Inverter single or 3 phase?

[joth]

1 hour ago, Nickfromwales said:

It’s obvious from the first few posts that the OP is being sent in that direction. Any mention of a 3-ph inverter would obviously require a 3-ph CU. 

 

Huh, I see.  I actually read the OP exactly the opposite way:

On 14/12/2021 at 19:21, ashthekid said:

I've just noticed in my solar summary report that it's planned to have a SolarEdge HD Wave single phase inverter.

 

 

1 hour ago, Nickfromwales said:

2 hours ago, joth said:

you also seem to be under the impression that net metering across phases does not work, and the house needs to equally divide its usage across all phases and supply generation equally across all phases to maximize self consumption? Why do you think this? 

Err, if a single phase inverter feeds into one of there phases, how would the PV generation get to ph-2 and ph-3 without that methodology?

 

It wouldn't. Why would it need to?

With net-metering you could put a constant 3kW into phase A, draw a constant 1kW from each of phases B and C, and still generate revenue on the 1kW of export (and charged no import).  Trying to balance generation vs consumption across the phases is pointless*, adds cost, and unnecessarily complicates pretty much everything.

 

* - for domestic installs. Larger commercial users are a different matter.

 

 

Edited December 16, 2021 by joth

[Nickfromwales]

16 hours ago, ashthekid said:

Wasn't planning MCS as I am planning to keep all generated electricity

@joth

Edited December 16, 2021 by Nickfromwales

[Nickfromwales]

7 minutes ago, joth said:

With net-metering you could put a constant 3kW into phase A, draw a constant 1kW from each of phases B and C, and still generate revenue on the 1kW of export (and charged no import).  Trying to balance generation vs consumption across the phases is pointless*, adds cost, and unnecessarily complicates pretty much everything.

Not without the MCS, as I said, read my way = no payment for export so net metering is out of the OP’s equation…..unless I’m missing something? 

Edited December 16, 2021 by Nickfromwales

[joth]

9 minutes ago, Nickfromwales said:

Not without the MCS, as I said, read my way = no payment for export so net metering is out of the OP’s equation…..unless I’m missing something? 

 

So in that case the  payment for export is irrelevant, but the import bill is still £0.

 

let me rework the example to make it easy:

.. With net-metering you could generate say, a constant 2kW all delivered into phase A (while drawing no load on that phase),  and extract a constant 1kW from each of phases B and C, and end up with a usage bill of £0.00.

2 - 1 - 1 = 0kW net usage.

 

There's no financial benefit from splitting generation across phases.

 

 

Edited December 16, 2021 by joth

[AliG]

Not saying they made the best decision.

 

But our installers split the panels across three single phase inverters each connected to one of the phases.

 

Don't know if there is any benefit versus a three-phase inverter. I assumed it was cheaper for them.

 

I think if you do this you can install 3.68kW on each phase without the G99 issue.

[joth]

1 hour ago, AliG said:

I think if you do this you can install 3.68kW on each phase without the G99 issue.

You are of course right. It looks like all the solaredge 3phase inverters have a G98 cert, and anything up to SE10K (10kW model) will provide <16A per phase so does not require additional permission https://midsummerwholesale.co.uk/pdfs/solaredge-g98-se3k--se17k-2.pdf

 

Still, a 3-ph SE4K is over £1000 vs under £800 for the HD-Wave single phase inverter.

 

[ashthekid]

Ok so to help with answering some questions on the layout of the property of my property, we have an ashp, an annex which has its' own kitchen/bathroom/lounge/bedroom and then a 4 bedroom property with 4 bathrooms, kitchen, utility, pantry and study etc. The whole property is on electric as no gas in the village so that means electric cookers too. The plan is to have an EV charging point although it will be unused for now. My thinking was to have the ashp/EV point on one phase, the main house on another phase and 3rd phase for the annex which seemed logical.

