@transparent I forgot to ask. What are you currently using to control the charging? I'm using a Sofar ME3000SP at the moment and it works really well. But it's limited to 3.2kw charging power

That's a very good question @batalto !

I'm working with others to develop a new control unit to operate heat-pumps off-grid. For obvious reasons I won't be giving much detail here on an open Forum, but I will say that it requires a Time-of-Use Tariff, and is not locked into the two main distributed-storage control systems currently under development in UK (Kraken from Octopus, and Flex from Kaluza).

The control unit must work with a number of different battery systems, BMSs and inverters. Unlike the classic YouTube off-gridders, that means I won't just stick with a favourite. I'll be amassing knowledge by mixing different pieces of hardware, each of which will have pro's and con's. 😖 

At this early stage I'm working with Growatt SPF5000ES hybrid units (5kW), but they are very difficult to get hold of. I currently have only one here. An order I first placed on 04feb22 for two more has still not yet been shipped from China. So be very wary of suppliers who will happily take your money and sit on it.

A 5kW inverter would be charging a nominal 50v battery stack at 100A max. That in turn informs the possible selection of battery and BMS sizes. Are you OK with that maths?

I also have here a 5500w inverter from MPP Solar in Taiwan. These are sold by many outlets with slight variations, badged Voltacon, Voltronic Infini or EAsun. However this inverter is currently still being used within another trial, so it won't yet form part of the HP-control development.

Courtesy of Western Power Distribution, I have been given a nice new 3-phase feed. Although initial R&D work centres on single-phase, I'm also genning up on the right and wrong ways to deliver 3-phase into UK homes. Unlike industry 3-phase users, domestic customers will be loading the phases unevenly... and that creates issues for DNOs.

Given that background, I would be interested to hear from others on RHH if they have good experiences with any other hybrid (mains & solar) off-grid storage inverter.

A typical mid-range heat-pump draws about 30A each time it starts (nominally 7.5kW), so it requires an absolute minimum of two 4kW inverters in parallel. I won't be spending time looking at anything smaller than that!

Now you know this, you'll appreciate why @Mars has just given me a shiny new gold cross icon... which we don't yet know the full meaning of. 😮 

@transparent interesting I'll take a look at them. Currently top of my running is a Goodwe unit. I also want the 100amp charging capability to maximise the time of use tariff from Octopus.

https://affordable-solar.co.uk/product/goodwe-sbp-series-3-6kw-5kw-ac-retrofit-storage-inverter-lv-battery-single-phase/

I can give you a good overview of the Sofar ME3000SP unit if you want it.

Thanks for the link @batalto. So I have three initial observations about the Goodwe unit you've mentioned:

1: It has grid-export capability. Even if you intend only to drive appliances from the off-grid 240v port, it is likely to still require an approved installer and G98 consent from your DNO (Is that SSE?). Otherwise, what's to stop you connecting back into the grid later? For this reason your DNO may withhold consent for the 5kW version with the 100A charge capability.

2: Inverters with grid-connection capability under G98/G99/G100 derive their 50Hz synchronisation from the grid. If the grid goes down, then they must cease their 240v output within 20mS. They may not rejoin until a random time interval has passed after the grid is restored. So it's possible that the Goodwe design is required to shut down its "off-grid" port too when there is an outage. Have you looked into this?

3: The spec refers to a WiFi link to a portal. If that link is non-operational, and you can't reach the portal, does the inverter continue to operate satisfactorily? Could you still see the data on the display, for example? Or is that only showing information supplied by the portal (as is the case with PowerVault batteries for example).

@transparent at these prices I might just buy two more Sofar units - it's basically the same price and just put each one with it's own battery!

Does the Sofar 3kW unit have the same issue as the Goodwe Point-1 (above)?

The system diagram has an arrow suggesting it has grid-connected export:

Even if the Sofar has an integral 50Hz generator to permit 'critical loads' to be operated during a mains outage, it remains the case that two such units in parallel would exceed the G98/99 export limitation of 3.68kW. Might you not face the possibility of your DNO refusing the connection?

