Parallel strings and overpaneling or how to maximize PV production on a single inverter.

[Calvin98]

Parallel strings and overpaneling or how to maximize PV production on a single inverter.

In the past I was told that you could safely add 20% more panels to an inverter than the name plate rating, i.e. on a 5kw inverter, you could put 6,000 watts of PV panels. New SMA docs say you can overpanel by 60%. i.e. you can add 8,000 watts of panels to a 5,000 watt inverter. On new SMA inverters that are coming out you can supposedly overpanel by 100% i.e. 10,000 watts of panels on a 5,000 watt inverter. From what I understand the extra power is just not used (whatever that means).The most it will ever put out is 5,000 watts.

Back in the day when panels were expensive, overpaneling was not practical, but now with prices so low, overpaneling and stings of different orientations are practical.

A 5,000 watt inverter is only producing max power of a few hrs. at local noon (assuming relatively sunny (shade free) location.

The question are: How can you maximize output on 1 inverter? Can you get it to produce 5,000 watts continuously from sunrise to sunset? What do you have to keep in mind?

In general, panels are oriented due south for maximum output. That leaves the mornings and afternoons with low power outputs.

Say you already have 6,000 w of panels facing South on 2 MPPT inputs. Say you want to add 2 more arrays, both vertical to capture early morning and late afternoon sun, but 1 facing due West and the other array facing due East. That is ok as long as the min max DC voltage and amperage conditions are met?

Can you put both arrays on the same 1 string? You only have to meet the min dc voltage? They will never both produce power. Or should you put 1 array on each input? i.e. South plus East and South plus West? Can you put 3,000 watts of vertical East facing panels to the same input as the 3,000W of South facing panels?

Can strings be different sizes, i.e. 1 string of 8 panels facing West and the other 10 panels facing East? Both connected to same MPPT input as long as the min DC voltage is met?

Can you mix strings of different voltages? If yes how? Can you have a string of 3 panels at 100V be connected to a 300V string facing a different orientation?

Are microinverters maybe a good solution for some of these situations? The ones on Amazon seem to overheat and shut off, plus they seem to fail rather soon.

If there is a sunny area, but it only fits 4 panels, can those 4 panels be connected in parallel to a different string. Keeping in mind not to exceed the max current.

Lets say you have an area that gets 2 hrs of good sun, how can/should you connect that to the inverter to capture those 2 hrs of power?

Example:
Sunny Boy 5.0-US Inverter
3MPPT trackers, 1 string/MPPT tracker
Max DC Voltage 600V
Max PV Power 8,000Wp
Rated MPP voltage 220V-480V
MPPT operating Voltage 100-500V
Min DC start Voltage 100V/125V
Max operating input current per MPPT 10A

I’m guessing there is a simple rule that you can apply to all of these situations.

 

[Mattb4]

Lots of questions.

Let's set some definitions so understanding might happen.

1. Solar panels do not push power. Loads draw it. People use term Clipping to describe when the load from the SCC is maxed out, but if more load was available the solar panels could produce it, because of panel wattage ratings. That unused potential power is "clipped". Actually it simply is not produced. If you were to graph power production you would have the top of your solar production curve (sun rising than setting) be cut off into a flat line as the SCC could not pull any more power.

2. Strings of panels are simply panels wired in series. Panels connected to one SCC are an Array (this could be one panel or several strings of panels wired in parallel (such as 12 panels wired 4S3P).

3. Overpanelling a SCC occurs at anytime the SCC max rated amps for charging (and supplying loads) is exceeded by the potential PV watts. The SCC does not know how much PV is out there it simply draws power based on its needs. If insufficient power is available it cuts back on how much it than supplies to the common DC bus.

4. Connecting panels in series will add total voltage while keeping the amperage the same. Adding panels or (strings) in parallel adds for the total current but keeps the voltage the same.

 

[Calvin98]

Right.
What is SCC?
I've always connected 10 panels in series to the Sunny Boy as that is in the mid range of DC volts - around 300V +/-
Another way to put it all is that the 6,200 watt South facing array (2x 10 panels x 310w) is only producing 5,000 watts between 11am and 2pm. Before and after that is it always less. How can you maximize the output on the 1 inverter with more panels since they are so inexpensive now.

 

[profesor79]

Right.
What is SCC?

SCC stands for: Solar Charge Controller

 

[profesor79]

to max out system production, those things that I will consider:

• add more panels (this is done per what you are writing)
• add tracking to corelate panels angle with sun position
• add more MPTT chargers, as they are cheap as well, and can offload your inverter
• avoid clipping, as that put hardware at stress (that kills the electronic inside) - so better is to have 20% buffer

 

[Calvin98]

SCC - ok, that is also built into the inverter already. The Sunny Boys have 2-3 MPPT controllers built in already. Trying to maximize use of those.

