Posted by: @majordennisbloodnok

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Nonetheless, assuming you were to get a fairly chunky compliant inverter - let's say a 12 kW inverter - my understanding is that it would be acceptable for the DNO to issue a G98 certificate on the understanding the inverter was limited to a maximum of 3.6 kW export to the grid. 

My understanding is that will require a G99 and a G100. It's common to refuse a large G99 in many areas. It's unfortunately becoming increasingly common to refuse or tightly limit even G100 on fragile rural networks.

I think there are 4 paths

- 3.6kW inverter, G98, no extra plug in stuff. DNO can be unhappy but it's their problem and they'll have to spend some of the shareholders yacht money fixing problems (as slowly as possible).

- G99 to try and get a bit more - but may be refused, may also be ludicrously small like going from 3.6kW to 4kW. That said 5kW for many people is a lot better than 3.6.

- Big inverter, G100 - may get refused but this is the nice grid tie option and gets you capacity, big batteries and options to balance it sensibly with loads of solar easily if you pick the right products.

- Big inverter, don't grid tie, use the grid as input only

The last option you get a big grid forming ("off grid") inverter, wire the grid as generator input, and run the house off it. There's no feeding back into the grid so there are no problems with you overloading anything beyond blowing your house fuse. You need a transfer switch so you can reconnect the house directly to the grid (in case the inverter dies), ground rod and some oddments, and you need an offgrid inverter which can field your house load, and also pass through your house load (so 60-80A). The kit for this is widely available - as you can imagine large rural farms hit this quite often and may well want to go well beyond 16kW with 50kW+ 3 phase options being widely used.

You are essentially then operating an off grid house, whose generator happens to be a wire to the national grid. The downsides of that are that a) your inverter needs to be big enough to field your largest demand so costs more, and b) you won't get any SEG payments, which I imagine isn't going to happen much anyway.

It is possible to build hybrid setups with a mix of on and off grid inverters sometimes sharing the batteries. However that tends to turn into an infeasibly large amount of electrician hours and lots of very expensive blue boxes.

OK, digging into this a bit further, distilling what @transparent has posted and trawling the ENA web site, I think the following are correct. The list is for clarity more than adding anything new (i.e. if what I think @transparent said matches what he thinks he said we're golden. If not, at least we know there's a misunderstanding).

• G98 approval is required for connecting to the grid something capable of exporting up to 16 amps per phase (about 3.68 kW for single phase, or 11 kW for three phase). The DNO cannot refuse this, so the process is for a qualified person to do the installation and connection and then use the G98 forms to notify the DNO. Even though they can't refuse, the DNO does, however, have to be notified so they know what's connected.
• If you want to connect something to the grid capable of exporting at more than 16 amps per phase, you'll need G99 approval which requires the DNO to be informed in advance and can take a number of weeks to get a response. You need to say what upper limit you're asking for and the DNO can say yes, no or offer a lower limit. If they say no, your G98 rights still apply so you're still entitled to an export limited to 16 amps per phase.
• If you have several things connected to the grid that are capable of exporting then the approval required is for all of them together as if they were a single unit, so if you have two inverters wired into the grid and a battery that can export to the grid then their combined export capacity is what needs to be taken into account.
• It doesn't matter how many inverters, batteries, solar panels and so forth you actually have; it's only what's wired to the grid that matters. Therefore, if you have a hybrid inverter (as I do) with solar panels and battery wired into it, it's only the inverter that's connected to the grid so that's the only thing whose export capacity matters.
• As long as anything connected to the grid is type tested and in the ENA database, if it has the capability to have its export artificially limited, it is that applied limit that matters rather than what the thing could theoretically be capable of. Therefore, an 8 kW inverter software limited to only be able to export at 3.68 kW is, from the DNO's point of view, the same as a 3.68 kW inverter.
• If you get something capable of exporting at more than 16 amps per phase and want to at least try to get agreement for using some of that extra capacity, you will have to seek G99 approval. If you don't want the wait, you can get it limited to 16 amps per phase, get it connected immediately and notify the DNO via the G98 route. If you later change or remove that limit without first applying for G99 approval, you're breaking the law.

I'm aware that G100 relates to import and export limits, but to be frank I'm not sure how it all fits in, especially since I've seen several FAQ lists saying whatever needs G100 approval also has to be compliant with G98/99 rules.