 

My previous contractor who has since disappeared had his recommended energy guy install 12 panels(4.320kW) but we sadly had to remove one due to velux being too close so down to 11 now (3.960 kW) with SolarEdge optimisers because I wanted to make sure it ran as efficiently as possible. After installing the panels the contractor and energy guy have since disappeared leaving me to finish off the project. In his solar summary it was planned to have a single phase SolareEdge HD Wave inverter, which is why I then thought it odd to not have a 3 phase inverter to match the input to the property.

 

I also have full planning permission for the panels which may or may not help with determining what inverter to use.

What would be the simplest thing to do here without having to touch the array as they are currently set up as?

And does it matter that I plan to use myEddi diverter for any unused generation directly into my DHW cylinder?

I have no intention of feeding anything back into the grid so does that mean I don't have to change my meter which is currently a standard 3 phase analogue meter?

 

 

[ashthekid]

Update:

Attached is the 3 phase meter setup which I believe a smart meter. 
 

My plan is to stick with the 1 phase SolarEdge inverter and assign it to the phase that will have the ASHP on as that’s likely to be the biggest draw. Would I be correct in assuming that? 
I suppose aI could put the main elelctric cooker and few appliances onto that phase as well. Then just have all lighting and minimal loads onto another phase. 
 

Would that seem logical?

[Dillsue]

3 hours ago, ashthekid said:

 

 

My plan is to stick with the 1 phase SolarEdge inverter and assign it to the phase that will have the ASHP on as that’s likely to be the biggest draw. Would I be correct in assuming that? 
 

Your ASHP is likely the biggest load but used at a time when youre generating the least. PV is not likely to make much of a space heating contribution in the winter, unless you have a big array.

 

If you want the best self consumption put everything on the same phase and stagger your demand over peak generation hours with anything left going through your diverter.

[joth]

Yeah it looks like a v2 smart meter with the Toshiba comms hub (per Ovo). You had said you were on an analogue meter, this changes things!

SMETSv2 polyphase meters use net-metering, so you can split loads and PV across the phases however you want - you'll be billed based on the net usage across all phases. Having one phase per "building" makes sense, and then drop the PV into whichever phase is on the building where the inverter sits.

Ideally the inverter should be as close to the meter head as you can, to reduce the voltage differential across the line to the inverter, which can cause it to cut out if you're in a location that regularly runs over 250V. The scribbling at the bottom of the meter board does indeed make it look like you are. (Volts = 250, 245, 250). 

 

 

 

 

 

 

[ashthekid]

@joth yes meter got changed when they moved it recently. I didn’t know they were going to change it to a smart meter. 

[SteamyTea]

On 13/01/2022 at 11:54, ashthekid said:

My plan is to stick with the 1 phase SolarEdge inverter and assign it to the phase that will have the ASHP on as that’s likely to be the biggest draw. Would I be correct in assuming that? 

Can you set your ASHP up to do the DHW between 11AM and 2 PM? (or whenever your PV is most likely to generate the most, on average).

And have a diverter to directly resistance heat the DHW in the summer.

On 13/01/2022 at 11:54, ashthekid said:

I suppose aI could put the main elelctric cooker and few appliances onto that phase as well.

Only if the diversity allows it.

Edited January 14, 2022 by SteamyTea

[markharro]

This topic is of interest to me. We have single phase atm but we plan to demolish and rebuild so presumably the cost to re-connect with 3 phase wouldn't be much more than single?

 

We have roof space for a 11kw plus array. I dont want the hassle/cost of getting permission to instal over 3.68 and had planned therefore to get an export limiting inverter and stick with single phase. But I then wondered if I could easily unlock the whole 11.04 by upgrading to 3 phase. The incentive would be to allow fast daytime charging for our electric car and not "waste" our export potential although do we get paid for this without a MCS instal?