 

[Hedges]

20% overpaneling is need to achieve full inverter output, because PTC or NOCT output is less than STC rating.

SMA has recommended a certain maximum wattage PV. Where that exceeds what an inverter can process, that is overpaneling.

But Sunny Boy Smart Energy is a hybrid. They say that you can put on PV panels for 200% of the 7.7kW output rating. It can also charge battery with up to 10kW. If they only recommend 15.4kW not 17.7kW of panels, maybe the (three) MPPT are limited to that. (Sometimes people connect excessive wattage of panels to MPPT A only, and don't get full output power. Paralleling A & B fixed that.)

SBSE has 15A per string operating limit, vs. 10A for SB. But some panels these days put out 10A. Two strings of 7A panels (same orientation) could better utilize it. Two string of 10A panels only if large angle between them.

I still say you can overpanel with multiple orientations, e.g. sqrt(2) = 1.4x peak wattage if array has two halves 90 degrees apart. So I would be inclined to put 22kW of panels on SBSE 7.7 (I won't because I don't have that much roof space.)

Obey string voltage limits.
String current limits, if exceeded, won't operate above the limits.
Over spec max short circuit current, reverse polarity would overload protection diode (so connect one string first, make sure it works, before connecting more that exceed Isc limit.)

More watts and conversion from some voltage extremes will generate more heat, shortening life.

 

[Mattb4]

SCC - ok, that is also built into the inverter already. The Sunny Boys have 2-3 MPPT controllers built in already. Trying to maximize use of those.

Sorry no. Your inverter is not a SCC. Your SCC has MPPT logic programed into it. Think of it like Windows being an operating system for a computer. AIO's have several components that interact but perform different functions. The SCC is not the inverter anymore than the LCD display is.

Your SB's* have 2-3 SCC's built into it. Why is this insistence of definitions important? Because it avoids misunderstanding principles.

ETA: Incidentally I would guess that manufacturers specifications on amount of OP is based on Duty cycle. The thing is power supplies can be a whole lot bigger than the load. Just like you can power 1-20w bulb in your house from a 200A, 240vAC supply. It will not blow up by doing this. However if it is heat constrained it might burn out if left lit for too many hours.

*Further edit. If you are grid tie only than your grid tie inverters are a different critter. They might utilize MPPT logic to find the Panel output at the best voltage for direct inverter conversion. There would be no SCC. However there is no device called a MPPT either. Also while connected to the grid you have infinite load and thus it will take whatever the grid tie inverters can send to it.

 

[Calvin98]

But Sunny Boy Smart Energy is a hybrid. They say that you can put on PV panels for 200% of the 7.7kW output rating. It can also charge battery with up to 10kW.

Meaning, for example, 50% can be sent to the grid and 50% (more or less) can be used to charge the battery? I think the SBSE uses a high voltage battery so it can send the high voltage DC directly to the battery while at the same time inverting the rest to send to the grid?

If they only recommend 15.4kW not 17.7kW of panels, maybe the (three) MPPT are limited to that. (Sometimes people connect excessive wattage of panels to MPPT A only, and don't get full output power. Paralleling A & B fixed that.)

That adds another question - you can parallel A & B but you can also split (or keep separate) A and B inputs?

I still say you can overpanel with multiple orientations, e.g. sqrt(2) = 1.4x peak wattage if array has two halves 90 degrees apart. So I would be inclined to put 22kW of panels on SBSE 7.7 (I won't because I don't have that much roof space.)

Obey string voltage limits.
String current limits, if exceeded, won't operate above the limits.

Can you mix a 300V string with a 100V string if panels are different orientations? What if they have similar (or even same) orientations?
The 100V string will not reduce output on the 300V string if and when the 100V string is not doing anything (low or no power output)?
or do all the strings on any input (A, B, or C) need to have the same voltage (i.e. same number of panels? Inputs A, B, and C already have 300v horizontal strings attached to them.

Over spec max short circuit current, reverse polarity would overload protection diode (so connect one string first, make sure it works, before connecting more that exceed Isc limit.)

More watts and conversion from some voltage extremes will generate more heat, shortening life.

Put it this way, if, for example I have a West facing vertical wall that gets sun between says 2pm to 5pm, and it fits only 4 panels placed long end to long end, which SB input is is best to connect to? 4 panels could meet the 100v minimum voltage and max amperage requirements of the SB.

I know it is a bit complicated and I may not be explaining it all correctly and probably making a lot of unstated assumptions with all of this.

 

[Hedges]

Spec appears to indicate it can process 15.4kW from PV (not confirmed.)
It can produce up to 7.7kW AC from PV or battery (surge to about 9kVA)
It can charge up to 10kW (from PV, and I would guess up to 7.7kW from AC)
I expect it to split PV power between the two - and ideal setup for someone with 7.68kW GT PV limit due to 120% rule. What PV can't go to AC goes to battery, so it maintains full AC output for many hours, or saves the extra for peak rate times.

