Starting from scratch, new design for grid-tied PV system, 8.5kW panels, 14.5kWh storage, 8kW inverter. G99 Form

[Smallholder]

Two thoughts:

If you don't go down the MCS route for any of it you can DIY all the electrics under a Building Notice and get it signed off by LA Building Control, this will give you peace of mind now and paperwork that is useful when you come to sell.

Although it is historical I have found the combination of some AC and some DC-connected PV to be very handy, as during the day the AC can supply the loads directly (esp the EV) in addition to what is avaiable from the Victron Multi. So 5k is adequate for me and the 8k Quattro might be enough for your setup. You might also find like me that WPD (now NG) limit your inverter power anyway, so best to check that out before going too far.

Again, great info, thanks very much. I'll contact building control when the time comes and see about them signing it off.

I am aware that I'll be pushing it with the inverter size in terms of DNO. I struggled to find much real info about what DNO's tend to do with larger inverter connections, but I did find a useful blog from an installer who said they get up to a maximum of 12kW inverters installed, but generally about 8kW is the limit. So this gave me some hope.
I know DNO's often allow you to connect, with the stipulation that the inverter output to grid must be limited. I'm not sure if software controls within the inverter would be accepted (I'd guess not), but I have seen hardware export limitation units that are DNO approved.

[Countrypaul]

Two thoughts:

If you don't go down the MCS route for any of it you can DIY all the electrics under a Building Notice and get it signed off by LA Building Control, this will give you peace of mind now and paperwork that is useful when you come to sell.

Although it is historical I have found the combination of some AC and some DC-connected PV to be very handy, as during the day the AC can supply the loads directly (esp the EV) in addition to what is avaiable from the Victron Multi. So 5k is adequate for me and the 8k Quattro might be enough for your setup. You might also find like me that WPD (now NG) limit your inverter power anyway, so best to check that out before going too far.

Again, great info, thanks very much. I'll contact building control when the time comes and see about them signing it off.

I am aware that I'll be pushing it with the inverter size in terms of DNO. I struggled to find much real info about what DNO's tend to do with larger inverter connections, but I did find a useful blog from an installer who said they get up to a maximum of 12kW inverters installed, but generally about 8kW is the limit. So this gave me some hope.
I know DNO's often allow you to connect, with the stipulation that the inverter output to grid must be limited. I'm not sure if software controls within the inverter would be accepted (I'd guess not), but I have seen hardware export limitation units that are DNO approved.

Is this not where G100 acreditation comes in, a sysem being accepted to only export to the agreed level and limited by a G100 device (such as an inverter with a CT)?

[sharpener]

Again, great info, thanks very much. I'll contact building control when the time comes and see about them signing it off.

I am aware that I'll be pushing it with the inverter size in terms of DNO. I struggled to find much real info about what DNO's tend to do with larger inverter connections, but I did find a useful blog from an installer who said they get up to a maximum of 12kW inverters installed, but generally about 8kW is the limit. So this gave me some hope.

I know DNO's often allow you to connect, with the stipulation that the inverter output to grid must be limited. I'm not sure if software controls within the inverter would be accepted (I'd guess not), but I have seen hardware export limitation units that are DNO approved.

WPD accepted my Multiplus 5k with as @Countrypaul says its G100 certification, and two separate software constraints:

inverter power limited to 3.68kW
Export limited to 3.68kW

I sent them screenshots of the relevant settings with my completion form and didn't hear anything back. It makes no operational sense as the export limit should be sufficient on its own, but it is not too much of a restriction as the max continuous power of the 5kVA unit is only 4kW anyway (4.4kW off-grid). However it would cripple a bigger one.

In an urban area you may well be allowed more than 3.68kW but it would be as well to find out first. I separately have 3.68kW of PV on the AC-Out, so the Victron keeps this synchronised (and earning FITs!) during a blackout and I can have up to 8kW of load on EPS, I don't think most of the Chinese ones will give you that.

Re sizing, it might actually be the pass-through capability that is the most important thing. The 5kVA unit has 50A which I decided was sufficient and so it has proved, the whole house is on AC-Out and this is enough for the cooker during the day and the EV at night. But I don't have an electric shower or two EV chargers, which would draw too much. In your situation I would have thought 3.68kW of MCS-installed PV would qualify you for SEG and then a DIY 8kVA Multi https://essandsolarsolutions.co.uk/multiplus-ii-48v-230v/ plus MPPT would give you what you want (it has 100A pass-through), see how the DNO react to that proposition!

If you can find the Pylontech Force L2 at a good price the stackable modules are very neat and they look much smarter in a domestic setting, IIRC the internals are much the same so you will already be familiar. I have used Node Red to taper the inverter power according to the voltage to get the last 10% out without tripping.