 

Anyway, having read this thread I suspect that there are other issues/costs I am blind to eg it seems if you use a 3 phase inverter you need permission? As @AliG said (hello btw) you can use 3 x single phase inverters and avoid this issue but I guess the cost of 3 of these inverters will be a lot more than one single phase inverter? Are there any other cost issues or complications with a 3 phase domestic instal that I am ignorant about?

 

I dont have a lot of technical knowledge in this area so it would be great to get a simple overview. thanks

[AliG]

Hi Mark,

 

You need to have an MCS instal to get paid for exported electricity.

 

You also need a letter from the DNO confirming they have been informed of the install.

 

There is not a big difference between the price of a three phase 12kw inverter and 3x4kw single phase inverters, depending on the make. You will save on instal time I guess. BTW two of our three inverters blew in the massive lightning storm we had two years ago. Luckily I got them replaced under warranty. One of the new ones then had to be replaced again. In our last house we had to replace the generation meter. this all eats into the returns.

 

The three phase inverter splits the output across all three phases so you can have 11kw either way.

 

Export payments are woeful, except on Agile Octopus. You are only getting around 4.5p per kWh at the moment. Octopus will pay 7.5p but not if you have Go or Intelligent. So the return on export payments is way less than the return on self use of electricity.

 

How often do you need the car charged quickly? you can get car chargers that can be set to use excess solar to charge the car. At 4kw you are going to be charging at about 11miles an hour. I would guess the sweet spot is in the 6-7kw range so that you can charge the car and cover background use of electricity. You could also set up your to heat your water during the day. Above this size the extra output will be exported. If you assume £1000 per kw installed then for 7kw versus 11kw you would get around an extra 3000kWh exported a year. So that is £135 of export on £4000 of investment. The other thing is that you can charge your car for 7.5p/kWh during the night from Octopus and heat your water then, so even then much of the electricity you use will only be offsetting 7.5p usage, not 30p.

 

As to other three phase costs, the consumer unit is a bit more expensive. Probably a few hundred pounds in extra costs.

 

We have 5kw of capacity and my average day looks something like this at the moment. Almost no usage during the day - depends on if the oven or washing machine is on. Way over half our usage at night at 7.5p

 

 

[joth]

4 hours ago, AliG said:

Octopus will pay 7.5p but not if you have Go or Intelligent

 

Could you explain? I'm on Octopus Go for import and Octopus SEG for export, and I _appear_ to be being paid.

That said, the payments are not automatic, it seems I have to raise a support ticket every time I want them to credit me, which they dutifully do and claim it's now fixed so I'll receive future payments automatically but I never do so I raise another ticket.

Is this what you mean by they won't pay if you're on Go?

 

Now I think about it, the automatic payments may have stopped around the time I moved from Agile to Go, but they never said this is the reason.

 

[joth]

5 hours ago, markharro said:

so presumably the cost to re-connect with 3 phase wouldn't be much more than single?

It maybe eye-wateringly more (10s of thousands) or maybe just £1k more. It depends how far you are from the nearest 3 phase supply.

Ours is under the main road right outside. They could move the single phase feed to a new location for £1500, all work done on our plot, but it was £3000 more to upgrade as they'd have to close the main road and dig a big hole in it.

Only way to know is put in an application and ask. With our DNO (UKPN) you can only request one quote at a time, but when the surveyor visited he was happy to sketch up and verbally give me price indications for all the options I was interested in, and then just take one away for the formal quote on.

[markharro]

Thanks Alastair, I must have been forgetting the MCS requirement to be paid for export. Ok so I need to factor in getting £0 for any export as I am not paying a premium for a MCS instal. I would rather wait to see if policy changes and becomes more enlightened over time. So back to the 1/3 phase thing. There are extra costs with 3 phase which I can't determine right now. If I ignore exporting then I think I am right that I can have a 20kw array for instance on single phase without any DNO permissions as long as I have an export limitation on the inverter? If this is correct then am I also right in thinking that if I was generating at full capacity on a sunny day AND I had sufficient load connected to self use the entire generation then I could theoretically use all the generation? It is only if I cant which I guess would happen frequently that the system would throttle to avoid export? If I am right so far then is the only real benefit of 3 phase for me the faster charging speed for an EV? And given that we don't drive every day maybe it would still top up enough over a few days to do a lot of its charging from the PV? I hear what you say about Octopus tariffs but I would rather not rely on these as they could easily change over time....I am much more focused on how I can minimise my draw from the grid whether this is during the day or overnight.

