Solar PV panels, iboost (PV diverter) and an air source heat pump (ASHP) is it cost effective?

  • 12 August 2021
  • 14 replies

Userlevel 4

Hi all,


Just starting the process of getting solar installed (ie getting quotes). I’m sure i will have more questions on this to come, but here is my first.

There is a product called iboost that takes excess electricity you are creating from PV, which is going back into the grid, and uses it / some of it to create hot water via the immersion.

The question is, as I now have a ASHP, is this of any use? My thinking on this is that if I had 2kWh of ‘spare’electricity from PV, then via the immersion I get roughly 2kWh of heat. But (assuming a HW COP of 2) it would only take 1kWh of electric to do the same via the heat pump. That leaves 1kWh spare which I might get paid 5p for, or at least someone else can use as green energy.

The only thing I can think is that the ASHP is only heating up to 50 degrees. The immersion could push that up to say 65 degrees. This has two benefits i think. 1) the legionella setting does this every 7 days, so I might be able to turn that off (maybe) and be using PV to power it. 2) if the water is 65 degrees then it would take longer to cool down to need reheating, therefore saving using the heat pump.


Overall not convinced the iboost is of value with a heat pump, but would value a more informed view

Any other general tips on ASHP plus Solar?





14 replies

Userlevel 7
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Thanks @Heatherd - that COP of 3.5 from an Air-source Heat pump is extraordinarily high. Participants on the current OVO-led Zero Carbon Heating Trial are struggling to get anywhere near 3.

Our current understanding is that COP readings above 3 are usually only attainable by Ground-source Heat pumps.

I have 16 panels on my roof enphase invertors under each one. they in turn provide electric to the house. then store in 4 enphase batteries. I also have an ASHP-diakin altherma. this supplements the heating at 50% so far. I do not have a heat store for my water with immersion in it. I use the boiler for that. My gas bill in the summer is low. bills are 30% below a house same size as mine. 


ASHP does not perform as well as i would like but after 5 years it is getting better. 75% eco will be set at next service to try and get a better savings. but it will use 1kw for every 3.5kw it produces. thus only set at 50%eco.

Userlevel 7

@Heatherd I’m tagging you in this thread as you may be well placed to advise. Without giving too much away, there’s some similar tech involved here...

Userlevel 7
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I’ve spent 5-mins this morning to find the following three parts on Amazon UK. So this will give others an idea of likely costs and how readily available these items are:



A ‘stack’ of 16 such Lithium cells would have a capacity  of 13.4kWh - the same as a Tesla Powerwall, but with much greater lifetime due to the cell chemistry.

Userlevel 7

As ever a really thorough analysis of the possible options by both @hydrosam and @Transparent


Any closer to getting that best answer, @James_N

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I agree with so much that @hydrosam has just written. There are certainly much better ways to implement this kit than has been done by the majority of the installers who were commissioned for the Heat pump Trial.

Problem A: The home-owners did not at that stage know these things and are discovering them too late.

Problem B: the Trial Managers probably should have known these things, but even if they did, they’ve left it to the installers.

Problem C: It’s still early-enough in the age of Heat pumps for Installers to design, install and commission equipment in a way that isn’t optimal. Few people are around who can challenge them… until it’s too late and they’ve been paid.

It’s annoying that the home-owners will be left paying the energy bills on this sub-optimal kit for years to come.

As the rules are currently written by the Electricity Networks Association (ENA), it is not permissible to have a solar-diverter to operate anything but an immersion heater.

In particular, you are not allowed to have a grid-connected Storage Battery operating as a PV diverter. I think that’s not right, ethical or in the best interests of  combatting Climate Change. But those are the rules.

However, if you implement your own electronics to send your own solar-generation to an off-grid battery, then the ENA rules don’t apply. It’s not grid (export) connected.

Moreover, if your off-grid battery and its electronics operate below 50v, then you don’t require a qualified electrician to install, test and certificate them.

If you then choose to de-couple your Heat pump from the grid and instead install an independent DC-AC 240v inverter to drive it, then this too falls outside the legislation.

Once you understand these rules enough to break free of them, it’s very attractive to run as much as possible off-grid.

Costs also plummet. None of that off-grid equipment has to be certified to G98/G99/G100 and you’re not paying for all the R&D which commercial firms have to fork out to get their products through those specification and safety tests for grid-connection.


I fully realise that these concepts are tricky to grasp.

