We have solar, battery and ASHP. Like you, our peak winter usage can approach 50kWh per day. We are able to cover this with a single Tesla PW3 battery at 13.5kWh (installed outdoors in the rain) by using the Octopus Cosy tariff which has 3 cheap rate charging slots.

The Cosy cheap rate is more than your current EV tariff, but we need a fraction of the battery capacity as we are able to recharge the battery 3 times per day so each recharge only needs to last 6h until the next available cheap rate slot. It also works well ion summer as a single 13.5kWh PW3 nicely covers a full day's usage (~12kWh) in summer, even if there is absolutely zero solar. We do not have an EV in that mix.

So for us the question was/is simple - would the cost of additional batteries be offset by moving to a cheaper overnight tariff (we could pretend we have an EV on regular Octopus Go). To achieve that, we'd need two or three PW3's so for us the smaller (cheaper) battery capacity works out better and gets us to the point of having zero bills (export in summer offsets import in winter)

There is also the risk that you may not see a return on capital if cheap overnight EV tariffs end (or prices normalise) as you now have large battery capacity that you can no longer cheaply fill.

Thank you. From what I see, it covers all building trades. VAT registered contractors or subcontractors can supply and install items in the list free of VAT. 

It's not as far as I am aware ever mentioned MCS and indeed many of the things it covers are simply not even MCS covered in the first place (eg draught strips).

For a vat registered operation the paperwork should be no more than usual for VAT. For a not vat rated operation I don't know but I doubt it's fun if it's even possible.

That is interesting. My understanding was that, to be VAT-free, it would need to be for renewables, so the installer would need to be registered with MCS. And when I reached out to an installer for my Fogstar+Solis system, I was given an estimate of £3200 (the installer was listed in the Fogstar website). They alleged there is a lot of paperwork involved. The VAT was significantly less than that, so I gladly paid HMRC.   

Or is any registered electrician now able to claim it? 

Is it not now referenced by BS7671 and the latest book (I'm too cheap to buy a new copy to find out). If it is as I understand then it's now got the force of building regs.

I hope that's not the case.

PAS63100 has a number of deficiencies.

1: It is concerned with protection against fire from storage batteries, and yet lumps all cells with lithium chemistries into the same category.
As such it over-emphases issues that should rightly be applied only to Li-MNC cells.

2: It makes assumptions about the battery being the potential source of fire, but omits the possibility of the inverter (or a charger) being a source of fire risk.

3: It assumes professional installation is a prerequisite, despite a qualified electrician not being legally required for voltages below 75v (ELV).

4: It doesn't apply to dwellings with a floor area in excess of 200m² (mine is over 300m²)

5: It states that DC fuses must be interlocked to an isolator, such that the fuse cannot be removed without the isolator being in the OFF position. That ignores all those installations where the fuses and isolators are external to the storage battery enclosure(s).

6: It introduces the term Power Conversion Equipment (PCE) which has no universality or legal definition. PAS63100 uses PCE to include both inverters and DC-DC converters.

7: It stipulates three instances where visual and/or audible warnings must be implemented. But battery condition monitoring is often a feature on the inverter, not integral to the battery unit.

8: It refers to Arc-flash protection to prevent burns. But then states that the battery manufacturer should be responsible for the methodology. That overlooks requirements for on-site maintenance, monitoring and fault diagnosis.

9: It asserts that the stored energy of all batteries should not be greater than 40kWh, or 80kWh if they are outside the dwelling, such a garage or outbuilding.

It's a fair attempt at creating a standard, but its wording is clearly created by those who haven't got sufficiently wide hands-on experience of how storage batteries are sold, installed and maintained. The sort of batteries it is trying to address are going to be a part of a larger installation which includes items not mentioned in the document.

There is no indication that the IET has the authority to produce such a document. Putting the DESNZ logo on it and having it published by BSI doesn't automatically endue it with that authority.

I also double checked and it does not seem this Solax inverter can use low volt 48-52V DC batteries. If it did, it could be an option to sell the Solax HV batteries (>100V). 

So yes, probably locked into Solax if wanting to avoid G99. My suggestion would be to pursue G99 with a second inverter able to use low voltage batteries..

"Have you come across PAS 63100, available free from British Standards?
This document provides Guidance on domestic battery installations, and is not mandatory at present.

