And I ahve just discovered that I can export that data from the Soliscloud.

I assume a single phase monitor would still work on a 3 phase board for single circuits but wouldn't be as accurate if trying to measure circuits on more than one phase as the voltages vary slightly.

Given inverters with the ability to export to the grid must, by law, stop exporting if the voltage goes above 253V (or thereabouts, I believe), I'm pretty sure all inverter manufacturers take care to ensure their kit can be accurate in its measurement of voltage. As a result, what I get via modbus from my inverter is, I believe, easily accurate enough for my purposes.

Agreed. In the Solis S6, the readings are via a dedicated meter that connects to an RS 435 port. Some of the data is relevant for export.

I say this, as the inverter also provides direct electricity data via modbus, about the electricity from its various ports. 

And in the case of @johnnyb triphasic Solis, the modbus integration would also add it for the other two phases.  

inverters with the ability to export to the grid must, by law, stop exporting if the voltage goes above 253V (or thereabouts, I believe)

The relevant statute is this:

The 'Declared Voltage' in GB is 230v RMS.

So the permissible range is 216.4v – 253v.

Equipment/appliances certified to G98/G99 must have detached themselves from the grid by the time the voltage drops to 186v.

What happens between 186v and 216.4v is subject to voltage/time graphs, which are submitted to the ENA as part of the evidence for G98 certification.

In practice the DNO doesn't concern itself with that.
They generally record the average voltage over an interval of up to 10 minutes.

Measuring line voltage on the far end of a supply cable from a consumer unit isn't going to provide useful data about the grid.
It tells me more about the size of copper cable connecting your inverter!

I assume a single phase monitor would still work on a 3 phase board for single circuits but wouldn't be as accurate if trying to measure circuits on more than one phase as the voltages vary slightly.

I have monitors 'out on the grid' and you'd be surprised just how much variation there is between phases (voltage and current).

It begs the question as to how a 3ph inverter 'decides' to use the phases evenly when re-charging a battery from the grid.
There would be a significant difference between a decision based on current than one based on kW.

This looks very interesting... using what would otherwise cause issues or be wasted. 

Perhaps you and @majordennisbloodnok may be interested in this:  

Via the Solis modbus integration (so data going onto a pc via the solis modbus) and that I believe is coming from the meter that they supplied me with (via modbus as well - I think), I see the following data points for A.  There's A, B, C so I think these are phases. In my single phase  inverters, B and C are 0.. 

All the data can be charted..

If this was from a Tuya clamp, I would not trust the figures. But they are probably from the Eastron meter that Solis supplied. 

To be clear, on my HA, this is flimsy, but perhaps that can be avoided..

@batpred, I have been pulling what the inverter believes to be the grid voltage for quite a long time now, and it has been VERY useful. My DNO - UK Power Networks - has been having some issues recovering from a problem back in June last year, and this has meant some significant swings in voltage, all picked up by the inverter. As a result, I've been able to use the data held in HA as evidence for UKPN, and they have been prepared to rely on it as reliable grounds for despatching engineers (albeit the engineers would, of course, double check themselves once on site).

Given inverters with the ability to export to the grid must, by law, stop exporting if the voltage goes above 253V (or thereabouts, I believe), I'm pretty sure all inverter manufacturers take care to ensure their kit can be accurate in its measurement of voltage. As a result, what I get via modbus from my inverter is, I believe, easily accurate enough for my purposes.

I assume a single phase monitor would still work on a 3 phase board for single circuits but wouldn't be as accurate if trying to measure circuits on more than one phase as the voltages vary slightly.

I have monitors 'out on the grid' and you'd be surprised just how much variation there is between phases (voltage and current).

It begs the question as to how a 3ph inverter 'decides' to use the phases evenly when re-charging a battery from the grid.
There would be a significant difference between a decision based on current than one based on kW.

This looks very interesting... using what would otherwise cause issues or be wasted. 

Perhaps you and @majordennisbloodnok may be interested in this:  

Via the Solis modbus integration (so data going onto a pc via the solis modbus) and that I believe is coming from the meter that they supplied me with (via modbus as well - I think), I see the following data points for A.  There's A, B, C so I think these are phases. In my single phase  inverters, B and C are 0.. 

All the data can be charted..

If this was from a Tuya clamp, I would not trust the figures. But they are probably from the Eastron meter that Solis supplied. 

To be clear, on my HA, this is flimsy, but perhaps that can be avoided..

I assume a single phase monitor would still work on a 3 phase board for single circuits but wouldn't be as accurate if trying to measure circuits on more than one phase as the voltages vary slightly.

Hah!

I have monitors 'out on the grid' and you'd be surprised just how much variation there is between phases (voltage and current).

It begs the question as to how a 3ph inverter 'decides' to use the phases evenly when re-charging a battery from the grid.
There would be a significant difference between a decision based on current than one based on kW.

The timeline below has four '240v' monitors at different locations, all supplied from the same Primary substation.

Ignore the red and purple lines because they're recording the output from two different inverters.

The other four lines show voltage variations all over the place.
However, I've been doing this long enough to be able to detect which sequences are the consequence of variations on an 11kV supply,
and which are due to changes being made by the DNO at the 33kV level (outside of the area being monitored).

I hadn't expected that the Eastron meter would be given its own 3-ph breaker!
Is that because the electrician decided to supply it by using thinner wires, I wonder?

I assumed they need to be connected to the supply to measure the volts to work out the kWh used so there needs to be a separate breaker. I guess it wouldn't need to be separate but the only other 3ph breaker is for the inverter.  The board is much larger than it needs to be so there is plenty of space for another breaker. The right hand side of the board is fairly empty.

@batpred I'm looking to monitor 2 circuits on a single phase board so the type you are using is probably what I need.  My son bought a 3ph Vue monitor from Amazon as a Christmas present which will monitor up to 16 circuits on this board.  I assume a single phase monitor would still work on a 3 phase board for single circuits but wouldn't be as accurate if trying to measure circuits on more than one phase as the voltages vary slightly.

You can either buy 'cheaply' from suppliers on AliExpress,
or get a certified 'industry level' current sensor from Sonnency in Germany.

The picture below shows a Battery Combiner Board being built with three different approaches to current measurement.
The red arrow points to a Hall Effect current sensor by Sonnency.

As you'd expect, the level of technical support from Sonnency is excellent!

Well, the Huawei CT clamps I have are 100amp and 230V.

So even if technically they would read a couple of triphasic live lines, the smartlife software would probably get confused. They are also meant to be supplied from the same power the clamps are reading from 

I did not find similar clamps for triphasic power.

But the Solis meter readings are available via modbus via the Solis so this could be an easy way to get them. In my case, I have the readings available via Home assistant (even if I have not found this setup particularly reliable for uninterrupted charting). 😉