This model supposedly available this month. They were going through certifications. Don't know when it will also be on CEC list.
An 11.4kW model is due 4Q 2024.

High voltage battery. For now they talk about BYD HVM, probably the others that Sunny Boy Storage supports will be supported. App notes about LG RESU-H say it is a low voltage battery with boost converter. The others like BYD are native high voltage, and you can add capacity by stacking modules (increases voltage, also wattage at the limited current spec.)

Sunny Boys and this SBSE have MPPT inputs A, B, C. The 11.4kW model will have A, B, C, D.
Each MPPT can have its own Vmp, different strings.
Inputs A & B can be paralleled for higher current and/or more watts. Inputs C & D can also be paralleled.


The following should be true for all inverters and SCC with multiple MPPT:
Separate MPPT inputs can each have a different voltage, within allowed MPPT range.
Paralleled strings need to be same/similar voltage. (You could mix brands with slightly different voltages, but usually it would be identical strings.
Orientation of parallel strings do not have to be the same.
Note that there are some products with different amperage and wattage limits for different strings. SMA's inverters aren't that way, but some limits such as short circuit current are quoted per string plus an overall sum of strings.

I think this means SBSE has similar capabilities to SolArk, EG4, others, except backup capability isn't built in.
It is 240V only and doesn't include transfer switch. It appears "ABU", separate auto-transformer and transfer switch, would be supported. SMA web site data sheet didn't mention, but a vendor had different version saying backup capability would come in the future with additional hardware. European version on SMA website already says that.

Another product on European website is a 3-phase version with transfer switch built in. It does not list 200% overpaneling, however.

I think the price is around $2500 for the 7.7kW model.
Only bad news is the price of BYD batteries, about 2x what server rack types cost. Is there a difference in quality, safety, flawless system operation straight out of the box?

 

[Calvin98]

Thanks. If the strings all have to be similar or same voltage, that kind of answers most all of it. I can find places to add 3,4,5, or so panels, but if I have to find places for 10 to fit, that would be a challenge.

Not sure if mircoinverters would be a solution. Enphase seems to need special AC connectors that you have to buy separately and they are not well suited to be connected to a wall plug or direct wire to a breaker. Assuming of course that the wall plug would not put out any ac power until is saw "grid" power (i.e. not be a suicide cord as soon as the pv panels see sun). The ones on Amazon might do it but they tend to overheat and fail rather soon. Setting up a separate inverter to feed power from 3 panels at 100V and say 900-1200watts into 1 input is probably too expensive to be practical. Growwatt has a 3,000w inverter for $500, but that still adds up.

I see that Growatt has a 750W inverter that might do the job, but not sure they sell it in the US.

 

[Kuma]

Please note I did not read this entire thread... For my SMA inverter 5kw-US-41 strings do not have to be the same size or voltage or amperage or orientation... if on separate mppt's./inputs. The panels on each string should be the same though. If paralleling strings refer to others comments. No experience with that.

I installed a SB 5kw, 3 mppt's, with 6.83kw of panels. Two strings, 6 455w (west) and 9 455w (south) panels using 2 mppt's. According to the SMA inverter design tool more panels could be added. Achieving the actual 5kw inverter rating should occur but for only a very small percentage of "total" output time. Less then 1% is my guess. In other words, not a big deal...

The SMA design tool is very helpful and takes into consideration panel orientation, inverter, panel size(s)...

If mounting on roof and RSD's are required that can change the design. I originally wanted 5 panels on west face but a minimum of 6 rsd's are required per string for SMA Sunspec using SMA rsd's.

 

[Calvin98]

Thanks! 5kw-US-41 is same as "5.0-US" or "SB5.0-1TP-US-40"? What is the max operating input current per MPPT? On these it is 10A, but some of the newer panels put out more than 10A. You can go over the max? by how much? Have to see what Sunny Design says. Just trying to see how to capture more early morning and late afternoon power w/o adding separate inverters for each string.

 

[Hedges]

Avoid Isc exceed max short circuit spec. That is the limit for reverse-polarity protection diodes.
SB has a max per string, and a max sum of strings.

If Imp exceeds, that just means current will be limited to 10A when sun is overhead. Follow the I/V curve on data sheet and see how much V x I changes. As soon as sun is off-angle, Imp will be under 10A so no clipping.

The -40 differs from -41 in that it does not have RSD keep-alive transmitter. Possibly, it also doesn't have the newer -SA or -SB grid-support functions, but not sure.

There is also -1TP and -1SP, difference being utility grade meter, I think.

