Here's my interpretation:

@transparent sounds about right!

I'll get my tools out and have a go at the website. Just needs a hammer!

T&E hopefully OK as it's not going to exceed 2kw currently. Could be extracted from insulation fairly easily and run differently - and Meter is under 10m away. Most of the cable is surface mounted and not buried in walls or insulation. Perhaps in 10 or 20 years or so the solar panels get upgraded to higher output. But even then unlikely to be more than say 3 or 4kw at peak. 

More importantly the twin 4mm cables sounds right. I could always open up isolator and/or inverter to see connections.

But for now I think it's probably most useful to see output on a sunny day when unshaded.

between the showers this am we have some bright sun. 
At one point the flat roof is producing the expected peak 6kw from 6.5kw nominal 

however the offending pitch roof continues to produce circa 400w! despite ideal, direct sun

So I guess its back to a clamp to see what DC input is actually happening?

If that is also low I will need to test individual panels? not sure how?!

It is likely that each panel connects into its Solar Edge optimiser using MC4 plug/socket.

Then the optimisers are joined into a string using another pair of MC4 plugs on flying leads.

So if you bought a pair of male-female MC4 leads, you could 'remove' each optimiser from the string sequentially.

That will show you if there is one faulty optimiser in the string.

To get the connectors apart, you require a matching MC4 tool

MC4 connect/disconnect tools can be bought cheaply online.
But beware that the cheaper ones are likely to be sent direct from China, with a resulting delay.

You can buy a complete set of solar-panels tools and a few MC4 connectors for around £20-£30, which is a 'better deal'.

The MC4 standard is defined by the manufacturer Stäubli.

There are other variants, such as the Amphenol H4, which can pass higher currents.
They interconnect with MC4s, but use plastic spanners of a different size.

@transparent many thanks for the helpful thoughts. Yes, seems worth getting a kit to help narrow things down.

Again today in reasonably good sunny conditions the 7 (8) panels are only producing around 400w when the flat roof system is around 5kw. So seems to confirm there is an issue.

I see the inverter records total production since installing as 10.5mwh. The meter is showing as 9.95mwh. I guess those 2 are close enough together. Very roughly 6 years production @ 1500kwh pa and the last 17 months are around 950kw in 2022 & just 192kW so far this year (31kw so far in May - which has not been sunny). Of course we don't know when the problem started but I'm inclined to think it's after summer 2021 and more likely early 2022 based on FIT readings.

Posted by: @tim441

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the inverter records total production since installing as 10.5mwh. The meter is showing as 9.95mwh.

If by 'meter' you mean the household Smart Meter or an MCS-approved meter on the mains-output side of an inverter, then there's no problem with those figures.

The 10.5MWh is the DC power imported from the PV panels themselves.

9.95MWh is the 240v AC 50Hz which is exported from the inverter.

That means you've got losses within the inverter running at around 5% which is quite acceptable.

@transparent that makes sense.

Yes, it's a MCS-approved meter on the mains-output side of the inverter.

The newer system has its own EM115. 

Both feed into a SMETS2 smart meter for measuring actual exports.

In the past I did not look at exports much as rates were so lousy. But since last year I've paid more attention as we can load shift & use batteries to help maximise. I was on Octopus Agile for export but have now changed to Octopus Standard Fixed (15p) as we are obliged (due to max inverter rate we can push into batteries being less than half the amount produced on sunny days) to export some of our peak production at times of day when Agile rates are not great. Makes it much simpler!

@tim441 I see you have solaredge inverter. As do I. If the solaredge is connected to the internet, and thus into the solaredge cloud , you can get full and comprehensive data as to what every single panel and optimiser is doing. Thats one of the reasons I chose solaredge, I'm surprised your installer didn't put it in.

its proved really useful to me as I've been able to spot a faulty optimiser and get it replaced. and keep an eye on what the shading does (as I have a high change of shading over time and over the array).

my suggested would be to pursue getting your solaredge internet connected and registered on their cloud.  IIRC you said your original installer was no longer in business, in this situation I'd hope that if you contacted solaredge directly they'd help you get it registered.  Then the data from that should give you a strong pointer as to what's not working.

here's what you can get - this is  mine, right now. info box in the middle is for 1 panel picked at random. colours indicate energy throughput today (Brighter is more).

Ian

@iancalderbank thanks Ian. Yes, would be a huge advantage.

Tbh I'm not sure if my kit can be retro-connected. It was installed in 2015 but you're right... I'll contact solaredge to check.

I think i could run an Internet cable if that was needed (if WiFi not an option).

@tim441 I just RTFM for your inverter. It has Ethernet as standard and wifi as an option. reasonable assumption is that the wifi option is not there . So run a  LAN cable to the RJ45 Network port of the inverter. Then call solaredge.

@iancalderbank thanks for checking. i'll get on the case