Solar panel design and installation

[Justloadit]

String 2 (1 and 1.1) + 2 (2 and 2.1)

String 1 - 5 panels x 49.75 x 5 = 248 VDC
String 1.1 - 5 panels x 49.75 = 248 VDC

String 2 - 6 panels x 49..75 x 5 = 298.5 VDC
No string 2.1

Just I note, and I stand to be corrected here, I am not sure if you can use 2 different voltages on each MPPT input on the inverter.

[Isetech]

My understanding is that is reason for 2 MPPT's, is MPPT1 and MPPT2 work independent of each other.

It is better to keep the 2 strings on the MPPT 1 as close as possible, but string 2 and 2.1 on MPPT 2 can be completely different from MPPT 1.

This is what makes those home made DC control boxes so dangerous when connected to Sunsynk inverters with dual MPPT's.

There should be a separate DC control box for each MPPT and there should be a 4 pole DC isolator in each DC control box, isolating both strings, which should also be a metal box, considering they have been identified as a fire risk.

There should never be power on the bottom of an main switch/ isolator, ever. I leant the hard way.

The 8 kw Sunsynk 2+2 in the Sunsynk manual identifies as MPPT 1 (with 2 strings) + MPPT2 (with 2 strings)

The 12 kw 3 phase unit has 2+1 MPPT 1 2 strings and MPPT 2 (only 1 string)

I am sure this confuses a lot of people, like the 50 amp pass through current, very few people understand what the pass through current is all about.

Just I note, and I stand to be corrected here, I am not sure if you can use 2 different voltages on each MPPT input on the inverter.

[Andrew_van_Zyl]

Perfectly fine - just don't exceed the maximum.

[Isetech]

You cant exceed the max power, the Sunsynk inverter automatically caps the power as a default setting. we normally adjust cap to suit the plant.

I dont know about the 26 amp current rating, but I do know that you must never exceed the VOC, in fact I make sure we never go over the 450 absolute max.

Perfectly fine - just don't exceed the maximum.

[Isetech]

Reading the 8 kw installer manual I found this rather interesting ... page 76 "Inverter commissioning info" under solar ... ensure both MPPT's are balanced.

Just I note, and I stand to be corrected here, I am not sure if you can use 2 different voltages on each MPPT input on the inverter.

[Andrew_van_Zyl]

Not sure what they mean by "balanced" but in my experience inputs to MPPT's are rarely exactly the same as so many factors come into play like roof design, module layout, orientation, old vs new modules, shading, etc.

Only on my own roof do I have EXACTLY the same inputs into my 2 mppt's but that's only because I am blessed with a vast north facing roof.

In my humble opinion the mere fact that (Sunsynk) inverters have 2 mppt's is to deal with precisely the above inconsistent roof issues.

[Isetech]

Attached is the 2023/2024 (1/07/2023-31/06/2024)

With reference to scale 3/4 residential tariff

The one that affects domestic dwelling sin suburbs.

Page 14 - Feed in tariff R1.44 inc vat

What referred to as the ancillary network charge (based on the inverter per month) - R126-86 per kw. An 8 kw inverter will cost you R1014.88 per month. This is why I want to get one of the 3.5 kw units from the UK, they can handle 7 Kwp of solar, R444.01 per month That all I need for my house.

For now I believe people of eThekwini are waiting for a the council to roll out the public awareness program and hoping the grace period for registration.

I am told that there are a couple of domestic installations which are registered , but not meters to fit on their property.


https://www.durban.gov.za/storage/Do...ff%20Rates.pdf

They say load shedding will end before 2025, I see there are nuclear power plants in discussion at the moment.

I also hear comments about stage 1- 16 load shedding, its hard to believe anything, rumors spread faster than wild fires.

My aim at the moment is to get solar solar installed so that I dont have to worry about eskom or our failing electrical infrastructure and everyone I know is going large with water storage and booster pumps.

[Isetech]

Let me start by saying, I haven't personally registered a system yet. It is a lot of hear say.

