What a great site this is. Many thanks for all those who responded to my request. I look forward to future correspondence when I get into our new home. It is very much appreciated that a community like this exists.

To those of you suggesting that I get a battery, yes I've considered that and just want to live with the system first before making any decisions, plus I'm trying to minimise extras on the building contract at the moment as the HP installer is a subcontractor to the builder who will add his bit and I get to pay stamp duty on the outturn cost.

@gz00 (heads up that some heat pump tariff providers will require you to submit your MCS certification when you apply. If not already in your possession, it’s time to chase the installer). Also ensure you have a smart meter and that it’s working (some can have signal strength issues). Being on the right tariff can halve your costs.

Posted by: @gz00

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To those of you suggesting that I get a battery, yes I've considered that and just want to live with the system first before making any decisions

Very sensible.  Also plug in batteries will hopefully become legal soon and if so are potentially an excellent option with the constant load that a heat pump gives.  I have had my heat pump for 2 years, I can't make the business case for a fitted battery work (so I haven't got one) but with plug in it will become a no brainer.

@old_scientist its not the size of the battery that limits my system, I can easily add a second or 3rd/4th one. Its stackable. What is the issue is the Inverter is limited to 2.5kWh, i was told thats a DNO restriction. Id need to ask supplier again, since its been a year and prices fluctuate, of can increase battery capacity and discharge level

thoughts?

Posted by: @agentgeorge

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@old_scientist its not the size of the battery that limits my system, I can easily add a second or 3rd/4th one. Its stackable. What is the issue is the Inverter is limited to 2.5kWh, i was told thats a DNO restriction. Id need to ask supplier again, since its been a year and prices fluctuate, of can increase battery capacity and discharge level

thoughts?

Are you saying that your DNO has restricted the maximum inverter rating to 2.5kW, or that is the hardware limitation of your battery system? 

I can imagine that 2.5kW would be quite restrictive as you would need to draw any additional power above the inverter rating from the grid. It maybe OK for small 5kWh 'plugin' type battery systems for low use households, but not really suitable for high use households with a heat pump. 

@old_scientist The restriction on how much you can export is set by the DNO and you need the G99 to export above 3.6kW per phase, which has to be applied for “before” installing a system.

Ive just checked PV log and can see the battery topped out at 2.78kW yesterday.

ill have to ask my PV provider what options are available to increase the inverter output

I found this in my manual:

Export limits are set through the inverter's settings. Do not attempt to change inverter export settings yourself - this can void your MCS certificate, breach your G98 or G99 agreement, and in extreme cases push voltage outside safe limits on the local network.

oop-errr

@jamespa 

You can find £80/kWh home batteries now on Amazon. Incredible value. 

Posted by: @papahuhu

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@jamespa 

You can find £80/kWh home batteries now on Amazon. Incredible value. 

The batteries are but the installation cost kills the business case, at least for me.  Hence why I'm waiting for plug in.

@papahuhu having seen

videos of houses burned to the ground because of cheap batteries, wouldn't buy one from amazoom or ewebay

@agentgeorge They are exactly the same cells as used in mid priced fancier units, so the risk remains the same.

Personally I’d never install any type of home battery within the internal fabric of the house, but I’m risk averse. 

Posted by: @old_scientist

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I can imagine that 2.5kW would be quite restrictive as you would need to draw any additional power above the inverter rating from the grid. It maybe OK for small 5kWh 'plugin' type battery systems for low use households, but not really suitable for high use households with a heat pump. 

That's where I disagree.  A heat pump represents a constant medium load like almost any other appliance and therefore is a perfect match for an inverter that has limited output, provided it charge sufficiently quickly.  It's true you will have to import during peaks but most people won't get a battery large enough to cover a whole day consumption in mid winter so will be doing that anyway.

For example most of the time my 7kW heat pump ticks along at 600-800 W consumption.   I have 18 hours at premium tarrif and 6 hours at low rate tariff.  So 18h*700W is 12.5kWh,  Thus even with the plug in limit of 800W I will discharge a 12.5kWh battery during my peak rate.  It's not worth me having a battery much bigger anyway, because there are relatively few days when my 18hr consumption is that high. So long as I can charge in 6 hrs, ie 2kW charge rate, that's just fine. 

Hence why I am personally waiting for plug in.

Posted by: @jamespa

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Posted by: @old_scientist

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I can imagine that 2.5kW would be quite restrictive as you would need to draw any additional power above the inverter rating from the grid. It maybe OK for small 5kWh 'plugin' type battery systems for low use households, but not really suitable for high use households with a heat pump. 

That's where I disagree.  A heat pump represents a constant medium load like almost any other appliance and therefore is a perfect match for an inverter that has limited output, provided it charge sufficiently quickly.  It's true you will have to import during peaks but most people won't get a battery large enough to cover a whole day consumption in mid winter so will be doing that anyway.

For example most of the time my 7kW heat pump ticks along at 600-800 W consumption.   I have 18 hours at premium tarrif and 6 hours at low rate tariff.  So 18h*700W is 12.5kWh,  Thus even with the plug in limit of 800W I will discharge a 12.5kWh battery during my peak rate.  It's not worth me having a battery much bigger anyway, because there are relatively few days when my 18hr consumption is that high. So long as I can charge in 6 hrs, ie 2kW charge rate, that's just fine. 

Hence why I am personally waiting for plug in.

I find my 13.5kWh is perfectly sized for our heat pump. On Cosy, there are 8h of cheap rate, split over 2x3h slots and one 2h slot. With a 5kW inverter or charge rate, the battery can be fully recharged in the 3h windows, and mostly recharged in the 2h window - at least enough to see through until the next cheap rate session. Because 8h are at cheap rate, the battery only needs to last for 6h max until the next cheap rate slot. So we do not need 50kWh of storage to cover our full house usage on the coldest winter days.

Our heat pump idles at ~1kW and probably draws 1.6kW during the coldest periods, not to mention the 3kW plus it's going to draw during a DHW run.

The most demanding time is the peak rate 4-7pm period in winter, where the heating is on, there is no help from solar and we are using the electric cooker/airfryer to prepare the main evening meal. We can easily be pulling 4-5kW for prolonged periods, so anything under 5kW inverter would be woefully underrated, 6kW I would view as a minimum and 8-12kW would be ideal (we also have a 9kW electric shower, so running that direct from the Powerwall is great)

Of course everyone's usage is different, and I completely get your argument - it makes little sense to spend £7k on a powerwall to ensure you are 100% running from cheap rate, when a £2k plugin battery may get you 95% of the way there and the additional £5k in costs yields marginal returns.

For us, siting a battery indoors was simply not an option (plus the wife would not have allowed it anyway on safety grounds), and the PW3 is class leading in terms of it's thermal management and weather protection ratings. Given where it is sited, fully exposed to the elements (the prevailing rain), I'm not sure I'd want to plug in an Ecoflow battery in my driveway where it may sit in feet of snow or be pounded by rain all year.