@etchedpixels, I've burrowed quite extensively on whether G99 applies to the inverter's theoretical export capabilities or its configured ones and everything I'm finding is suggesting it's the configured limit that matters, hence what I said above. If you have found something else that suggests differently, I'd be grateful for a link.

All that said, @transparent, does that all seem to be accurate or am I way off base?

@davidb, hopefully this list along with any refinements or clarifications others make will help with your decision making.

theoretical as in the worst case scenario a user or likely mess up can cause. So setting a device to "0 export" doesn't count - without G100.

herefore, an 8 kW inverter software limited to only be able to export at 3.68 kW is, from the DNO's point of view, the same as a 3.68 kW inverter.

No - that's what G100-2 gives you. An 8kW inverter that's export limited to 3.68kW is 8kW without G100. An 8kW inverter that is *output* limited to 3.68kW and locked so the end user can't change it however may well be G98 but that's basically useless except it lets people make standard inverter sizes and software lock them to similar smaller ratings.

G100 is the usual pathway to "I've got a big inverter feeding lots into the house but only 3.68kW or whatever into the grid". There are a load of additional requirements the installation has to meet to make it very very unlikely it crosses that limit more than momentarily. If the local grid is fragile though the DNO can refuse a G100.

Posted by: @majordennisbloodnok

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G99 approval which requires the DNO to be informed in advance and can take a number of weeks to get a response

There are two pathways through the G99 approvals process.

A: G99FT (Fast Track) covers three different categories of Small Generation Installations (SGI). The three SGI categories are well explained on this National Grid website.

If your proposed installation is covered by SGI-1, SGI-2 or SGI-3, then you should receive a response within 24hrs using the online forms. SGI-3 allows up to a whopping 60A per phase!

G99FT still requires the DNO to be notified within 28-days of the installation having been completed. That allows the installer some wriggle-room in case he/she has to make changes once they are actually on site.

B: G99 Full is for anything not covered by SGI-3. That includes equipment which doesn't already have G98 approvals, and hence doesn't appear on the ENA database.

For domestic installations you would still expect a response within 28 days, and often a lot sooner.

Whichever route you take, the DNO uses in-house software to calculate the maximum export they can accept on your particular electricity supply. The Area Network Planner will add to the information you've provided, with data known to the DNO. Typically this includes.

• cable size connecting to your house
• cable-size between your in-road connection point and the local substation
• cable material (usually copper or aluminium)
• capacity rating for substation transformer
• available export capacity of transformer which isn't yet allocated
• distance between you and the local substation
• number of houses on the same 3ph feed as your house
• number of those houses between you and the substation
• number of houses which already have Low Carbon Technology (EV chargers and heat pumps)

Please note: There is a major difference between the application methodology used by network planners, and the way in which electricians apply the Wiring Regulations.

A Network Planner is a qualified engineer who knows your particular area. They can adjust the output of their in-house software to better reflect what is actually the case where you live. They are "on the same side" as you. If you know that you will make another change to your electricity demand in the next few months, then tell them. For example, a network planner might wish to allow a higher export current if you are going to install an EV charger in a couple of months time.

Never try to withhold information from your DNO. They have specialist equipment which can be brought to a local substation to seek out a customer who has installed something without obtaining the right type of consent.

Posted by: @transparent

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Never try to withhold information from your DNO. They have specialist equipment which can be brought to a local substation to seek out a customer who has installed something without obtaining the right type of consent.

They can also obtain smart meter data with a warrant and reasonable suspicion so definitely good advice.

@majordennisbloodnok bummer, it’s happened again - missing draft.  I returned to review quite a long draft I had started yesterday and again I can’t find it.  I know it was saved and the system was taking regular backups.  I imagine there must be a draft folder somewhere against my log in, I just don’t know where to look.  Any advice very welcome.

Unfortunately, @davidb,this last reply of yours will have superceded that other reply you were creating so I don’t think you’ll be able to retrieve it. @editor, I don’t suppose you have any further thoughts?

Posted by: @etchedpixels

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theoretical as in the worst case scenario a user or likely mess up can cause. So setting a device to "0 export" doesn't count - without G100.

herefore, an 8 kW inverter software limited to only be able to export at 3.68 kW is, from the DNO's point of view, the same as a 3.68 kW inverter.

