OK, I'm working on the premise:

1) Panels are cheap and it's best if you install all you can at the start while there's scaffolding etc.

2) Ideally I would like to be off-grid, but PV production in the middle of winter is b....ks, so I'll still use a grid connection then. But I would like to maximise PV production in the shoulder seasons.

3) I have a 1P / 100A grid connection to UKPN.

So, I'm planning about 48kWp of panels split across 4 roofs (in 2 pairs) in various orientations. And then:

1) Asking the DNO to allow a 12kW 1P inverter with grid connection for the house (ASHP, DHW with immersions, kitchen etc) which will support 24kWp of panels on the house (2x clipping). This will be integrated single stack (probably Sig) with as yet unknown amount of battery.

2) Having a separate 12kW 1P for the other 24kWp of panels on the 'cart lodge' but disconnected from the rest of the house circuits and the grid. This will only be used for car charging.

What do you think?

Edited October 25Oct 25 by Alan Ambrose

Firstly, with a 100A connection why start with the premise of only exporting 12kw? At least ask the utility if they can support higher export? Then it's simple and easy. Even if they limit export below generation it seems like keeping things simple would be better. Having said that, you might not even get 12kw so that then creates another problem for you.

I'm not sure your approach does anything more than make things complex. A 24kwh battery is not enough to fully charge a car and even if you are mostly just topping up you will likely come across times when you empty the batteries in the off-grid system before you charge the car. Also, if the off-grid system had spare solar stored in it's battery but the house did not, you would have no way to use that capacity. So I would look for everything to be grid connected (with ability to continue operation without the grid). That way you get the full capacity of all batteries available.

If you have significant excess generation that can't be exported what are you going to do with it? Seems like a huge waste to spend a fortune on install and not have anything to use it for.

If you plan a workshop with potential for 3p machines then an alternative setup would be to use inverters to generate the 2 other phases (all connected to the same battery). 1 inverter is connected to the grid, the other two phases purely rely on solar/battery (inc charging from the grid connected inverter). I doubt this is a common option but I know you can do it with some inverters (Victron). Still I'm not sure it makes much sense from a financial POV.

Just over clock the inverters, fit PV to run at max amps or max voltage. We have 6.5kW PV on a 3.6kW inverter and 2kW on a 1kW inverter. Any excess is just clipped, but rarely does it clip anything.

But if going with hybrid inverters and battery you can put as much PV as you want on the DC side as long as you stick each array in it's own MPPT.

Battery and inverter I would investigate if you are running on a cloud and walk away if you are. Our GivEnergy is a good example it can be run in full local mode, so when GivEnergy went belly up everything just carried on working. A cloud based system you stuffed. Sig I believe need a web connection to have full functioning - so if that's the case, I would look elsewhere.

Also if you want full house functionality in a power cut, look for a system that has a gateway, that will allow full inverter output to house with grid off. Most normal hybrid just give a small ups output which is great if you just want a light on.

@Alan Ambrose

Have you done a decent usage analysis yet?

You may find you need less than you think.

>>> Battery and inverter I would investigate if you are running on a cloud and walk away if you are.

Good point - looks like Sig needs a connection every 90 days.

>>> Have you done a decent usage analysis yet?

Ah, ROI is not the objective - the objective is to be as nearly off-grid as possible.

>>> If you plan a workshop with potential for 3p machines then an alternative setup would be to use inverters to generate the 2 other phases (all connected to the same battery). 1 inverter is connected to the grid, the other two phases purely rely on solar/battery (inc charging from the grid connected inverter).

Yeah I do have a 3P lathe :) I wrote off this option though as, possible technically, but too tricky for UKPN / my chosen sparks to get their head around.

>>> Firstly, with a 100A connection why start with the premise of only exporting 12kw?

Good point, I believe the max on 1P is 17kW. I think Sig's max 1P inverter is only 12kW though.

>>> A 24kwh battery is not enough to fully charge a car

Yeah, I was planning on only charging direct from PV - it seems overly complex and expensive to me (I know that's ironic) to charge a static battery and then use that to charge a car's battery. That means either charging from the sun during the day or from the grid when the sun's not shining. That means having the car charger connecting either to the off-grid inverter or the grid via a (probably manual) changeover switch.

1 minute ago, Alan Ambrose said:

>>> If you plan a workshop with potential for 3p machines then an alternative setup would be to use inverters to generate the 2 other phases (all connected to the same battery). 1 inverter is connected to the grid, the other two phases purely rely on solar/battery (inc charging from the grid connected inverter).

Yeah I do have a 3P lathe :) I wrote off this option though as, possible technically, but too tricky for UKPN / my chosen sparks to get their head around.

I'm not sure why UKPN would need to know. Only the first phase inverter would be grid connected. The other inverters would share a neutral but not be connected to the mains line voltage at all.

