Solar PV plan for 3 phase supplied house - Is there an idiot's guide?

[SteamyTea]

2 minutes ago, Radian said:

I don't think any commercial offerings are as dumb as that.

Long time ago when I did some work with small turbine manufacturer we had to go to site as there was a troublesone turbine.

Transpired that the SMA WindyBoy had been configured as a SunnyBoy.

There was a few seconds delay before the inverter reacted and then shut down.  Only happened when the power was ramping up.

Apparently the inverter reaction time had to be a lot faster for small turbines than for a similar output PV system.

[Radian]

1 hour ago, PhilT said:

A solar immersion diverter is hard to justify if you have a heat pump which uses one third of the electricity and you get 15p for export - it could COST you (15-33/3)=4p for every kWh solar you divert to the immersion heater

You could say the same for gas-fired hot water heating. Except for the shortfall in efficiency in converting 1kWh of gas into hot water. Whether by luck or judgment, the utilities seem to have accounted for this on average. Similarly your argument depends on the COP you're getting at the time excess PV is on hand. Some cold, bright, clear days in winter might favour divert to immersion over ASHP if it's struggling to get up to the required temperature.

 

[joth]

4 minutes ago, Radian said:

Similarly your argument depends on the COP you're getting at the time excess PV is on hand.

Actually not the case: Phil is saying you can run the ASHP whenever you want, perhaps timed to maximize COP, and if the COP is better than the ratio of import to export price, you're better off exporting (all) excess PV and (potentially) paying to import all energy required to run the ASHP.

With current prices at 33p vs 15p you only need to achieve COP of 2.2 for this to hold. If you can time the ASHP run to be around max PV generation, so around midday which happens to be likely time of best COP, the case gets even better. 

 

For my part, I have turned off my PV divert for more or less this reason. (Additional motivation is I now have a battery, and I'm on octopus Go so some import is at 7.5p)

 

 

[PhilT]

20 minutes ago, Radian said:

You could say the same for gas-fired hot water heating

Good point and I must confess I haven't yet completely ruled out the idea of a solar diverter. One other element in the mix is the viability of a battery, mentioned earlier, as an alternative to a diverter in being able to mop up excess solar and use it to power the heat pump to heat the water later on. You can't do that with gas. But I still haven't got my mind around all of these kind of logical issues yet which is one of the reasons why this forum is so useful.

Edited January 29, 2023 by PhilT

[Dan F]

2 hours ago, PhilT said:

A solar immersion diverter is hard to justify if you have a heat pump which uses one third of the electricity and you get 15p for export - it could COST you (15-33/3)=4p for every kWh solar you divert to the immersion heater

 

A solar diverter makes no sense if you have a ASHP and a battery (to act as a buffer), but if you have ASHP without a battery solar diversion still makes sense because:

i) A heat pump will have a minium power, where as solar diverter and divert small amounts of excess into hot water.

ii) The sun comes and goes but you can't instantly turn heat-pump on and off to react to the available power, where a solar diverter can deal with this.

iii) Even if you try to turn heat pump on/off to react to available power this might not be ideal for the ASHP.

 

There may be some days where PV production is both enough, and constant enough, to dire up heat-pump at midday to heat hot water but, particiauly in the UK climtate and considering lower levels of PV output in spring/autmumn this will be rare.

 

I don't currently have solar diversion or a battery, but as soon as I've installed a battery I will be using Loxone to decide when to fire up the ASHP based on i) SoC of battery ii) tarriff iii) PV generation and expected sunshine hours the next day.

 

 

 

Edited January 29, 2023 by Dan F

[Radian]

47 minutes ago, joth said:

With current prices at 33p vs 15p you only need to achieve COP of 2.2 for this to hold. If you can time the ASHP run to be around max PV generation, so around midday which happens to be likely time of best COP, the case gets even better. 

 

I hope that's the case! I don't have enough of a feel for the likely range of ASHP water heating COP, I do know it's quite a big 'ask' to get water up to 55^oC when it's some 60^oC lower than that outdoors. Fortunately these products are getting better all the time. We might all have to use them one day!

[PhilT]

2 hours ago, Dan F said:

if you have ASHP without a battery solar diversion still makes sense

With apologies to the OP for diverting (pardon the pun), it depends on the tariff/SEG. So, for an example without a battery, say for my hypothetical day of 12kWh total consumption I get 12kWh of solar, a diverter does all the water heating using 6kWh, with the remaining 6kWh used elsewhere, net running cost impact for the day = 0. Alternatively my heat pump would use only 2kWh to heat the same water requirement, leaving 4kW exported, net running cost impact @ 15p = 60p (credit). Not forgetting the up front cost of the diverter, would any combination of circumstances and tariff/SEG calculation make one financially justified?

