@transparent it’s akin to gas welding, no penetration or a big hole in the subject, trying to weld with a mig is worse even with a good machine good wire and argon gas.  A tig is more flexible but I spent a week in a cubicle welding, eventually then it was sent away for testing before I could be registered with Lloyds of London. But if your machine isn’t a high end you will struggle. You get a spray to apply first and it helps for spatter etc.

Just had Scottish power out checking my three phase installation following the main neutral wire falling out.  They are somewhat confused as to why the entire house is on a single phase and likely near the top end of rating, fitting another solar array with inverter wouldn’t be a good move he said.  The guys that ran the three phase were reluctant to do anything and wanted to install single phase although Scottish power surveyed and specified three phase.   It’s never ending and pissed off. 

I contacted growatt with the details of my inverter and the panels I have installed and they say each string can take 10 panels so a further 14 can be added. So 4 to the original string and ten on the garage.   Something doesn’t sound right and that’s a lot of panels. 

@david999 we did the calculations for the number of panels you could connect earlier in this topic here.

It depends on the rating of the panels, so I don't understand why Growatt would be supplying an answer based on the number of panels.

But there's no point adding more panels to an inverter which has already got more power than it can handle.

Unlike the situation with @bretix installation, your Growatt SPH-series inverter cannot operate in parallel with another unit.
So you've got yourself constrained by the maximum export parameter which Scottish Power will allow.

With 3-phase now available to you there are lots of possible ways forward,
but the useful options won't work for you unless you change inverter.

@transparent I gave them the details so figured they gave those numbers based on that but looking like a figure for panels max rather than max output.  I will ask them to confirm what they mean but I’d rather take your advice than theirs to be honest. 

I was working out the maths for another, off grid inverter to top up my batteries but cable runs present issues in terms of distance and cost. My first job is to get three new tubes for my thermal panel then consider my options for all this other stuff. Electricity is to increase dramatically soon as will coal, oil and gas so never a better time to invest. 

I got them to confirm the details but I don’t entirely understand the answer. Are you able to explain please

The problem with the maths is that you're starting with two different types of panel.

I proposed (purely as an example) 425w panels manufactured by Perlight which had Vmpp of 34.6v and Impp of 12.3A

But you later posted a photo of the label on your existing Canadian Solar panels.
Yes, they are also rated 425w, but with Vmpp of 31.8v and Impp of 13.37A

You can't just add together panels with different specifications and arrive at answer as a whole number of panels.
When I described how to calculate the number of panels which could be connected to your inverter, I had no idea that the existing ones were rated 425w.

Did you provide both those sets of panel specifications when communicating with Growatt?

When your SPH3600TL inverter was originally designed, it's unlikely that any panels were available with an output as high as 425w.
So what data is Growatt looking at when they gave an answer?
The original design specifications or marketing literature couldn't give an answer for products that didn't yet exist.

A similar issue arises when you mention the distance and costs of cable-runs.

I'm unable to recommend anything without knowing what you intended putting at each end of a cable.
Are we talking about 500v DC at 13A from a string of solar panels...
... 52v DC at 100A to connect a battery...
... or the 240v AC mains output from an inverter?

Feed me data! 🤪 

@transparent I gave them the tags off the panels and inverter, but they know my system it was registered through them and they can go online into it. I posted the tags on here earlier to help. When you get data back from the manufacturer like that it kind of throws you off. I can fit 20 panels but the output will only be marginally more in ratio to the 6 already on. So 2.6 for 6 and I think they said 3.6 for 20.  I’m confused and that’s their tech dept

In relation to cable runs currently my hall looks like a sub station and doubt the wife would appreciate anything else. My initial thoughts were keeping everything confined to the garage but I need to get into an already too busy cupboard in my hall. Looking into purchasing a book but haven’t chosen one there are a few. Do you recommend any?

I'm struggling to follow that.
I need units 😉 

So you have 6 Canadian Solar panels, each rated at 425w and connected in SERIES to one of the two available MPPT inputs on your SPH3600TL inverter.
That equates to 2550watts maximum; (6 x 425w = 2550w).

That same MPPT input can accept 550v DC max.

The existing CS6R-425T panels have a peak open-circuit voltage of 38.8v

550v / 38.8v = 14.175
which means you could add a further 8 CS6R-425T panels in SERIES to that same MPPT input port (14 total - 6 existing = space for 8 CS6R-425T panels available).

If you did have another 8 of those panels to add, then you'd have 5950w (peak) available in full sunshine (14 x 425w = 5950w)
of which your inverter would be prepared to accept 3600w, because that's its maximum rating.

So there's no advantage in having 14 panels on that MPPT input on a day with full sun,
but you would have more power available on overcast days, when the PV panels were yielding lower output.

And that's before we consider the second MPPT input port.

The greatest constraint you're facing is the 3600w maximum power handling of the SPH3600TL inverter.

Any number of panels you add whereby their total max output is greater than 3600w will only be of benefit on dull days.

You could have 9 CS6R-425T panels in SERIES on just one port,
or you could have the existing 6 panels on port-1 and another 4 to 14 panels on port-2,
and all of those combinations are above 3600w.

@transparent I’m honestly struggling with the technical side of this but bare with me.

1. am I correct in saying that each port has its own values as described on the information tag. 
2. if I fit all those panels as explained and the output exceeds the inverters max does it not damage the inverter. 3. If the inverter sees 5950w and rejects that do the panels not get damaged producing current that isn’t being used. 
4. the production of solar power in winter is more important than summer since my demand will be greater and currently with the 6 panels and two batteries we use little grid. We could save a little bit more on the feedback with more panels but I expect the export tariff to be discontinued anyway. As you previously mentioned the panels produce one 7th of summer power in winter so an increase in panels might resolve that. 

it’s heavy going at the minute but stick with me if you can 

Ok done some research on the information I have so far, see what you think. 

running a solar system above the capacity of the inverter is helpfull in producing power, it ramps up in the midday and drops off slower in the evening. No more than 30% is recommended.  The inverter dumps the excess energy using a process called clipping. So in reality my 3.6 inverter running at 2.6 will only ever output that max and clips the remainder. Therefore having more panels ensures it reaches that max figure during overcast days and more importantly an ability to produce power during winter. Most systems rarely reach max production if sized as per spec and really should be oversized from the panel aspect.

So if I’m correct here, if you can only get 6 panels on your roof and with no prospects of further panels then your inverter should be 30% smaller than the rating for those 6 panels if you want optimum production, any surplus will be clipped by the inverter and overall it will be much more efficient. Which brings me to my installer who knows none of this and said I can only fit two perhaps 3 more panels.  

Scottish power just emailed me with an excuse for not using the three phase tails although fused them all.  Just in case I get a burn out they can use one of the others without digging up my garden again. I thought I was fairly near the top end of my use in the house with all my gear.  Should I push them do you think or leave it as it is. They claim I need a three phase meter which is true but also say I need to rewire to a three phase consumer unit which I don’t believe is true. 

@david999 they wanted to charge me for the 3p metre but subcontract to Siemens I believe. When Siemens contacted me to install I rearranged a date directly with them and  potentially avoided a fee, unless that happens if I change supplier.....