It seems at this point in time there is a lot of speculation, I am not aware of one domestic installation that has been registered, complete with the correct meter.

I am not aware of any public awareness programs currently running to educate the public on how what to do with a solar system of their property.

I have noticed there has been an upgrade to the website.

https://www.durban.gov.za/pages/residents/energy-office


Trying to figure out what to do is like a maize for me at the moment, just imagine what it must be like for the man on the street trying to figure out what to do.

It is time to employ a person to manage registrations, this is going to become big business. I dont have time to try figure out what is going on, the website keeps updating, unless you follow it on a daily basis, it will be difficult to keep up. I have around 15 site that require registration.

[Isetech]

I am busy with another project. The roof is big enough to double stack the panels. The reason I am sharing is because I get asked these questions at least once a week.

"What is the max amount of panels you can connect to an 8 kw Sunsynk inverter"

We plan to install 32 x 545 watt panels,

8 per string on the east facing roof and 8 per string on the west facing roof.

string 1 top row (east) - 8 panels x 49.75 VDC = 398 VDC
String 1.1 bottom row east - 8 panels x 49.75 VDC = 398 VDC

String 2 top row (west) - 8 panels x 49.75 VDC =398 VDC
string 2.1 bottom row (west) - 8 panels x 49.75 = 398 VDC

Optimal voltage 370, so it is just above.

Looking at the STC values which are higher than the NOCT values so it should be ok, max power for a 545 watt panel is 13.04 x 2 = 26.12, border lining the 26 amp for the MPPT.

The short circuit current is 13.93 amps.

Which brings me to another very common question asked all the time "what size fuse must you install? " If the panel has a short circuit current of 13.93 amp, why do people fit 30 amp fuses, the answer because they use a 6 mm sq solar double insulated wire. Is the fuse the panels or the wire, according to any electrician to speak to, it is protect the wire. What do you think?

Back to the panel:

So we have 17440 kwp on the roof, you cant do that, actually you can, your voltages are within the VOC rating which is the most important factor when designing installations and your currents are within the limits.

If these panel were all north facing, I probably wouldnt suggest this many panels, however there are a few things to consider, the east west facing roof, the angle of the roof and the losses in winter (which could be as much as 40 %) which means you can only produce a total of around 11kwp in june/july. If you are registered and feeding back into the grid to maximize ROI, best you push as much power as you can.

Considering the price of panels, I am looking at only installing 1 x 5.1 kwh battery for now, just to keep the lights on and the rest of the money will be pushed into panels and inverter.

There is talk that the load shedding will be done by the end of the year, I know I heard that back in 2008, so I wont be holding my breathe, however even if they do fix the load shedding. my goal is not to install a system for load shedding but rather saving money on my bill.

Our utility charges a fee per kw which is around R143.75 per kw x 8 kw (hence the small grid tied) = R1150.00 per month.

So you need to push as much power back into the grid to recover these cost. Having 6 panels on the roof with an 8 kw unit was a nice load shedding solutions, to top up your batteries without using the grid power, but will become a waste of money once they force you to register your system.

Just think if you install a 16 kw unit x R143.75 = R2300 monthly charge, with a feed back tarriff of only around R1.40 , you gonna need a lot of panels.

[GCE]

So you need to push as much power back into the grid to recover these cost. Having 6 panels on the roof with an 8 kw unit was a nice load shedding solutions, to top up your batteries without using the grid power, but will become a waste of money once they force you to register your system.

Just think if you install a 16 kw unit x R143.75 = R2300 monthly charge, with a feed back tarriff of only around R1.40 , you gonna need a lot of panels.

You may get a surprise using the above logic
Nersa restricts to 25% of CB size being allowed to feed back and at the end of the day municipalities are going to have to restrict feed back to enable transformers to stay loaded at at least 25% of capacity otherwise the network will be unstable.

If you look at Cape Town there is a limit to the amount of people that can feed back