[Dillsue]

Staying within the G98 rules youre limited to 16A/3,68kw export per phase so 1 phase export limit is 16A and 3 phase export limit is 3 x 16A. For a 3 phase system you can use a 10/11ish kw inverter with a G98 cert OR 3 x 3.68kw inverters each with a G98 cert.

 

If you want to go more than 16A on any phase then its G99 application. 

 

If you want to have 20kw of panels on a single phase Im not sure youll find an inverter that can cope with 20kw input and have a 16A G98 limited output 

[AliG]

7 hours ago, markharro said:

If this is correct then am I also right in thinking that if I was generating at full capacity on a sunny day AND I had sufficient load connected to self use the entire generation then I could theoretically use all the generation? It is only if I cant which I guess would happen frequently that the system would throttle to avoid export?

From a bit of Googling, it does appear that you can buy an extra box that limits export from the inverter. So you could install more PV and limit export to 3.68kw when not using all of the power. I couldn't actually find one to buy, but I only had a quick look.

 

I do wonder if batteries are the better solution for what you propose. The problem with PV is that it produces power over a very limited time period. As you can see from my Octopus chart the moment we produce lots of power between 9am and 4 pm. This will get better as we move into summer. If I go back to February (before I had Intelligent Octopus) the charts look like this.

 

 

For 4 months you get very little output and also only for a very short time during the day. You may also want to consider putting some panels up facing a different direction in a large system to expand the amount of time that you are generating. I have considered adding a west facing array to generate in the late afternoon or east facing for the morning. But it is a hassle now the roof is built, the other panels are integrated into the roof.

 

From using the power perspective, one phase is better than three phases, as the three phase output has to be split and so you have to try and balance your usage across the three phases. This is not easy. The car can use all three phases to charge, but for example, you would maybe want to try and split your appliances across phases which is awkward. I need to actually try and figure out if the oven etc are on the same phase as the washing machine/dishwasher/tumble dryer in our house (Edit: I had a look and things are reasonably well spread)

 

So if you can limit export, what is the most efficient amount of PV to install. Admittedly in a non MCS install, the cost per kw may be quite a bit less, maybe £600-700.

 

On one phase you can charge your car at 7kw. This allows you to charge up 20mph give or take. I have a three phase charger, but the cable has recently become iffy and I have been using a three pin 2.4kw charger for a couple of months. This has so far always sufficed. It depends on your origin habits, but the number of times we drive more than 50 miles in a single day is quite rare. And if you drive a long way, you are probably charging on the road. So the reality is that you could charge your car in two-three hours max on 7kw. Nothing else int the house comes close to using 7kw of power, maybe a power shower, but then if you shower when getting up or going to bed there won't be PV generation most of the time. The next largest instantaneous power draw would be the oven or a kettle both of which draw 3kw.

 

Thus other than car charging you will rarely have instantaneous power draw of more than 5-6kw, even the oven which draws 3kw to heat up, only actually uses around 1kWh an hour so it cycles on and off once up to temperature.

 

You can install a smart car charger that would draw 7kw when available and then reduce this when using power elsewhere. Thus I suspect the amount of use that capacity over 7-8kw would get is fleeting. Meanwhile unless the oven etc are on you would have the full PV capacity available to charge the car much of the time.

 

The only way more than 7-8kw capacity would be useful is if you install batteries so that the excess power generated during the day can be used to run lights, showers etc when it is dark. You might get a little longer coverage during the day and into winter with more  generating capacity, but I suspect the benefits are small.