It is, of course, possible that the ENA has checked out the export-limitation feature of the Sofar system and approved its inbuilt method of constraining it below 3.68kW. But you'd need to check with your DNO first because they'll look it up on the database.

There's no fee to submit a G99 Fast-track application for G100-certified equipment, which the Sofar unit is. So you could just send in the application as a householder and see what they say.

Take this back to first principles @batalto

The following diagram is permitted because it cannot export back to the grid. You would not even require G98 certification because there is no possibility of export to the grid. The inverter supplying the "off-grid" devices is not synchronised to the universal 50Hz. It creates it's own 240v 50Hz AC and operates during outages from the battery.

Curiously, under the current UK regulations, you would still need to apply for consent to connect the heat-pump in the above diagram. The LCT application process hasn't embraced the possibility of anyone ever running a heat-pump or an EV charger entirely off-grid! 😟 

I think, @batalto, you need to spend more time in the pub, doodling diagrams on a napkin. That's how I get my more 'adventurous' ideas knocked into shape by like-minded renewable-energy enthusiasts. 🍺 

For others trying to follow this in future, here's the type of inverter which I think @batalto is really trying to implement.

It has no possible export back to the grid, as shown by the arrows. G98 certification and DNO approval isn't required.

Its 240v output runs from an internal 50Hz generator, and is not synchronised to the grid.

The difference between this and my earlier diagram is that the hybrid charger is now also the inverter. These functions share common electronics.

Due to there only being one inverter section in the box, when it's put into 'charging from the grid' mode, the off-grid devices are switched across internally to be directly grid-connected.

(Yes, I've left out all the safety devices and requirements for BS 7671 IET Wiring Regulations complience).

@transparent as an FYI I do have the old G59 as my panels are over 4kw and it's applied to my current Sofar ME3000SP

That's good news @batalto - the existing Sofar ME3000SP had its grid-limitation system approved by ENA in addition to it having G59 certification. Those are two entirely separate concepts.

Nevertheless, one of the reasons that G59 evolved into G98 was because multiple installations in an area could invoke a cascade failure. I'm not going to describe here how that was invoked because it could threaten the stability of the UK grid(!). Suffice to say that it was an unforeseen consequence of G59-certified units responding to the grid temporarily straying outside of its statutory frequency specification.

In one particular geographical area known to me, the threat was deemed sufficiently serious that MSC-approved installers were required to revisit customers and modify the firmware of those G59 units. It's likely that the earlier Sofar units fall into this category because their datasheet states compliance with G83/2. G83 is the overall accreditation for grid-connected export devices. Supplement 2 was a requirement to (re-)test units to ensure that the cascade effect wouldn't occur.

So my recommendation is to submit a G99 Fast-track application to your DNO for the theoretical increase of your installation to three Sofar ME3000SP devices. (G99 is for more than one export-capable device at the same site).

The DNO will check the characteristics of the distribution grid in your area as part of the process. They will ensure not only that the export-limitation system is approved for 3 units in parallel, but that the grid cannot be  compromised by the possibility of a cascading outage.

[Moderators is this too technically deep for this forum? Is it sufficiently clear, or do we need diagrams/tables for the G certification categories?]

I wrote a 'story' demonstrating a potential grid-outage due to a cascade effect with renewable energy devices over on the OVO Forum. That might make the issue more comprehensible.

Posted by: @transparent

[Moderators is this too technically deep for this forum? Is it sufficiently clear, or do we need diagrams/tables for the G certification categories?]

Diagrams are always helpful, if you have them on hand. 

We have 4kWp of (mcs) solar, and subsequently a V2G unit by Indra for which there is a G99 A1-1 form stating the overall export is <16A per phase, even though the V2G unit is capable of 6kW.  I assume from this that our area is constrained somehow.  I've never noticed an issue, we never get power cuts, so I guess all is well.  Given the 16A overall limit imposed, could this have been done in a simpler(quicker) way? - the G99 application did take quite some time I recall. 

In future we are likely to reconfigure the V2G to be V2H, and hopefully add a house battery with a hybrid inverter so that I could add more PV 😊.  I think this would be acceptable if it too is G100 limited with an overall house export of 16A per phase so that we still will not export more than 16A overall.  What is the simplest application process?