Edited January 29, 2023 by PhilT

[bob the builder 2]

HI All

I am just at the point of ordering a solar + battery system.

 

If possible i would like a 10kw system with battery.

 

Unfortunately i have about 12 weeks max before the scaffold comes down and am concerned i will not get through the DNO process approval.

 

Could anyone advise

 

1) Can i install / commission without DNO approval

 

2) Any good suppliers for black Mono on standing seam roof ?

 

Thanks

 

Bob 

[Roger440]

On 28/01/2023 at 12:19, Omnibuswoman said:

 

Just watched the video - a perfect idiot's guide to how this works. So in effect, it matters not how we set up the house demand across the three phases as long as we have a vector sum meter totting up total in and total out. 

 

It does indeed explain a lot.

 

However, one thing i dont grasp is, if you are consuming 4kW on one phase, and exporting same from batteries, whats making thar decision to export and balance the incoming v outgoing?

 

Is the the thing on his diagram marked "gateway"? If so, what is that?

[Dan F]

3 hours ago, PhilT said:

With apologies to the OP for diverting (pardon the pun), it depends on the tariff/SEG. So, for an example without a battery, say for my hypothetical day of 12kWh total consumption I get 12kWh of solar, a diverter does all the water heating using 6kWh, with the remaining 6kWh used elsewhere, net running cost impact for the day = 0. Alternatively my heat pump would use only 2kWh to heat the same water requirement, leaving 4kW exported, net running cost impact @ 15p = 60p (credit). Not forgetting the up front cost of the diverter, would any combination of circumstances and tariff/SEG calculation make one financially justified?

 

Maybe, but this is a simplifcation that won't apply every day due to the weather and clouds!  Practically, using ASHP when the sun is out means:

- ASHP controls to i) turn it on when certain PV output is reached ii) ensure ASHP stays on for a minumum run time.

- Some high-tarriff import when the sun goes behind a cloud.

 

Quote

would any combination of circumstances and tariff/SEG calculation make one financially justified?

 

Would have to run some models. But if the consumption of your AHSP is 2.5kW and clouds mean your PV is fluctatuing between 1kW and 4kW then you will be importing more on an expensive day-time tarriff, with this approach.  (Turning ASHP and and off based on PV output isn't a good idea.)

Edited January 29, 2023 by Dan F

[Dan F]

24 minutes ago, Roger440 said:

However, one thing i dont grasp is, if you are consuming 4kW on one phase, and exporting same from batteries, whats making thar decision to export and balance the incoming v outgoing?

 

Is the the thing on his diagram marked "gateway"? If so, what is that?

 

Yes, in the case of the Tesla product it is the "gateway" that does this using a meter on the supply.  Other products have different equipment to do this, but they all need to use a 3-phase import/export meter.

 

With the Victron system, you use three seperate charger/inverters (one of each phase), and then it has a seperate contro lsystem called the "Cerbo GX" which connect to all inverters and the 3-phase meter and does this.

 

[Roger440]

1 minute ago, Dan F said:

 

Yes, in the case of the Tesla product it is the "gateway" that does this using a meter on the supply.  Other products have different equipment to do this, but they all need to use a 3-phase import/export meter.

 

With the Victron system, you use three seperate charger/inverters (one of each phase), and then it has a seperate contro lsystem called the "Cerbo GX" which connect to all inverters and the 3-phase meter and does this.

 

 

Thanks

 

Thats clever.

[SteamyTea]

On 26/01/2023 at 21:30, Radian said:

Solar PV + battery installed together attract no VAT. Otherwise they will be subject to the standard 20% VAT rate

There is a good chance that battery systems will be 20% cheaper in a few years time.  There won't be the huge price drops that PV modules saw, but there will be one.

Also give some time to see what is happening across each phase over a decent time period.

And then here is how tariffs will vary.  At the moment, basing costings on Octopus's confusing array of tariffs is a big gamble.

 

If imported energy prices drop down to what they were 3 years ago, plus 25%, gas will be around 5p/kWh, normal rate electricity about 18p, and for E7 users, somewhere around 28p during the day and 14p at night.

I doubt that these silly number of 15p/kWh to export PV and 5.5p/kWh (or water the best deals are) to import during a short window at night will be about in 2 or 3 years.

Edited January 30, 2023 by SteamyTea

[Carrerahill]

On 22/01/2023 at 10:54, Omnibuswoman said:

I have reached the point of having obtained a quote for our PV system, attached, from a local company via my builder. Happy to hear any feedback on it - I thought it seemed competitive for an 8KW system with REC 365w twinpeak black panels. We won't be going with the batteries at this point for reasons of limited budget, but plan to add those in at a later stage. The total cost without batteries is £10,500.

