I don't have the same battery as that so can offer no opinion on the specific supplier but on the way of using it: I run everything exactly the way you describe. you still get your FiT.  doing house battery to EV battery is less than ideal, 10% loss each time, but you'll be on a cheap night tariff so you just charge your EV from grid on the night tariff as well. of the last 60 days I've had 16 days where the battery + solar had not enough capacity to run the heat pump all day. most of my heat pump use therefore at 7.5p/kwh.

make sure you do the maths on how big battery + inverter you need.

I have a solax system, I actually have x6 of the 5.8kw batteries.  

does it work, yes, very well.  I charge at night I’m on eon drive, fixed tariff.  9.5p for 7 hours.  I moved from octopus as they only did 4 or 5 hours off peak and that wasn’t long enough to charge 6 batteries!

FIT should remain, mine does, no impact.

I wouldn’t charge your car in day or from batteries.  I would charge at night and export any spare at a higher kWh rate.  Again, eon do 16.5p export, plus FIT with original agreement 

I have a eon referral code if you use it you get £50 credit, so do I!

https://share.eonnext.com/noble-ant-49

it all works well and the only catch is the cost!  charge battery at night or via solar and the battery powers the house and heat pump on reduced or free electricity 

you will want more than 5kw of battery though but really that depends on your needs.

Posted by: @geo3geo

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What I'm looking at is 

Triple Power 6.0kWh + X1-FIT 5.0kW Battery Bundle £3840

The issue I can foresee with this is how it can satisfy the G99 rules for a grid-tied connection.

Each piece of the apparatus which can export to the grid must already have G98 certification.
That covers basic safety and protocols to cope with a power-outage, which are summarised here:

G99 comes into play when there is more than one device with export capability.
As I understand what you propose @geo3geo you will have the potential to export from both solar panels and batteries.

There are various methods of satisfying G99, and it also depends on what level of export is permitted by your Distribution Network Operator.
They may be prepared to allow you more than the basic 16A specified in the diagram above.
You get to learn that when you make an application to them.
As it's free, that's where I'd start.

Fill in the online form as if you are having the Triple Power 6kW and battery bundle added to your existing solar system, and see what they say.

@transparent Thanks for the G99 info, just wasn't aware of that. I do have two G98 approvals for my two PV systems - about 2.5KW total. I'll do more investigating, obviously the first thing to check out.

@geo3geo Don't be daunted by what your DNO may say.

There are many possible solutions to this problem.
For example, consent from your DNO is not required if the heat-pump were to be run from storage batteries in an 'off-grid' manner.

The G99 rules only apply to the installation of equipment which has the capability of export to the grid.

And please tell us roughly where you live.
We can then see who your DNO is, and better understand the electricity supply constraints in your area.
Thanks.

I did my own g99, region SW Scotland so Scottish power.  My understanding, if you stay under 16amp and can only export from one inverter, g98 is fine.  

you would have to get a hybrid inverter, connect battery and your solar to it, stay under 4kw generation and you would be fine.

though. I would get a 5kw hybrid inverter or battery charger, as having the extra bit of power to draw from on batteries is worth it.

google the G99 in your area, basically it’s a form, confirm all your stuff, state your export potential, produce a basic schematic, refer to all the safety standards of you system, all the solax stuff has conformity to relevant standards.  If you are nearby a substation you can generate / export well over 10Kw on single phase.  

SP did charge me £250 plus vat for my G99 approval, basically they checked the grid could cope with my extra export capacity.

@transparent I'm at DY103JG
the inverter I'm looking at for the 5.8KWh battery system is  Solis 3.7kW X1-FIT G4 AC Charger so I guess it's capable of exporting 3.7KW.

I already have two Solis 1.5KW inverters (roof 1KW PV + garden 1.2KW PV).  Both G98 registered. Only registered one of them last week as it happens, it replaced a 10yr old inverter that died. DNO didn't mention G99.