No - that's what G100-2 gives you. An 8kW inverter that's export limited to 3.68kW is 8kW without G100. An 8kW inverter that is *output* limited to 3.68kW and locked so the end user can't change it however may well be G98 but that's basically useless except it lets people make standard inverter sizes and software lock them to similar smaller ratings.

G100 is the usual pathway to "I've got a big inverter feeding lots into the house but only 3.68kW or whatever into the grid". There are a load of additional requirements the installation has to meet to make it very very unlikely it crosses that limit more than momentarily. If the local grid is fragile though the DNO can refuse a G100.

It seems I need to get a lot more familiar with the G100.

Posted by: @etchedpixels

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theoretical as in the worst case scenario a user or likely mess up can cause. So setting a device to "0 export" doesn't count - without G100.

herefore, an 8 kW inverter software limited to only be able to export at 3.68 kW is, from the DNO's point of view, the same as a 3.68 kW inverter.

No - that's what G100-2 gives you. An 8kW inverter that's export limited to 3.68kW is 8kW without G100. An 8kW inverter that is *output* limited to 3.68kW and locked so the end user can't change it however may well be G98 but that's basically useless except it lets people make standard inverter sizes and software lock them to similar smaller ratings.

G100 is the usual pathway to "I've got a big inverter feeding lots into the house but only 3.68kW or whatever into the grid". There are a load of additional requirements the installation has to meet to make it very very unlikely it crosses that limit more than momentarily. If the local grid is fragile though the DNO can refuse a G100.

This is what the Tesla PW3 can do.

Nominally, it's an 11.04kW (48A) inverter, that can be software locked to lower ratings (3.68 through to 11.04kW in 1kW steps). This is a 'hard' software lock that once set by the installer cannot be changed by either the end user or installer - only Tesla support can reset and change it, and they will only do that upon sight of an appropriate G99/100 approval (most commonly where the installer made a mistake and set it incorrectly at installation).

Then there is a separate export limitation, which again can be set by the installer during commisioning through their installer app but this is not 'hard locked' and can be changed by the installer through the Tesla installer's app should permission subsequently be granted for a higher export limit.

So the PW3 can have it's inverter rating set (hard locked) at 3.68 - 11.04kW, and the export limitation can be separately limited to any value. When I had my PW3 installed, the DNO's initial offer was for the inverter to be hard locked to 7kW (roughly matching the output of my array), and for export to be limited to 1kW. 

Posted by: @old_scientist

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Nominally, it's an 11.04kW (48A) inverter, that can be software locked to lower ratings (3.68 through to 11.04kW in 1kW steps). This is a 'hard' software lock that once set by the installer cannot be changed by either the end user or installer

NB @majordennisbloodnok  - there's no single definition of what constitutes the G100 designation.

It's specific to that product and method of installation. The key factor is that it's been accepted/approved by the ENA.

We can't tell much by someone simply identifying that a product has G100 certification. We have to ask further questions about the feature and what it's intended to achieve.

To be frank, my interest in burrowing into the G98/G99/G100 details is purely personal. My original reason for raising the whole issue is to clarify for @davidb what is and isn't possible.

It seems from @davidb's posts so far that his potential installer is suggesting that (assuming because of his locality that the DNO refuses any export greater than 3.68 kW) he is going to have to accept an inverter of that size, thereby limiting his ability to draw from his battery and his solar. All these discussions have now made it clear that doesn't necessarily have to be the case, and that there are ways of having a beefier inverter installed in such a way as to satisfy the DNO whilst allowing higher household draws. Unfortunately, all these discussions have also made it clear that the devil's in the detail and that there are plenty of configurations that would not be acceptable to the DNO.

Nonetheless, from the point of view of the original post I've highlighted what I felt was necessary. From my own point of view, I've highlighted how much of what seemed a straightforward subject I still need to learn, so I'm off to do some more reading.

@majordennisbloodnok thanks for your post - really not what I wanted to hear but I do appreciate the interaction.
When I joined the forum again this morning, there was no sign of last night’s version.  I only sent you the message which you suggest overwrote last night’s missive because I could find nothing to update.  I am really very confused just now so any help from Mars or others would be wonderful.