Good luck with it all!

[Smallholder]

Again, great info, thanks very much. I'll contact building control when the time comes and see about them signing it off.

I am aware that I'll be pushing it with the inverter size in terms of DNO. I struggled to find much real info about what DNO's tend to do with larger inverter connections, but I did find a useful blog from an installer who said they get up to a maximum of 12kW inverters installed, but generally about 8kW is the limit. So this gave me some hope.

I know DNO's often allow you to connect, with the stipulation that the inverter output to grid must be limited. I'm not sure if software controls within the inverter would be accepted (I'd guess not), but I have seen hardware export limitation units that are DNO approved.

WPD accepted my Multiplus 5k with as @Countrypaul says its G100 certification, and two separate software constraints:

inverter power limited to 3.68kW
Export limited to 3.68kW

I sent them screenshots of the relevant settings with my completion form and didn't hear anything back. It makes no operational sense as the export limit should be sufficient on its own, but it is not too much of a restriction as the max continuous power of the 5kVA unit is only 4kW anyway (4.4kW off-grid). However it would cripple a bigger one.

In an urban area you may well be allowed more than 3.68kW but it would be as well to find out first. I separately have 3.68kW of PV on the AC-Out, so the Victron keeps this synchronised (and earning FITs!) during a blackout and I can have up to 8kW of load on EPS, I don't think most of the Chinese ones will give you that.

Re sizing, it might actually be the pass-through capability that is the most important thing. The 5kVA unit has 50A which I decided was sufficient and so it has proved, the whole house is on AC-Out and this is enough for the cooker during the day and the EV at night. But I don't have an electric shower or two EV chargers, which would draw too much. In your situation I would have thought 3.68kW of MCS-installed PV would qualify you for SEG and then a DIY 8kVA Multi https://essandsolarsolutions.co.uk/multiplus-ii-48v-230v/ plus MPPT would give you what you want (it has 100A pass-through), see how the DNO react to that proposition!

If you can find the Pylontech Force L2 at a good price the stackable modules are very neat and they look much smarter in a domestic setting, IIRC the internals are much the same so you will already be familiar. I have used Node Red to taper the inverter power according to the voltage to get the last 10% out without tripping.


Good luck with it all!

Thanks very much indeed, I really appreciate all this info.

So do you know what they'd have said if you had a larger array and inverter? It would be madness if they insisted in throttling back an 8kW inverter to 3.68
Presumably they'd have no way of knowing if you were in this situation, and then altered the inverter power to max, but left the export at 3.68?

As far as I understand, the only way to see what size connection the DNO would allow, would be to fill in a G99 application?

I'm not at all familiar with the EPS approach, but I'll read up.
I'm afraid I'm not yet sufficiently clued up to understand exactly what you're suggesting adding a 8kVa multi to an MCS installed system. I'll learn though!

Hypothetical idea here (sorry if this was what you were suggesting). Get a 3.68kW MCS approved system installed, with export and feed in, also have battery storage installed. Then add my own DIY array, say 5kW, with MPPT that charges the same battery storage. Meaning the original install can draw on the power from the battery, which is topped up from the secondary install.

[nowty]

Hypothetical idea here (sorry if this was what you were suggesting). Get a 3.68kW MCS approved system installed, with export and feed in, also have battery storage installed. Then add my own DIY array, say 5kW, with MPPT that charges the same battery storage. Meaning the original install can draw on the power from the battery, which is topped up from the secondary install.

Some on here do that and its legit.

Assuming you mean the MPPT is a DC connected charge controller.

[Smallholder]

That sounds encouraging.

I wonder if the MCS installer would do a G99 form and install a larger inverter? Even with a 4kW PV array.

Even with a 3.68kW inverter at full chat, I reckon you can get £4 a day from Octopus Flux just during their 3 hour peak export window. Assuming you've got the PV generation (11kWh a day spare) to support this. Or the battery storage to top up at night and sell back.
With a bigger inverter it could be a good income.

[sharpener]

Thanks very much indeed, I really appreciate all this info.

So do you know what they'd have said if you had a larger array and inverter? It would be madness if they insisted in throttling back an 8kW inverter to 3.68
Presumably they'd have no way of knowing if you were in this situation, and then altered the inverter power to max, but left the export at 3.68?

No but they might be put on guard if you agreed to do it in the first place!

As far as I understand, the only way to see what size connection the DNO would allow, would be to fill in a G99 application?

I think WPD may have changed their processes over the last year since re-branding as NG, also I believe the forms are now different from when I did them, IIRC SG 1,2 and 3 (see elsethread). But I think there is a way of consulting them informally, try ringing their no. when you are back in the UK(!) but they answer reasonably quickly and were quite efficient when I wanted their fuse pulled so I could install an isolator.

