Good luck with this... should be worthwhile. A couple of general comments:

For EPS make sure you compare like with like

• I assume Tesla will be with Gateway for automatic changeover
• Make sure quote for other makes includes capability for automated changeover... for whole home. Not just part of consumer unit etc
• Also that inverter size is capable of max draw. Tesla will likely meet that - I don't know about others. e.g. Start up of heat pump is a surge? 
• Probably requires an earth rod - decent installer will know that! 

Hopefully good tariff options open up. Agile could be worth considering if your battery is big enough and if charge/discharge rates are maximised. 

I have a very similar set up in almost all respects, except that  I do have an ev.  Just a few comments.

1 Eon didn't check that I have an EV when I went to next drive.  Of course they can probably tell from usage pattern, but I doubt they bother.

2 With a Vaillant you can get the OVO heat pump add on, 15p for your heat pump usage.

With my situation I couldn't make the business case for a battery stack up, but without an EV and if the OVO rate doesn't suit (I don't think they do a good export tariff) then it likely does.

i remember hearing about that HP tariff. im wondering how they work out what power the heat pump is using and what the house is using separately?

one installer of PW3's and solar who had suggested the Myenergi, i think due to the cheaper pricing and possible DNO limits not making a bigger inverter useable, is MCS for solar but not for battery storage. what are the implications if i use such an installer? will i have issues going onto some of the tariffs that control the battery for instance?

A few comments:

1  MCS - my solar/battery installer wasn't accredited initially and I couldn't open an export tariff with Octopus until they were.  It took 6 months or so.

2  I just looked at the Ovo HP tariff and they use the cloud account to determine HP power usage.  I have a Mitsi Ecodan which they don't support yet - I am on the waiting list.

3  I believe that it is possible to have import and export tariffs with different suppliers.  Therefore you can have the Octo export at 15p without an import. (I haven't done this yet though so the rules may have changed)

Posted by: @adamk

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wondering how they work out what power the heat pump is using and what the house is using separately

They use the info from the heat pump which they obviously deem good enough.

Posted by: @downfield

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 I believe that it is possible to have import and export tariffs with different suppliers.  Therefore you can have the Octo export at 15p without an import. (I haven't done this yet though so the rules may have changed)

Yes it is but some suppliers will only offer the best export tariffs if you also import from them.  You need to check the small print before doing this 

We have a Tesla Powerwall 3, recently installed, so I maybe biased.

With a Gateway, it will provide full home backup. A separate earth rod must be installed when doing whole home backup, but that is easy enough to do and the installer should be very aware of this. We have tested the full home backup and it switches to the battery seamlessly, no lights flicker, flashing clocks or internet drops. The only way you'd know is the notification on your phone in the App.

The PW3 provides 185A LRA (Locked Rotor Amps) of load start capability, so should in theory have sufficient capacity to meet any startup loads when an ASHP starts up during a grid power cut/outage. I have not tested this yet - maybe that is something I could do tomorrow if anyone is interested

Once you have reasonable battery capacity, a tariff such as Octopus Cosy becomes viable where you can recharge the battery 3 times per day in winter and there is 6h max between cheap slots. Current rates are around 13p for cheap rate. This will be our first winter with a battery, but I'm hoping the 13.5kWh capacity will be sufficient to see us through the most challenging 4pm-10pm period (where usage is heavier and there is no solar to help). Adding sufficient battery capacity to charge once overnight and then last the day is probably not financially viable for heat pump owners, but the Tesla PW system can be expanded by adding up to 3 further 13.5kWh expansion packs.

I think each battery system option provides pretty much the same, but check with individual installers on your final choice that it meets your needs.

If I had to criticise Tesla, it would be the App. It's control is simplistic to say the least - 2 modes and the App uses it's inbuilt algorithms to decide what's best for you. For many folks who like a set and forget system, this will work absolutely fine, but I like fine control and being able to tell the system what I want it to do, and when I want it to do it, so I find this lack of control frustrating.

The only other potential consideration I can think of is that Tesla are now approved for the Octopus Intelligent Flux (IOF) tariff, which can be highly lucrative during summer if you generate more solar PV than you use. They do not currently support Sigenergy or Myenergi Lbbi systems. We use Cosy in winter whilst we are net importing energy and IOF in summer when we are net exporting energy.