So looks like I'd be up to 6.7KW potential export.

Well that postcode makes life easier for us @geo3geo

I have a partnership agreement with National Grid Electricity Distribution (NGED). That enables me to see their network maps and data.
A lot of this is also available to the general public.
Enter your post code into their Carbon Tracer site, and you can discover all sorts of useful statistics about the Bulk Supply Point (BSP) transformer at Kidderminster which provides your electricity.

NGED staff are extremely helpful.
If there's a way for you to achieve what you're after, then they'll find a way through.

However, I need to point out that the houses in Gladstone Place are supplied through a privately-owned distribution network (IDNO) operated by The Electricity Network Company Ltd. who are a subsiduary of GTC. Different constraints might apply if your house is within that zone.

... and you'll now appreciate why this forum is a good site to learn about such issues!

It's worth having a go at calculating your expected daily electricity use. Everybody is different, but I hazard a guess that the national average is around 8kWh/day, assuming heating by gas. If heating with a heatpump, this figure will increase perhaps significantly - not that this should put you off, but some basic sums will give a better outcome.  I'd guess in future you'd want to add to the quantity of batteries.  Are the ones you link to "HV"?  I expect that makes the system more efficient, the downside being safety(I would not diy HT batteries) and perhaps ease of increasing the battery size (LV batteries being relatively similar to each other in contrast).

Battery storage is a very fast moving area, and the price of the batteries is falling fast.  Our install was diy, so "free"- however there's no VAT now on batts I think, which levels the playing field.  I don't think there is any reason to get an MCS installer in your case, as you should already have an MCS certificate I expect with the existing FIT install.

We're an all elec house, with  a 4kWp FIT PV (now measured export, but still FIT generation), a tiny 2.5kW heatpump, as the house has loads of insulation, and an Elec car, charged overnight. 3.6kW Sunsynk hybrid inverter + 15kWh of battery. The battery has been large enough to cover our daytime use almost everyday - there have been maybe 5 or 10 days when they ran out sometime in the evening, and we end up on expensive elec rather than stored 7.5p/kWh "Go" elec.  I've measured the in-out round trip efficiency, that is AC->sunsynk->battery->sunsynk->AC, with a mid certified meter, and I get 76% with our typical use pattern - not as good as hoped, or as the "up to 96%" the inverter manuals will have you believe.  This includes standing (inverter) losses, battery resistive loss, inverter operating efficiency, everything.

January's bill was:  £56 (off peak) + £11(peak) + standing ->£79 total.  That £11 peak charge is due to three things, of similar magnitude:  Exceeding 3.6kW - eg toast & kettle at the same time and we import a bit.  Then the batt running out a few times late evening.  Another one is the small signal system accuracy - the sunsynk has a CT sensor on the main tail, and a long twisted pair cable back to it - any noise on this cable results in a little import, and it all adds up.   We're neutral money wise through the year due to the summer FIT payments (17p/kWh), but still import elec over the year - we need some more PV to fix that 🙂

I'll give you an L-Xl version for comparison. 16kw heat pump that generates 9kw of heat using 3-3.5kw of electric when its sub-zero. 15kw of battery inverter capacity (covers any peak time usage in terms of peak power), 41kwh battery storage. fully electric kitchen. 2 EV's. 11kWp PV. Inverter capacity was an important factor for me (5kw not enough, 10kw maybe , 15kw definitely).

january:

2200kwh imported, of which 2000 kwh offpeak @ 7p, the rest @30p. the peak import is all due to the battery running out in the evening on the sub-zero days. Above about +2C , the reduced heat requirement is such that all load is fully met by the battery.

300kwh generated

average import is therefore around 75kwh/day. but that's absolutely everything.

I agree with @iancalderbank about the level of power which is likely to be required.

15kW of inverter(s) is much more likely to be required, rather than the 6.7kW which @geo3geo is presently proposing.

There are number of ways to achieve this.
I don't yet see an issue which is insurmountable.