I'm not at all familiar with the EPS approach, but I'll read up.
I'm afraid I'm not yet sufficiently clued up to understand exactly what you're suggesting adding a 8kVa multi to an MCS installed system. I'll learn though!

Hypothetical idea here (sorry if this was what you were suggesting). Get a 3.68kW MCS approved system installed, with export and feed in, also have battery storage installed. Then add my own DIY array, say 5kW, with MPPT that charges the same battery storage. Meaning the original install can draw on the power from the battery, which is topped up from the secondary install.

Not quite, but close. That isn't going to give you the capacity to invert from the battery at more than the (small) MCS inverter rating. Also there is the warranty to consider, if you add more batteries to the original MCS installation (which you would need to do, there is a big profit margin added to batteries) and MPPTs to charge them they may not honour the warranty, but if you connect to it only at 230V it will remain unmodified.

As you originally asked how much was DIY-able, and mentioned Victron, my suggestion was to get the minimum possible done on an MCS cert - a cheapo 3.68kW PV generation only inverter, no battery - and DIY the larger array/battery charger/inverter/MPPT to give you the 8kW in the title bar to power your EVs during the day and the house at night.

IMO splitting the project this way is preferable to having a massively big charger/inverter/battery fed by big MPPTs and no AC generation, and it is more resilient if something packs up.

[Krill]

That sounds encouraging.

I wonder if the MCS installer would do a G99 form and install a larger inverter? Even with a 4kW PV array.

Even with a 3.68kW inverter at full chat, I reckon you can get £4 a day from Octopus Flux just during their 3 hour peak export window. Assuming you've got the PV generation (11kWh a day spare) to support this. Or the battery storage to top up at night and sell back.
With a bigger inverter it could be a good income.

Some hybrid inverters (Sunsynk) can be parralled and could provide the significant inverting capacity wanted, as can some battery inverters (Lux etc).

[sharpener]

That sounds encouraging.

I wonder if the MCS installer would do a G99 form and install a larger inverter? Even with a 4kW PV array.

Even with a 3.68kW inverter at full chat, I reckon you can get £4 a day from Octopus Flux just during their 3 hour peak export window. Assuming you've got the PV generation (11kWh a day spare) to support this. Or the battery storage to top up at night and sell back.
With a bigger inverter it could be a good income.

Some hybrid inverters (Sunsynk) can be parralled and could provide the significant inverting capacity wanted, as can some battery inverters (Lux etc).

Good scheme if you can prevent the second one exporting to keep the DNO happy, then one can be installed under MCS and another added as and when. But I don't think there will be massive scope for export. On @Smallholder's original figures his average daily usage is 20kWh a day so that is 7.3MWh/yr. The limiting factor is roof area and in Somerset you will get maybe 9.5 MWh of production annually, though not evenly spaced so there will be a deficit in winter and some surplus in summer - perhaps 50% of output on a good day but there won't be too many of those.

[Krill]

That sounds encouraging.

I wonder if the MCS installer would do a G99 form and install a larger inverter? Even with a 4kW PV array.

Even with a 3.68kW inverter at full chat, I reckon you can get £4 a day from Octopus Flux just during their 3 hour peak export window. Assuming you've got the PV generation (11kWh a day spare) to support this. Or the battery storage to top up at night and sell back.
With a bigger inverter it could be a good income.

Some hybrid inverters (Sunsynk) can be parralled and could provide the significant inverting capacity wanted, as can some battery inverters (Lux etc).

Good scheme if you can prevent the second one exporting to keep the DNO happy, then one can be installed under MCS and another added as and when. But I don't think there will be massive scope for export. On @Smallholder's original figures his average daily usage is 20kWh a day so that is 7.3MWh/yr. The limiting factor is roof area and in Somerset you will get maybe 9.5 MWh of production annually, though not evenly spaced so there will be a deficit in winter and some surplus in summer - perhaps 50% of output on a good day but there won't be too many of those.

Agreed that export is less of an issue unless there is the possibility to fit in a second PV array to cover the winter shortfall that leads to significant summer over production, but the recent solar pergolas have shown this should be a consideration, even if minor.

Having just gone through the process last month, I think this would be a better set up than I have ended up with (Separate standalone PV inverter and paralled battery inverters) because it would have been a cheaper up front cost and there would be enough redundancy if something broke.

Personally I'd have a look at the Sunsynk 3.6kW hybrid inverters, they don't look that expensive and have greater inverting power then the name suggests. It's probably worth considering to just get two (or even three if you really want to be paranoid about ability to not cap total production) installed by a professional and get them to do the paper work, and then connect up additional battery banks and PV arrays as any when needed. That way everything is legit and can be sold on if needed.