Posted by: @old_scientist

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The PW3 provides 185A LRA (Locked Rotor Amps) of load start capability, so should in theory have sufficient capacity to meet any startup loads when an ASHP starts up during a grid power cut/outage. I have not tested this yet - maybe that is something I could do tomorrow if anyone is interested

Having had a chance to test this now, I can confirm that my Tesla Powerwall 3, whose spec sheet states to be able to supply 185 LRA, is able to supply sufficient current for our 12kW Samsung heat pump to start from a cold start. I've no idea how much current it initially draws, but at least we know the heating can turn on from battery backup during a power outage.

Posted by: @adamk

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i remember hearing about that HP tariff. im wondering how they work out what power the heat pump is using and what the house is using separately?

Many of those tariffs are simple time of use tariffs, where the off-peak rates apply to all of your household consumption during those times. It is just that the times are more suited to heat-pumps, with off-peak periods within the day.

From when I've looked around - I think the Octopus Cosy, EDF Heat Pump tariff, Good Energy Heat pump tariff all appear to be simple TOU tariffs that apply to the whole house.

Worth comparing them as they all adopt a slightly different approach - off-peak times, peak-prices or no peak prices etc.

OVO had a Vaillant heat-pump specific tariff which was tied just to the heat-pump, with a rate which applied only to the energy used by the heat-pump, I've not investigated how it worked.

I'd look at these before investing in a battery. Also, for the battery, I'd be wary about whether prices are likely to drop in coming years, the component costs have been falling sharply, there's new makes/models of batteries on sale now with prices of just £1,500 for 10kWh, so with ~£500 for a suitable battery inverter, £2000 for 10kWh plus the installation. Many of the established products have held their prices in the UK for a long time and look very expensive.

Posted by: @temperature_gradient

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I'd look at these before investing in a battery. Also, for the battery, I'd be wary about whether prices are likely to drop in coming years, the component costs have been falling sharply, there's new makes/models of batteries on sale now with prices of just £1,500 for 10kWh, so with ~£500 for a suitable battery inverter, £2000 for 10kWh plus the installation. Many of the established products have held their prices in the UK for a long time and look very expensive.

I think you need to be sure you are comparing like for like.

It is true that you can buy a Fogstar 16kWh battery for around £1750 whilst a comparably sized Tesla Powerwall expansion pack will cost in the region of £4000-£5000 (Octopus are offering them at £3773 installed)

However, the Fogstar is not IP rated for outdoor use and has no active thermal management so will find it's usable capacity and charge rate significantly diminished in winter, just when you need it most if relying on it to power a heat pump. Not everyone has the capacity to locate a battery indoors or wants a battery inside the house (I'm looking at my wife), at which point you are going to pay significantly more for a product with IP67/IP55 rating and that has active thermal management to keep the cells in an optimum temperature range when it's -5C outside.

IMHO these are two very different products, even though both are essentially 15-16kWh batteries.

Posted by: @old_scientist

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I think you need to be sure you are comparing like for like.

Yes I was thinking of outdoor, IP65 batteries with built-in heating, these:

Dyness G2 - 10kWh, IP65 outdoor rated with temperature control. £1500

https://www.bimblesolar.com/Dyness-G2-Lithium-heated-battery-IP65

Bit more expensive, but similar spec-ed Fox EP11, outdoor rated and with a built in heater at £2400 inc VAT.

https://www.solartradesales.co.uk/fox-ep11-h-10-36kwh-384v-battery-10-year-warranty-heating-element

There's the battery inverter cost on top, these are typically around the £500 range, plus the installation (but that should remove the VAT as they're zero VAT if installed), so component cost starting at ~£2000 plus install for the Dyness, £2500 for the Fox.   

@adamk what you need is enough battery size to cover 3 hours of peak prices mid winter that you get on some tariffs so probably >10kWh, with enough output capacity to run your ashp and rest of house together so probably >4kW. 
Tesla nearly always get good reviews other than price. Our Givenergy has been fine but other users have had troubles so they get mixed reviews. I’ve never heard of the Myenergy ones. 
Certainly you are correct not every battery manufacturer supply the interface to allow them to be controlled by some of the more innovative tariffs. Eg Octopus