5.0-US, unless it is -40 or -41, could be an earlier model completely different in appearance and some functional differences.
But I think it is used as an abbreviation by some advertisers:

SMA Sunny Boy 5.0-US 5kW String Inverter w/ SPS 3 MPPT

DetailsSMA Sunny Boy 5.0-US 5kW String Inverter w/ SPS 3 MPPTThe SMA Sunny Boy 5.0-US is a transformer-less PV inverter which converts the direct current of the PV array to grid-compliant alternating current and feeds it into the utility grid. The SMA 5.0 inverter is suitable for indoor and...

www.renvu.com

"SMA Sunny Boy 5.0-US 5kW String Inverter W/ SPS 3 MPPT
SKU#: SB 5.0-1SP-US-40"

 

[Calvin98]

Thanks. Looks like Isc (max short circuit current) is 18A for these Sunny Boys. Ioc (max operating current) is 10A, so it sounds like you can use the newer panels that put out more than 10A. Sounds like anything over 10A is just "clipped" (i.e. not used). For the Sunny Boy 5.0-US is data sheet says max 8,000 watts of panels (max PV power). I guess that is how you can put 10,905 W of panels on a SB 7.7 or 8,000W on a 5.0.

SMA does not list all the manuals, or nothing specific for -40 or -41 inverters that I can see anyway.

 

[zanydroid]

Not sure if mircoinverters would be a solution. Enphase seems to need special AC connectors that you have to buy separately and they are not well suited to be connected to a wall plug or direct wire to a breaker. Assuming of course that the wall plug would not put out any ac power until is saw "grid" power (i.e. not be a suicide cord as soon as the pv panels see sun).

Microinverters are indeed an excellent way to deal with situations where you can only fit a few panels per facing.

Weird way of expressing concerns. Free associating for learning?

You should start with the installation manual. They are exceptionally good for Enphase and they will walk through all the accessories that will get the microinverter into a breaker.

You can plug a microinverter into a 240 receptacle and it will work and backfeed, but this is not accepted in U.S. code. They are hardwired into dedicated branches.

 

[Hedges]

SMA does not list all the manuals, or nothing specific for -40 or -41 inverters that I can see anyway.

Looks like SMA just cleaned up the "Products" tab of their web page, removing reference to discontinued model.

Using their site's search feature for the model you listed I got:

https://files.sma.de/downloads/SBxx-1TP-US-40-IA-xx-12.pdf

You'll get plenty of google hits too, may vendors have a link.

 

[Kuma]

Thanks. Looks like Isc (max short circuit current) is 18A for these Sunny Boys. Ioc (max operating current) is 10A, so it sounds like you can use the newer panels that put out more than 10A. Sounds like anything over 10A is just "clipped" (i.e. not used). For the Sunny Boy 5.0-US is data sheet says max 8,000 watts of panels (max PV power). I guess that is how you can put 10,905 W of panels on a SB 7.7 or 8,000W on a 5.0.

SMA does not list all the manuals, or nothing specific for -40 or -41 inverters that I can see anyway.

The 455w Solarever panels output 11 amps. The SB 5.0-41 does "derate", clip and this has happened, al biet a very small percentage of total output time, on the larger 9 panel string. With Hedges assistance, I paralleled inputs A and B for the 9 panel string. No more derating. The total string amps is now shared/split with A and B mppt's. SMA does specify that derating is normal and does not negatively impact the inverter. Just stay stay below 18amp mppt max. This may not be true for all inverters.

One thing I am unsure of, does paralleling A and B use more calculation cycles and possibly lower inverter efficiency.

 

[MattiFin]

The question are: How can you maximize output on 1 inverter? Can you get it to produce 5,000 watts continuously from sunrise to sunset? What do you have to keep in mind?

If you have lots of space you could put 1 vertical array of bifacials in south-north direction and one array of non-bifacials east-west pointed to midday sun.
Vertical array of bifacials would capture both the morning and evening sun with just 1 array.

In tropics this won't bring much benefit as sun rises rather quickly but here close to Polar Circle the sunrise&sunset takes forever in summer. (and in winter we don't need to worry which way to point our panels, works as good even inside pitch black cave)

Math gets bit complicated with multiple arrays pointing to different directions and angles and I'm not aware of any tool that would plot the combined output of multiple arrays at every possible combination of sun location. Would be fairly easy task in to do in excel..

 

[Calvin98]

If you have lots of space you could put 1 vertical array of bifacials in south-north direction and one array of non-bifacials east-west pointed to midday sun.
Vertical array of bifacials would capture both the morning and evening sun with just 1 array.

Now that is an interesting idea. Bifacials out put on both sides is the same? Have never looked into those, but this is interesting.

This was the reason for the question to begin with. A 5kw inverter for example only puts out 5kw (max power) if overpaneled some but for only maybe for an hour or 2. Before and after that it is always less and adding panels at different orientations could max the output from sunrise to sun set. But you have to know exactly what the criteria are as to how to do it.