 

Before I seek further quotes,  I really need to better understand how solar systems work for houses, and in particular with a 3-phase supply. I'm not certain that the quote provided is suitable for a 3-phase supply.

 

My knowledge of electrical and electronics is really poor, and I need to have a look at an idiot's guide. Can anybody point me to the most basic of basic explanations of what each bit does, and where it goes in the system?

 

Thanks!

 

I would question the use of those panels. 22 x 365W panels - that is a pretty low energy density these days. When I first looking at PV 350W or thereabouts was about the norm, then when I came to buy it was about 400W now I am looking at some 500W panels.

 

If it was me I would want fewer panels, a slightly higher buying price but reduced installation time, less brackets & rails, and less panels on my roof. 

[SteamyTea]

33 minutes ago, Carrerahill said:

I would question the use of those panels

200W.m^-2

May need the same amount of rail.

Really all depends on how well they fit into the roof.

At last, a reason to use Factorising that we all learn at school.

Edited January 30, 2023 by SteamyTea

[Carrerahill]

52 minutes ago, SteamyTea said:

200W.m^-2

May need the same amount of rail.

Really all depends on how well they fit into the roof.

At last, a reason to use Factorising that we all learn at school.

They have got wider but the difference for me was 30mm, but about 290mm taller, so same rail requirement.

 

I agree it all depends, but I would really question these panels until I was satisfied they were the best choice. 

[Roger440]

Dont think i will be doing 3 phase. Had to have a lie down after costings from National Grid.

 

Shame as seemed to be some good advantages.

[joth]

34 minutes ago, Roger440 said:

costings from National Grid.

Yeah you should ask your DNO for a quote instead. If National Grid are doing it, it'll be coming in hot at 400,00 V and get categorized as a regional infrastructure project. 😉

But yeah shame its not practical, as the trend is towards encouraging more 3ph installs generally.

 

Edited January 30, 2023 by joth

[SteamyTea]

2 hours ago, joth said:

National Grid

Western Power are now know as National Grid.  I don't know why.

[Roger440]

As above, western power is now national grid.

 

The electricity distribution (11kv?) runs across my field. My house and next door have a transformer on a pole behind the barn. Sadly, its a single phase transformer. If i want 3 phase, i need to pay for a new transformer. Just for me.

[Omnibuswoman]

My experience with Western Power was that they initially quoted me almost £15k for something that, when challenged, may not have been justified. I spent a lot of time gathering information about local loads on the line as I was convinced they were wanting me to fund a local infrastructure upgrade that was already badly needed. 
 

After asking them for detailed figures on this, as permitted under the relevant regulations, they dropped the cost to £950. It’s worth taking a close look at what they are quoting for and it’s necessity for your project. 

[Omnibuswoman]

21 hours ago, Carrerahill said:

I would question the use of those panels. 22 x 365W panels - that is a pretty low energy density these days. When I first looking at PV 350W or thereabouts was about the norm, then when I came to buy it was about 400W now I am looking at some 500W panels.

 

If it was me I would want fewer panels, a slightly higher buying price but reduced installation time, less brackets & rails, and less panels on my roof. 

As the roof is under construction, the panels will replace slates, so I’m not too worried about area as there is a cost offset there. 

 

My main concern is that the project is priced in the right sort of ballpark, as I know that PV costs have fallen a lot lately and finding information on the web is more challenging than I expected. 

[SteamyTea]

9 minutes ago, Omnibuswoman said:

My main concern is that the project is priced in the right sort of ballpark, as I know that PV costs have fallen a lot lately

Back in November 2019, installed PV in the UK was, on average, £1465/kW_p on a typical 4 kW_p system.

March 2022 it had risen to £2000/kW_p.

https://www.statista.com/statistics/499507/average-solar-pv-cost-per-kw-installed-uk/

 

9 minutes ago, Omnibuswoman said:

finding information on the web is more challenging than I expected

The only way is to get quotes from installers.  Realistically the only information you can get from web searching is component parts.

Edited January 31, 2023 by SteamyTea

[Omnibuswoman]

@SteamyTea in that case the cost looks quite reasonable. 8kWp for £10,500. Still, I will get some quotes for comparison. 

[SteamyTea]

7 minutes ago, Omnibuswoman said:

@SteamyTea in that case the cost looks quite reasonable. 8kWp for £10,500. Still, I will get some quotes for comparison. 

Yes, I don't think it is excessive.

I assume you have scaffolding up, if so, that is quite a saving.