 

Assuming that you use around 3-400 litres of hot water a day, this will need around 15kWh to heat it. If you have free PV electricity you can heat it just using an immersion, but in winter this will mean paying out to heat it. The alternative is heating it using an ASHP, but the COP onto water heating is likely less than 3. Considering that you don't need an ASHP for heating, the immersion probably wins out.

 

Assuming an average of 6 hours a day of generation, you may want to add 2-3kw of capacity to heat hot water when you have PV. This gets you up to around 10kw as a good number. Again this can be set up to use excess generation when available.

 

I would still want to be on an Octopus type deal as this will cover energy use during the night and can be used to heat hot water more cheaply at night in the winter when there is no PV. Nighttime electricity should be cheaper for the foreseeable future due to the drop in demand overnight.

 

 

 

 

 

[SteamyTea]

I am seeing a lot of kw, should be kW.

[joth]

52 minutes ago, AliG said:

From using the power perspective, one phase is better than three phases, as the three phase output has to be split and so you have to try and balance your usage across the three phases. This is not easy.

Not easy, but also not necessary.

 

Install a smart meter and it records net-usage across all three phases. So you could put generation on one phase, all your house hold usage on a different phase, and it would still only charge you for the difference between import and export at any given moment.

 

(It's still good to try and balance phases as best you reasonably can, as it will reduce local voltage drift, which can avoid overvoltage cut out in the inverter, but this is a secondary consideration)

 

[Dillsue]

If you want to go down the route of an oversize system with export limitation then youll need to get permission from your DNO with a G99 application. If you propose an export limitation device or the DNO asks for one, then youll also need a G100 application. Both G99 and G100 aplications each have their own costs. On top of design verification fees our DNO wants £375+vat to witness an export limitation system.

 

If you want more than the 3.68kw G98 limit then speak to your DNO as they may well be able to accomodate you generating more. Ours was very approachable and we seem to have informal agreement to have 2 x 3.68kw export. Hopefully get that formalised when they process our G99 application.

Edited April 6, 2022 by Dillsue

[AliG]

25 minutes ago, joth said:

Not easy, but also not necessary.

 

Install a smart meter and it records net-usage across all three phases. So you could put generation on one phase, all your house hold usage on a different phase, and it would still only charge you for the difference between import and export at any given moment.

 

(It's still good to try and balance phases as best you reasonably can, as it will reduce local voltage drift, which can avoid overvoltage cut out in the inverter, but this is a secondary consideration)

 

Good point. Totally forgot about net metering.

 

Actually it must be working as we very rarely use electricity during the day despite having less than 2kW per phase.

 

I don’t know how helpful or not DNOs are re allowing more than 3.68kW. Few people seem to ask. Costs nothing to ask though.

 

As to the costs of three phase it will depend on how close the cable is. We have a three phase cable in the street and had to connect to it anyway so the extra cost was small. If it requires a new supply cable to be connected, however, you are looking at £200+ a metre in the road. Scottish Power we’re very good at quoting though. It only took days from me making an application. May as well request a quote and see the costs of single versus three phase.

 

Without an MCS install though you won’t be paid for exported electricity. The only reason to need to be approved to export more is if the cost of that is less than the cost of the export limiter and any approvals. I also wonder if the DNO may treat non MCD differently.

[joth]

11 minutes ago, AliG said:

Good point. Totally forgot about net metering.

 

It seems easily forgotten!

 

I've explained it 3 times on this on page  alone, and umpteen times elsewhere on this forum. But this myth keeps popping back up

[SteamyTea]

2 minutes ago, joth said:

 

It seems easily forgotten!

 

I've explained it 3 times on this on page  alone, and umpteen times elsewhere on this forum. But this myth keeps popping back up

Makes it worth getting a 3P meter as it offsets then imports at 35p/kWh.