Is it not now referenced by BS7671 and the latest book (I'm too cheap to buy a new copy to find out). If it is as I understand then it's now got the force of building regs.

Only thing I really wanted to add though - if you get a sparky to do the install get them to buy the kit too. Firstly they get VAT off, secondly they can sometimes (and understandably) be really funny about components they don't know definitively are to spec and from a proper trusted source.

Posted by: @weoleyric

↑

• I have export capability with G98, and am open to G99 if needs be.

That could be worth, as a new inverter may give you more ability to export more when it makes more financial sense. 

Clarification needed. Here goes...

G99 approval is required if the home has more than one G98-certified device which has the potential to export to the grid.

Here I've taken the terms used in the G99 documentation, and presented them as diagrams:

Thus in your case a G99 (Fast Track) application would be required if you were to add a second inverter. Your Solax is the first one, and the Ecoflow systems don't have export capability and are therefore excluded.

Adding more storage outside the house wouldn't require G99 per se, but the inverter which connects to that storage does.

The problem I foresee is how any new inverter could operate satisfactorily in tandem with the existing Solax in the attic. Their protocols are proprietary, so the new inverter must also be a Solax.

There is a PARallel port in the Solax X1 inverters, but I don't know if they have limited the length of data cable which goes between the inverters.

Secondly, the diagrams I've found online suggest that installers are fitting Solax X3- Hybrid-G4 inverters when parallel operation is required.

You have an X1 unit, and those diagrams appear to suggest an additional box is required in order to facilitate parallel operation. In most diagrams it's called a Solax G1 Boost.

Thirdly, the storage batteries designed by Solax are 'high voltage' which means they operate above 75v DC.

That means they can only be installed by a qualified electrician.

Buying a second Solax inverter means that you'd also need to buy Solax storage batteries. There is a dfedicate communications cable which runs between the inverter and the BMS electronics in the batteries.

In short, choosing the first inverter to be Solax has left you 'locked in' to a Solax-only system.
That's compratively expensive, and a DIY route isn't an option.

• I have export capability with G98, and am open to G99 if needs be.

That could be worth, as a new inverter may give you more ability to export more when it makes more financial sense. 

Personally I avoided full DIY but managed to go further by doing some of it myself. 

Like I am with the UKPN DNO and when I approached them to register my inverter, they "offered" 5kw export and said "just fill in the details in the initial G99 form as if G98". 

• The Home Assistant setup is working reasonably well. It's not too intelligent, but thanks to Wonder Watt, our EV's don't drain the batteries when an Octopus Intelligent Go smart session kicks in. I've only got minimal automation setup with the Ecoflow plug-in batteries, but I'm hoping I can automate that in the future, too.

Personally I try to use the inverter builtin local capabilities whenever possible and home assistant for monitoring. 

• While the Solax capacities could technically be expanded outside the loft, if I understand the technical limitations in the Solax system, I could only add one additional slave battery (1 Master + 2 Slaves (maximum 3 batteries total).

However, your suggestion did prompt me to do more searching, and a SolaX BMS Parallel Box would apparantly expand that limit to 6 batteries (~34.8 kWh).

But... SolaX officially recommends adding extra batteries within one year of the initial installation. Because lithium-ion cells degrade naturally over time, adding a brand-new battery to an older stack can cause capacity mismatches, where the older cells limit the performance of the new ones.

It's been three years since the original Solax system was installed, so I think we're back to square one and exploring a 3rd system in order to expand our capacity by another 30kWh.

Certainly if you would replace some cells within a battery the BMS would need to be able to manage that. Arguably a good BMS needs to be able to do that as no two cells even if the same age perform exactly the same. My battery has a Seplos BMS but uses a jk balancer to balance the cells. 

From reading other contributions in this forum, I understand many (most?) BMS systems allow batteries to be charged independently including batteries of different nominal capacities.
I am surprised Solax if could not do this. But I could also imagine some BMS system that would automatically account for some battery degradation and would need batteries to have the same age (and supposedly been used the same number of cycles). Something to check carefully. I believe Solax is mainly an inverter manufacturer. Perhaps put less focus on battery management features and instructions on using them. 

I posted a bit about adding more batteries to a single BMS here, but I have not yet done that. @bash has that his Solis inverter, the menus may be different from ASolax but it is essentially the same. @transparent has multiple inverters and the ability to switch battery banks across them! 

Not exactly what you are looking for, but hopefully of some help.