Agree finding an electrician who is comfortable might be challenging. But maybe look up what’s involved in doing it as you might want to keep the option open for future and that might impact the design

1 minute ago, Alan Ambrose said:

>>> Firstly, with a 100A connection why start with the premise of only exporting 12kw?

Good point, I believe the max on 1P is 17kW. I think Sig's max 1P inverter is only 12kW though.

Why limit yourself to one inverter on grid? Nothing to stop you having multiple. Just need to be setup the right way so they honour the export limits. (There are simpler and less simple ways of doing this depending on design).

1 minute ago, Alan Ambrose said:

>>> A 24kwh battery is not enough to fully charge a car

Yeah, I was planning on only charging direct from PV - it seems overly complex and expensive to me (I know that's ironic) to charge a static battery and then use that to charge a car's battery. That means either charging from the sun during the day or from the grid when the sun's not shining. That means having the car charger connecting either to the off-grid inverter or the grid via a (probably manual) changeover switch.

And what you propose isn't complex? I think it's worse. Solar only wouldn't work. You need the battery (or live connection to grid) to smooth solars variation. Many car chargers can be set to limit output to approximately match what is coming in via solar while still being connected to the grid, so just use one of them with a normal grid tied system.

I think you should step back, start from the basis of sizing the inverters to be big enough to power the house + be able to export the maximum allowed/generated. Any bigger than that is pointless. Ensure you have enough MPPT's to enable proper segmentation of the roof (limit shading impacts). If there is a mismatch look at alternate systems/arrangements (eg, Victron that decouples MPPT's from inverters - also sells combined models). Work out the amount of generation you expect from the solar at the darkest time of year. You have an awful lot planned so it's likely that you will still generate a meaningful amount. Direct attention to areas that may lower demand to match this winter generation as much as possible and then look up data to see what to expect in terms of multiple cloudy/sunless days in a row and determine how much battery you need to ride through that. Get a car charger (and immersion controller) that can adjust output based on solar generation.

Obviously your plan doesn't make much financial or environmental sense, but if you want to do it that's the approach I would look at.

I should note, that if you plan to own an EV and keep it relatively charged up then the need for big batteries diminishes if that car has a built in ability to export it's energy. Many cars these days can output 2-3kw that could be directed to a generator input on the appropriate inverter (if it exists) or a battery charger direct charge from the car into the house batteries (easy if 48V system, likely not possible on a Sigenergy system). Having said that, I think Sigenergy are one of the few companies that claims to be able to directly interface with the DC side of the EV battery so could take charge that way.

It is both cheaper and more environmentally conscious to use the EV battery as the long duration emergency backup as it saves oversizing the house battery for the extremely rare case. The car doesn't need to be able to support the max load of the house. Just trickle (3kw) charge the batteries over many hours. That should get you through most extended outages when combined with even limited solar output (worst case scenario you conserve energy a little to extend the time). Advantage of intentionally building such a setup is that you end up with a very large portable battery. So in the extremely extreme event you have the potential to get in your car and drive somewhere else to charge it and then drive back home where you can use that energy.

15 hours ago, -rick- said:

Many cars these days can output 2-3kw that could be directed to a generator input on the appropriate inverter (if it exists) or a battery charger direct charge from the car into the house batteries (easy if 48V system, likely not possible on a Sigenergy system). Having said that, I think Sigenergy are one of the few companies that claims to be able to directly interface with the DC side of the EV battery so could take charge that way.

There maybe others but I think Victron do an inverter with a genny input that could be hooked up to an EV V2L output...maybe Victron Quattro or Multiplus?? I think it was discussed on here a while back??

1 hour ago, Dillsue said:

There maybe others but I think Victron do an inverter with a genny input that could be hooked up to an EV V2L output...maybe Victron Quattro or Multiplus?? I think it was discussed on here a while back??

It's the Quattro.

But you can do it on any 48v system by wiring a separate battery charger direct to the batteries (requires buying the 3kw charger though)

Sorry to be slow to reply - I'm been busy building and also mulling over your replies...

Is V2H & V2G really a thing now?

@-rick- >>> I think you should step back, start from the basis of sizing the inverters to be big enough to power the house + be able to export the maximum allowed/generated. Any bigger than that is pointless. Obviously your plan doesn't make much financial or environmental sense ...

That's a bit harsh 😄. Actually I've done quite a lot of thinking and analysis. I think I'm just chafing against the DNO over-control of kit. Export limits I get, constraints on inverter sizes I don't. That's like forcing owners to govern their cars to 70mph - OK, let's not go down that wormhole.

One thing that would help would be a 1P inverter with >4 MPPTs and/or high MPPT current/voltage limits (say over 600V / 16A) and/or PV overcapacity support > 200%. That would allow me to load up the inverter(s) with panels for winter use while keeping the actual inverter size down. Gemini says there are no 1P - maybe only 3P.

