I am a new PV householder - advice please. Unbalanced strings and wrong inverter

[juniperz]

I was quoted for a Growatt-3600MTL with 16 Suntech STP 245S panels. We have a large roof, but it is hipped, so the installer proposed 12 panels on the house roof and 4 on a garage roof with same orientation and similar roof elevation. I did a bit of research on the inverter, even had a reply from Growatt saying that the panels should be connected 8 + 8.

So, today they came to install. The electrician has done all the internal wiring but they need to come back Monday to finish the panels and complete the electrical side.
2 Problems:
1) I just looked at the inverter in the attic and found it is not Growatt, but Samil SR4K4TLA1 (also badged Solar River and also known as Samil 4400w).
Any opinions on whether I am better or worse off compared with the quote? Unfortunately I am a new member so cannot post links, but I did find a test that said "A serious competitor - Samil Power's SolarRiver SR4TLA1 replaces Growatts's 5000TL as the best Asian inverter to be tested by PHOTON Lab".
(google "as the best Asian inverter to be tested by PHOTON Lab" and you will find it).

2) I don't think the panels have been wired as 8 + 8. I would like to attach a photo, but can't, but it shows 1 pairs of wires going into a hefty junction box labelled "Garage Roof DC" and another single pair of wires going into a junction box labelled "House Roof DC". Each junction box has leaving from the other end, a single pair of wires. These 4 wires go into the bottom of the inverter. Now I may know nothing about this, but that looks like 12 + 4 to me...
Am I in big trouble? If anyone wants to PM me an email address, I could send the photo if it would help, though it's pretty straight-forward.
Thanks in advance.

 

[sedgy34]

Ask the company to stick to the quote and the choice of materials

 

[ktech]

Never come across Growatt although some on here might have. Wouldn't like to say 100% but on all the inverters we have used 4 panels on a seperate tracker doesn't sound like a good idea. From memory the minimum (depending on wattage / voltage of the panels) is five. I'm assuming that they are using a twin tracker inverter hence unequal panels on the strings, or maybe that are linking them in one string although the pitch and orientaion needs to be the same to do the latter.

I would say the more concerning issue is that you have found a different inverter in the loft to that ordered. This is not good as you should get what you have paid for / been quoted for or have agreed to a variation in the contract. Generally you get a bad smell of the job when this happens. I would question this first and ask for detailed information on the configuration of string / tracker arrangement and also why you have not been consulted on this change of hardware.

 

[juniperz]

Thanks ktech - confirmed my feeling, but wanted a second opinion.

 

[FB.]

I imagine that a 3600W inverter (as originally spec'd) is G83 compliant - capped at 16A output current.

An inverter with 4400W sounds like it may p!$$ off the DNO - and perhaps be ordered to be disconnected from the grid.

Additionally, an oversized inverter will not be as efficient. Generally speaking, an inverter should be about the same as the panel wattage for best efficiency, otherwise on dull days a big inverter can use up everything the panels produce just to remain powered-up.

So I would refuse to accept the installation.

 

[MorganPVI]

There is no way you can balance 12 + 4 - even on a dual maximum power point inverter, let alone a single MMPT. A 4400w is very much over sized, I'd say send it back, but tbh your still going to face issues with a 3.6 Growatt.

The only credible design would be to use 2 inverters, even then you'd really have to split it 11+5 as you'd probably want to use a 1200SB for the smaller inverter (1.2kwp).

I suggest you halt the works immediately given the different inverter supplied, have the installer come round and prove technically that his design is correct and the system is correctly installed, tbh it sounds like your installers have not correctly designed your system and are pulling some kind of a fast one on the equipment - ask for a copy of the variation notice that the installer is required to produce under the terms of the MCS accreditation, you should have already had a copy and signed it!!

 

[cbw]

Hmmmm....If I was quoted for a BMW and was delivered an Audi I would at least expect an explination

Let them finish it and then ask why???

At least you have working kit and have only paid 25%

 

[ktech]

From a customers point of view I would say you are quite within your rights to 1. get an explanation on why you are having different kit fitted from what has been quoted. 2. You should be given detailed design information for the array. If you get 2. feel free to post it for an objective opinion. The type of thing you should ask for is a design tool report. Many invertor manufactuers have tools for designing arrays. However I would suggest playing it cool but to the point and stick to your point. If you work with your supplier but under your terms you may end up with a good system but don't be fobbed off. Hopefully you havn't had to pay a deposit so will have the upper hand. Deposits, in my view, are not required due to reduced system cost today. We always term it as balance on completion to your satisfaction and this effectively becomes as sales tool as it gives customer confidence in our work . The customer is always invited to take a look a the roof work once the hook and rail system have been fitted and also after panels are mounted. If they don't want to venture up the ladders we take photo's anyway so can show them the job - cleaning out the gutters also helps customer satisfaction as well

 

[FB.]

Although I would reject the system on the basis of the inverter, is it not possible that the installer has wired four of the roof panels with the four garage panels on one string, then the other eight roof panels on a second string?
I would ask to see the schematics of the system. The string voltages should also give an idea of how many panels per string. A four-panel string would be in the 100-150Volts range, while an eight-panel string would be in the 200-250Volts range, and a twelve-panel string would be in the 350-400Volts range.

As above posters have said: a string of four panels needs careful consideration due to limited options for inverter, although it may be that your proposed inverter can handle unbalanced strings without a major loss of efficiency.

And, as per the earlier post: I would still be concerned about an oversized inverter which might be able to exceed 16Amps output and annoy your District Network Operator, who can legally order your system to be disconnected from THEIR grid.

Whereabouts in the country are you?

 

[ktech]

I'm my view unbalanced string on a single tracker inverter are a non-starter. If the two arrays are put together in series then it might be ok. But if the two arrays are on different pitches of roof, again non-starter. It's not good to mix different pitches on a string. You only have to use a irradiance meter and alter the pitch to see the difference it makes. Hopefully you can use this guidance and apply it to your array and work out if it is relevant (ie on the different pitch).

 

[whinmoor]

If you'd like some photos posting, email to me and I'll try to put them up: [email protected]

 

[juniperz]

Hopefully you havn't had to pay a deposit so will have the upper hand.

I have paid 20% deposit...

 

[juniperz]

Whereabouts in the country are you?

Bristol

 

[juniperz]

(5th post)

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(6th post

 

[juniperz]

When I contacted Growatt, their Tech Support person said:
"Max. number of parallel strings, means that you could connect 2 strings per MPP tracker. This is usefull when working with low power modules.

This particular inverter can work with the panels STP245S-20/Wd with an string between 5 and 8 panels, so 8+8 will work fine. If you 12+4 you are overloading one of the MPPT trackers (see screen shoots ).
Unless both roof are at facing the same azimuth and with the same tilt, you could physical split them 12 +4 but electrical 8+8.

Here's the 8+8 config

And the 12+4