT optimizes power)? I had thought that movement along that curve was based on a load or resistance managed by the MPPT controller rather than sunlight intensity. But I haven't quite understood what's going on electrically with the panel, nor how the controller does its controlling of the power. I follow that the sunlight impacts electrons on either layer of the cell, creating a voltage potential between them, and inducing a current flow. But what happens to those variables as the sunlight intensity changes is not clear not me (nor how the charge controller manages it). Can you all help me figure this out?

 

[gnubie]

Idealised, the current that can be drawn increases but the voltage remains constant. There's a lot of detail chopped out here for simpliticy's sake, cell temperature, charger performance etc etc. I can explain it all in detail but I'm not so good at explanations so it'll get a bit confusing. Someone else that is better at explanations might turn up and post some detail for you.

If you look at the IV graph you will see a fairly flat maximum level of current to the left of Vmp. That line will go up and down in relation to light exposure. The unloaded voltage of a cell will rise to around 0.6V depending on the cell. It's up to the MPPT controller to scan a range of voltages to determine the optimum wattage point by drawing more and more current to load up the cell and pull the voltage down. The MPPT controller can only do this as long as it has a load to provide the energy to. Without a high enough load it may not be able to draw enough current to locate Vmp.

Vmp itself will wander around a bit as the cell temperature varies.

 

[GAmountain]

Thanks, Gnubie. And the load for the MPPT to do its thing would be essentially the same whether or not the battery system were 24v or 12v?

 

[gnubie]

More or less. The charger doesn't care in this regard so long as what ever is on it's output terminals will draw all the current its making available. This is why the array voltage goes up when the battery approaches fully charged and its charging current goes down as a consequence of high SOC. It's also why the array voltage goes up if the controller hits its output current limit. Without the load being able to draw enough power the charger can not hold Vmp.

With a typical MPPT SCC the array voltage must always be higher than battery voltage and since we want maximum power that means Vmp must always be greater than battery voltage, by some margin too.

 

[GAmountain]

Thanks, gnubie. Your last comment is part of why I ask about 12v vs 24v battery systems. If the array has to have higher voltage than 24v to charge a 24v system, would that not mean that an overcast day might be able to charge a 12v system, but not a 24v system (voltage might be under 24v, but above 12v)? Or are you saying that voltage output has more to do with panel design (voltage specs) than sunlight intensity? Voltage has to do with panel specs and sunlight intensity just changes amps available to charge?

 

[gnubie]

Even on a cloudy day the array voltage will rise up to Voc but the current available at a given voltage will be lower, ie it the array will still be able to charge a 24V battery just at a lower amp rate. Once there is enough light for you to read a book there is enough light to bring the array to full voltage with a very light load. Panel voltage is more determined by the number of cells in series once that low level of light is reached, but obviously if we place too high a load on the panels the voltage will collapse. It's the MPPT charger's job to balance this.

 

[GAmountain]

So no appreciable difference when it comes to often cloudy installations, between 10 each 200w panels vs. 20 each 100w panels? And no appreciable difference when it comes to cloudy installations between 12v battery systems and 24v battery systems? My question is about which options performs better in cloudy conditions, all else being equal.

 

[gnubie]

Watts is Watts, how you get there doesn't matter with in the limitation that the array configuration must produce voltage over battery voltage to permit charging. Cell surface area being equal, power produced is equal.

 

[Hedges]

In the case of shading, I've heard that if you shade one cell completely, the whole panel loses output. So if I had one 200w panel, it would all go dead. However, if I had two 100w panels, and shaded one cell, I would lose just that 100w panel and the other panel would function, correct?

You don't lose the entire panel, just the diode-bypassed section with the shaded cell.
Some panels might have just one bypass diode. Two is typical. Larger panels usually have 3. Maybe some are more.
The string of cells with a bypass diode apply their voltage to the one which is shaded, reverse-biasing it. There needs to be a bypass diode every so many cells to keep that voltage from being high enough to cause breakdown.
It is very likely a 300W or 500W panel has more bypass diodes than a 100W panel, so while loss from a single shaded cell may be greater, probably not something to worry about.

Get the best deal per watt on the quality of panels you're interested in. Make sure the voltage and current ratings support a series/parallel configuration good for your inverter or SCC. Make sure they fit physically, and that you can manhandle the size and weight.