You can get 1P with 3x MPPT, with high volts and high amps allowed. Screenshot of data for a 7kW fox grid tied inverter other sizes available

True, although I was already looking at the Sig 1P 12kW which has 24kW / 600V / 16A.

BTW here are the expected numbers from PHPP / PVGIS (ignores the odd efficiency factor).

I'm 'self-sufficient' except Jan & Feb with a shortfall of a total of ~725 kWh.

1 hour ago, Alan Ambrose said:

Is V2H & V2G really a thing now?

V2G not so much. Maybe look at the SigEnergy offering as I think they can do it on some cars. But a lot of cars have what is effectively an off-grid/generator output of 2-3kw at least as an option. With thought you could use that as an emergency battery during extended outages and trickle charge your main battery (wouldn't support peak load of the house but could likely provide sufficient power over a 24hr period to maintain your main house batteries.

1 hour ago, Alan Ambrose said:

Actually I've done quite a lot of thinking and analysis. I think I'm just chafing against the DNO over-control of kit. Export limits I get, constraints on inverter sizes I don't.

Where are you seeing this? I know people on here have talked about that but I didn't think the rules allowed that so long as the kit was appropriately certified to limit output.

1 hour ago, Alan Ambrose said:

One thing that would help would be a 1P inverter with >4 MPPTs and/or high MPPT current/voltage limits (say over 600V / 16A) and/or PV overcapacity support > 200%. That would allow me to load up the inverter(s) with panels for winter use while keeping the actual inverter size down. Gemini says there are no 1P - maybe only 3P.

As I mentioned there is the alternative of adding additional panels via DC connected MPPTs. Not generally available with high voltage batteries but if you go with a 48V system you could get your inverter with 2 MPPTS and add another 10 connected straight to the battery.

Lots of small scale ones available. Some bigger ones:

https://www.victronenergy.com/solar-charge-controllers/smartsolar-mppt-rs-450-tr

You can put multiple inverters on a single phase.

then in the summer, disconnect one.

Thinking a bit more about disconnecting at peak generation, how do micro inverters deal with this?

Do they all disconnect virtually together as soon as they sense over voltage, or is there enough variation in them that the 'quicker' ones disconnect and the rest carry on?

Looking at your numbers, I am generally thinking you are trying to hard.

Why do you need a 20kWh battery, why do you need 48kWp of panels?

Apart from your graph, which is pretty, have you actually done any cost benefits analysis?

Be ready for when being charged for summer export, becomes more a thing. Even a couple of weeks ago exports rates were between -1p and +1p for me for a few days. If summers are getting hotter and less rain fall, that generally means less cloud and more production - and no where to dump except to a grid that doesn't want it.

I would cut your PV by at least half maybe 3/4. If your near passive cut your battery to 15kWh.

There is a risk that maximum power requirement and mean energy usage are getting confused.

Here is my maximum power usage for this year.

The peak is at 8.5 kW. So if I was to run off an off grid system, I would only need an inverter that is capable of delivering 9 kW.

Now while the Mean Power chart looks similar because I am on E7, the mean power does not go over 4.1 kW.

So if I only had an inverter that was capable of delivering say 5 kW max, then it would cut out (if off grid) or import from grid for a peak times.

9 hours ago, Alan Ambrose said:

I think I'm just chafing against the DNO over-control of kit. Export limits I get, constraints on inverter sizes I don't.

Why do you think they constrain inverter sizes? AFAIK they're only interested in what you push to the grid so nothing wrong with a 12kw G100 certified inverter set to whatever export limit the DNO will allow.

If you're needing to speak to your DNO then I assume you're desire to be off grid has gone and you'll be having a grid tied system. If that's the case and you've got batteries, then grab a cheap overnight tariff and use that to charge the car and batteries when there's limited PV.

If the goal is off-grid, and you're serious about installing a descent level of storage to support this, the simplest thing is to forego grid-tie and plan on most of your house supply coming from an off-grid (DC coupled to grid) inverter sized appropriately for your peak demand. This is 100% outside the control of the DNO. If you need 20 kW inverter then great, you can do it! Just don't grid-tie it.

Then separately if you want to engage in the VPP/grid arbitration market get another inverter that is grid-tied, at the maximum size your G99 will allow, and export through that at times when it pays to do so.

If winter output is a constraint, consider panels orientated vertically - they'll give better output in Winter, slightly.

@joth

Your point is very well made - you can do whatever you want with DC as it’s out of the control of the DNO.

But, as you imply, that probably means not a Sig-style set-up as their batteries are fairly tightly coupled, but a Victron-style more open system.

Sorry - I’ve pretty much just repeated what you said - it’s an important thought though.

@Ed_ we’re pretty much baked re panels with the LPA approving a design with 40 on one dual pitched roof and 32 on the dual pitched ‘cart lodge’ roof. Also, the aesthetics will work out well that way.