@James_N  is quite correct to say that he doesn’t follow my argument.

I also sometimes have difficulty understanding what I’ve just written. :thinking:

Userlevel 7

Hi @James_N 

I’ve had the electrician back today to sort out my Solar PV diverter because the setup was tripping.

The integration of a solar PV diverter (need a short hand solution for that) is not straightforward for heat pumps it turns out,  unless they get a special tank with a secondary immersion dedicated to the Solar PV. Someone in the early days of the scheme had such a tank fitted, a mixergy tank (look them up, it’s fancy) but I have had a regular tank fitted with just one immersion heater. Because the heat pump needs  to control the immersion heater to reach 60C during the legionella cycle integrating the pumps need to control the immersion and the Solar diverters desire to dump ‘excesses’ energy there hasn’t been straightforward. A timer and relay have needed to be fitted to stop the RCDs tripping when both systems are calling for the immersion to be on. 

I tried to get the mixergy tank when they were replacing mine but they wouldn't do it, didn’t get the reason why. In hindsight it would probably have worked out better for them to buy the slightly more expensive tank and not pay two days of the sparkies time, plus special parts.

The lesson is make sure the installers are in the loop if you get an iBoost, Eddi or other solar PV because it might involve some work from them to integrate in to the setup. Mine was fitted pre-heat pump (gas with boiler having no control over the immersion) so they committed to getting it functioning again.  Alternatively, but more expensive, is to fit a battery setup to solar PV, but for me the cost is prohibitive and doesn’t stack up financially yet (and I’m also not convinced on the environmental side with all the lithium and other chemicals needed in the battery). As ever, going green isn’t straightforward.

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@Jess_OVO I’ve postponed getting PV until I have improved the building’s insulation. I’d rather reduce my electricity use as far as possible, than bolt on another piece of tech !

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Not sure I 100% understood Transparent’s post.


Did you take a look at @hydrosam’s explanation here - from my understanding of things it sounds like both are suggesting that the ideal solution would be using a PV diverter to charge up a home battery and use the stored energy to power the heat pump rather than using a much less efficient immersion heater. 


Obviously the up-front costs of installing a battery might not make this feasible -  Any other heat pump trialists considering these options? @juliamc, @sylm_2000 


@JohnMcG - know you’ve got a PV Diverter which you use to power hot water production and an EV charger - any pearls of wisdom to share? :slight_smile:


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Not sure I 100% understood Transparent’s post. The Eddi is the same as the iboost (and actually looks better).

I will be exporting to the grid with the new PV array. And in fact on the Eddi website it does say it works with heatpumps. It’s obviously better to use electricity I create than export, and the Eddi does this by using the HW immersion. But me question is if it’s better to heat the HW via the heatpump than the immersion (as triggered by the Eddi)

Userlevel 7
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Well I’m obviously happy to claim the points for providing a Best Answer…  :slight_smile:

… however, for the sake of completeness, can I point out that we have another topic about the general principles behind Load Diverters. It’s going to be tricky to merge them and still make sense of the conversational flow…

… but thankfully that’s a job for the Moderators!

Userlevel 7

What’s your latest thinking, @James_N? Shall we ask more heat pump trialists for their take on using iBoost? 


Happy with Transparent’s advice for the ‘best answer’?

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I think the iBoost is heading in the wrong direction @James_N 

Firstly as you have no storage for space-heating, the DHW cylinder is your only heat store. The higher its temperature is, the harder your Heat pump must work to put any of its output into that cylinder.

The whole point of Heat pump technology is to keep it running 24hr/day.

If you’ve got ‘spare’ PV capacity then it could certainly be re-routed somewhere, but to do so you must a Diverter approved by the Electricity Networks Association (ENA). The Eddi from My Energi is such a device, and I’m assuming that the iBoost is too.

An approved Diverter means that you are allowed to have a PV Inverter capable of sending more than 16A AC to the Distribution Grid, but that the Diverter has a G100 accreditation to prevent this happening.

An alternative is to install an Inverter rated at 3.68kW (16A) max, but divert the excess energy from your PV Solar Panels as DC. This wouldn’t need to be ENA accredited because they only regulate the 230vAC side of the installation.

You would still need to store that excess generation of course. But that’s a different issue.

Userlevel 7

Hi @James_N - I’m looking forward to hearing from our heat pump members shortly. 


Have you seen this PV panel thread? You might find it worth a read, it’s been kicking off there this week: