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immerSUN installation and first impressions

Posted by Jane Richardson Hawkes on 27 January 2016 at 9:40 am

Jane Richardson Hawkes shares her first impressions of her new immerSUN unit. She recently had this installed in her home in the South East of England where she lives with her partner. This is her first report in a series of three:

Why an immerSUN?

Using as much of our PV generated electricity as possible in the home has been exercising our minds since the panels were installed in October 2012. 
We have a 3.84kW PV system installed on a south facing roof which has generated 11,955 kWh* of electricity over a full three year period to date.

We try to use high energy consuming appliances like the washing machine only on sunny days and during peak PV output times such as 11.30am – 2pm.

We have an energy monitor which glows green when the house is not importing grid electricity. This is helpful for gauging when the use of other appliances tip us into importing electricity.

Having heard about the immerSUN, we decided to trial it as an energy saving option for optimising our use of PV generated electricity.

What our immerSUN is supposed to achieve

The immerSUN monitors the electricity which is about to be exported to the grid and diverts it to the immersion heater. It then automatically directs any excess generated electricity, not used by the house, to the immersion heater in the hot water cylinder.

If the electrical load from the house increases the immerSUN reduces or stops the electricity diversion to help maintain independence from the grid.

Installing our immerSUN 

The immerSUN was installed in early December by one of the company’s electrical engineers. The installation took about 3 hours and went smoothly.

It wasn’t possible to connect the immerSUN with 4 eco’s server at the time of installation due to server maintenance. Once this maintenance was concluded the immerSUN automatically transmitted data and we were able to register on the 4eco immerSUN website and view the data from our PC/android Smart phone.

The results so far

During sunny winter days, the immerSUN diverts electricity to the immersion heater, as long as our base load electricity demand is reasonably low. One clear sunny morning the water was up to the required 60 degrees centigrade temperature by midday.

We normally heat our water via the gas boiler for 15 minutes a day to provide our normal daily hot water needs. Consequently, if the forecast is sufficiently sunny I can confidently rely on the immerSUN to heat the water without needing to use the gas boiler, but not on cloudy days.

Here are some graphs showing the diversion of solar electricity to the immersion heater in action over the course of a day - these are taken from the myimmerSUN app, which allows you to monitor your results on a tablet, PC or smartphone.

  immerSUN readout   Graph: A sunny winter day when we were at home all day    

Graph: A sunny winter day when we were out of the house

Pleasing performance

We’re really pleased with the way that the immerSUN is performing, and it has been quite gratifying to hear it quietly whirr into action when the sun comes out. It’s fitted neatly into its location next to the consumer unit in the cloak room, and the data screen on the front of the unit is clear to view and easy to understand and use.

The online My immerSUN monitoring data has proved to be an unexpected bonus in that it also allows us to visualise the amount of electricity the panels generate over time as well as what’s diverted by the immerSUN. When the PV was installed we didn’t opt for the generation visualisation software due to the cost.

We’re pleasantly surprised that we’ve only had to resort to using the gas boiler to heat the hot water intermittently, even though we’re currently in mid-winter. I confess that we wouldn’t contemplate using the immerSUN ‘Boost’ function to use grid electricity to heat the water if needed, basically because the boiler is so quick and the fuel much cheaper.

We’re expecting that by spring with the longer, and hopefully sunnier days, we won’t need to use the gas boiler at all. We’re also hoping on longer sunny days to be able to heat the hot water before noon and then use the electricity generated during the peak sunshine of the day for the washing machine or the oven/microwave. So we’re planning on making some lifestyle changes to maximise the advantage the immerSUN has given us. We’ll let you know how it goes!

About the immerSUN

The immerSUN is designed to help you get the most value out of your solar PV, especially if you are often out much of the day. It does this by diverting your surplus solar electricity to an immersion heater in your hot water tank (or to an underfloor heating system) at times when it would otherwise export that electricity to the grid. This means your surplus solar electricity is providing higher value hot water you can use instead of lower value export tariff returns.

* Not 3,268 kWh as wrongly stated in the first published version of this blog article.

More information about Solar Electricity on YouGen. For more information about the immerSUN see here.

Find a solar PV installer.

Need help with any Jargon?

About the author: Jane is a specialist in sustainable energy policy with responsibility for advising local authorities on sustainable energy technologies and policies in the context of land use planning and climate change strategy. She has a degree in Environmental Studies, a post-graduate diploma in Urban Planning and is a member of the Royal Town Planning Institute. Currently she works for the National Energy Foundation as a Senior Energy Specialist.

If you have a question about anything in the above blog, please ask it in the comments section below.

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19 comments - read them below or add one

Sandra Hayes

Sandra Hayes from Comment left on: 6 February 2017 at 11:17 am

I had an Apollo GEM installed with my solar PV system in September 2015.  I am also very pleased with it.  I didn't use my gas boiler once for three months last month and still had all my hot water needs met.  

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agrimaComment left on: 2 November 2016 at 9:57 am

I have solar panels and an immsesun unit all fitted a year ago which I find excellent. I like the way it diverts spare power to heat the hot water and when that is "full" it heats the towel rail. I also have an immersun iL bridge to monitor the system via my android phone. Again this is excellent as I can put the unit on holliday mode and put it back again when away as well as use the boosts functions. Unfortunately since 4Eco stopped trading in July I have been unable to use the app and altough a new company has now taken over I am not gettinh help and the app does not work. The immsesun unit does find the bridge but the Gateway status light on the bridge is orange/red and the Immsesun status light is steady green. Can anyone help please.

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Tim1965Comment left on: 24 September 2016 at 8:20 am

I'm pleased to report that 4Eco appear to have been taken over and the immerSUN is back.

We've had ours for nearly a year after our PV installer included it with the PV system, although what neither of us had spotted was that the cylinder didn't have an immersion heater although wiring was present.  We subsequently replaced the 40 year old cylinder with a modern foam insulated one when we replaced the boiler, ensuring that the cylinder had a bottom-entry immersion heater to maximise potential use.

I think that we average around 5kWh daily into the cylinder and have relied on it for hot water almost exclusively through the summer.  However given that we're replacing gas then the payback on the immerSUN is long when providing hot water alone.

However the immerSUN can also control on/off loads switched on via a contact when a given potential export power is available, and I use this to enable the charger for my electric car so that the car charges from my own solar power.  The immerSUN's Boost and Timed Boost functions also allow topping up the car from grid power when necessary.

The immerSUN also has a third output which could be used for a second immersion heater or some other form of resistive heating, but we don't currently use this.

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bklvrComment left on: 24 August 2016 at 8:45 am

We were just about to have an Immersun diverter installed when our electrician discovered that the company has gone into voluntary liquidation.  

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GnattycoleComment left on: 18 August 2016 at 8:19 am

We had a pv system installed In 2012 just after moving in to our current home. The roof is SSE facing with 25 degree pitch so our summer yield is disproportionately higher than the winter when the sun is lower. I specified an immersun as part of the installation as we had a traditional indirect hot water system at the time. The unit itself worked well but due to the top mounted immersion heater and cylinder volume around 80 litres, we made minimal savings because we couldn't absorb the surplus energy available. Last year, I fitted a 300 litre thermal store as a twofold upgrade. Firstly it gave me mains pressure hot water so I no longer had to run around in the shower to get wet but also had a 3kw immersion heater mounted low in the tank. This is now capable of absorbing much more of the surplus energy from the pv system due to both the larger volume of water but also the higher temperature the Immersun thermostat is set (85 degrees) although in clear summer days, the thermal store is still satisfied by early afternoon. My intention is to fit a 2nd thermal store next year with solar thermal coil and use the 2nd immersun output to boost the solar thermal panels. Intention is to run underfloor heating from this although the main central heating is still backed up by the existing oil fired 120,000 btu boiler. I found out today that 4eco, the manufacturer of the Immersun has sadly ceased trading as of July 2016. I hope that a buyer can be found because these units are superb little devices in my opinion. I think you just need the right conditions to maximise it's contribution.

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Graham Marshall

Graham MarshallComment left on: 18 March 2016 at 12:57 pm

"In a household with a PV array generating a 2kW surplus, just 2.5 hours is needed to raise a 100ltr tank temperature from 20 ˚C to 60˚C giving a saving of approximately £0.50 per day based on average energy prices. (Feb 2013)" - a quote from the sales pitch of Solar iBoost. 

If it requires 5kWh to raise a 100 litre tank through 40C then it presumably requires 15 kWh to raise a 300 kW tank by the same amount. Assuming I have a 15kWh excess in summer then I can save £1.50 per day or £180 over an avarage three month British summer. Pyback for the switch would, therefore be around 18 months.


I am sure no one here believes that and, of course, the seller does not make such a payback claim. Based on the actual figures kindly provided by the correspondent from Leeds I would calculate the likely payback to be nearer 8.5 years, great if you use enough water in the summer to justify all that water heating.

My logic is a little different. I have a 300 litre tank and must be able to produce as much excess electricity as our friend in Leeds. I will shortly have underfloor heating and should, therefore, have a decent demand for hot water during Spring and Autumn. On that basis I have now judged it worth a gamble to invest the £250 required for an installation. 

How can I achieve and installation at that price, well, I am hoping someone out there will have identified a less expensive switch than the Immersun and be prepared to share ther knowledge with those of us yet to fit. Based on the few connections required and an electrician who knows our house I can't see installation taking more than 2 hours.

I'm off to make a coffee while those wiser than me work out what is lwrong with my calculations and post a response.


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GaryComment left on: 8 February 2016 at 6:35 pm

The boost function is either a manual switch on for X minutes in 15 minute chunks or programmable in the same way as your central heating system. Take a look at the user manual

I am not sure whether you are talking about the boost (override) function or the priority of output 1 over output 2 etc.  The former is described above; the latter is fully programmable.

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paul53Comment left on: 5 February 2016 at 3:43 pm

the  one  i have  the  boost  is  manual so as  long as the the  time of boast is  set to the time the first  device  needs it  will  only  feed  this , or  simply  turn the storage  heater off at the  switch  

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trstrangComment left on: 5 February 2016 at 11:50 am

I'm considering an Immersun. I have an Evance 5.5kW wind turbine which produces about 10,500kwh per annum - its been in 5 years and a bit. 

Am thinking of installing an immersion heater and a 2kw storage heater, cascading from the first to the second in case of enough surplus. I don't want to use the boost ( if I should have to ) outside the off-peak hours. I have dual rate supply, and use timers to run dishwasher etc at night. 

Is it possible to arrange to only be able to ' boost ' the immersion heater during cheap rate hours? Anyone an expert on this?!

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NEF Gordon

NEF GordonComment left on: 2 February 2016 at 10:28 am

Jane has provided a correction - the three year total electricity produced from her PV system is 11,955 kWh. The system is perfectly located and south facing. Thanks very much to our readers below who spotted that the 3,268 kWh figure previously given ought to be an annual rather than a 3 year production figure - you were right!

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GaryComment left on: 30 January 2016 at 7:17 pm


The system will only divert when the immersion heater requires heat.  If you look at the good day chart, I would suggest that approx 1kWh diverted in the first two hours brough the water up to temp.  This would suggest that either an alternative source had provided some heat or very little hot water was used that morning.  The later two blue blips would then be re-heating the water afer drawdown for, say washing the dishes etc.


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AshtreexComment left on: 30 January 2016 at 7:15 pm

With a 3.44 kW PV array, on a London rooftop, we have a highly pragmatic solution. From about March - the end of October we simply switch off water heating by our gas boiler and instead switch on the immersion heater from 11.00 - 13.00 GMT. Some days the solar electric has to be supplemented from the grid - other days there is sufficient electricity from the PV to run the 1kw immersion heater.

Not perfect of course, but with no installation costs for an Immersun or similar I bet we get 80% of the overall cost benefit.

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John Rickerby

John RickerbyComment left on: 30 January 2016 at 9:28 am

I am intrigued by your figures. My 3.2kW array in Mid Wales generates about 3000kWh in a year, so yours seems to be underperforming big time?

The blue bars in your graphs would seem to show minimal levels of diversion even when there is significant export.

I can't see how this justifies your conclusions...or the significant cost of the device.

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HarmsyComment left on: 30 January 2016 at 8:55 am

Whilst such systems increase the use of renewable energy within a single location, they cannot be considered "green". This is because exporting the electricity to the grid has a lower carbon footprint - at the moment at least - than the alternative. Why? Even old gas boilers, which most households use to heat water, will be upwards of 70% efficient. Meanwhile, new ones are 90% or better. On the grid, gas is used inefficiently in power stations - half that of the best domestic boilers - and then you lose 5% or more in transmission. The exported electricity from PV will go straight to your neighbours with negligible losses. So, perversely, these PV diversion systems actually increase the carbon emissions of the UK, not decreasing them. The only thing that decreases is the hot water heating bill in the household where they are installed.

But people must not kid themselves that spending the extra money on these devices makes them "extra green", the reverse is actually the case.

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GaryComment left on: 29 January 2016 at 9:31 pm

Pleased to hear your initial thoughts are as positive as my experience 2 1/2  years in.

I had the mk1 installed with my system in June 2013 and paid £350 as they agreed to absorb the fitting into the PV install quote.  I was pleased with the results and keen to have the monitoring system and took the plunge and the upgrade offer when the mk2 and monitoring system came online.

The mk2 is much better, not only in terms of looks but also the way it works and what it diverts.  I guess this is the Truesine technology but who knows. But my water heats better and takes more of the available excess, presumably because of the eddies and water circulation differences in teh tank between the way the fire burst and truesine technologies provide power to the heating element.

The mk2 also has 3 outputs, not two, and wireless connectivity to the monitoring unit, the sensor if you want that and to other immerSUNs.  I have a 13A socket on outlet 2 of my main immerSUN and use it with a 2kW oil-filled radiator to warm my office when I work from home.

Two and a half years into using the mk1 and now mk2 I reckon my return on investment is about £100 per year.  I record savings at leccy rates which some would say is wrong as gas is cheaper but having done some comparisons with old bills I reckon the losses in my pipework to heat only hotwater mean that it costs as much in gas to heat the water as it does in electricity.  Either way, my gas is off for 5-6 months each year and saves me about £80 during that period based on what my old bills show over the summer hot water only months.  My guess is I get about £10 to £20 of top-up over the other months as well, hence my £100 estimate.  And of course I get a warm room when working from home.

You say you would not use the boost.  I do use mine on those days when the water is not quite up to temp to avoid those losses in the pipes if i power up the boiler.  15 or 30 minutes on boost at the end of the day to get the water to temp might take an hour using the boiler and heat the floors in the process...

At the end of the day the immerSUN (and those likie it) is a clever switch. I invested in a second unit at a good price when upgrading to the mk2, which I use downstairs in the in-between months to power 2 x 2.4kW oil rads.  I have mounted it on a nice wooden board (see link below) and having spoken with 4eco have a trailing lead with a 13A plug to connect it anywhere in the house.  I then have 3 trailing sockets for the 3 outputs with cable rated for the 3.2kW max output of the device.  I plug the rads into these.

This works amazingly well and means we can turn the central heating off about a month earlier and on a month later and still have a warm living room - and who cares about the rest of the house at that time of year anyway?  On a good day the rads can pump over 10kWh into the room.

I have 2 rads because a single unit will quickly reach its max internal temp and its thermostat will turn it off. You then have to wait for it to cool down a bit.  With 2 rads attached, the second one turns on and gives continuous heating. 

The immerSUN, plus wiring, plus 2 bottom end rads (you dont want fancy electronics as that would not cope with the varying output of the immerSUN) cost less than £400. 

Return on investment?  Hard to say as only installed in June last year, so only used in the autum but pumped about 200kWh through the set-up so far.  That could be 400-500kWh per year at whatever rate you want to apply to that - leccy rates could be too high but what are the losses of heating the pipes and at least warming some of the other radiators when you don't need heat in any of those other rooms at that time of year?  So, payback in perhaps 6 or 7 years at a guess?

I am a fan of the immerSUN and think it is the best of the bunch which I was lucky enough to decide on when my PV went in - I was not advised by anyone as the idea was quite new at that time.  I am sure the other makes also do a good job but for me the immerSUN seems to have the edge, not least its truesine technology and full compliance with all of the regs...

By the way, my system is 5.25kWp split 1/3 SSE-facing and 2/3 WSW-facing, with an annual generation of about 4.5mWh.

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MuppetKeeperComment left on: 29 January 2016 at 9:29 pm

Hi Graham,

I have a 4kw installation, eastish facing and installed my immersun2 in mid March 2014. I live in Leeds, so further North than the case study, and have diverted 1526 kWh to water heating since installation. 

When heating water, almost 100% of electrical energy turns to heat, with a gas boiler it is less. So with a good boiler, below 60 degrees you may be getting 90%, but above 60 degrees I believe the condensing stops, so efficiency drops to around 80%

I think your estimates may be a little low. It's likely you'll get nothing diverted from November to March, but I turn my hot water boiler off in April, and rarely need to turn it back on until end September...  which seems to be good.

Finally, in summer I'm generating 25kwh per day, but it only takes about 5kwh to heat my tank, so I still end up exporting!!


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paul53Comment left on: 27 January 2016 at 4:01 pm

i  have  a  similar  device  fitted  which  can  supply  to  2  loads has  reduced my  gas  bill  by  13 pounds  a month and  then  turns wall heaters  on . if  was  going to  fit  one  now would  want it  to  switch  at  least  3  loads,  expect  to  pay  about  300 pounds  fitted,700  is way  over  the  top

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smithgtComment left on: 27 January 2016 at 2:41 pm

I had the immersun installed with my solar PV array and the cost was around £500, but the online app and data upload module wasn't available at that time. The unit is currently on promotion at £325, and £99 for the online access to see the generation vs the import costs.

As much as I'd like to see data on the unit working, like Graham, I can't really justify this additional cost. I have online access to the PV Array, so know how much it's generating, and certainly through the summer months there was no need for gas to heat my hot water.

I've had my 4kWh system for 16mths and generated 4.39MWh, with the immerSun ensuring that only nominal power is uploaded to the grid.

As a product it does work well, but I'd like see be able to see how much of the generation has been diverted without spending any more money.  

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Graham Marshall

Graham MarshallComment left on: 27 January 2016 at 11:05 am

The timing of this article is very useful as we have recently had a 3,75 kW array installed and hope to maximise the use of the electricity we generate. The PV array is one of a number of energy saving measures we have undertaken another of which is to install a 300 litre heat store to provide domestic hot water and a heat source for our underfloor heating.

The main source of heat is a new 35 kW condensing boiler which uses piped gas purchased at 2.98 p per kWh. It seems silly to us to export to the grid any electricity which could be used to heat water so I an currently considering how to access this power costeffectively. Clearly, an automatic controller is the ideal answer but it needs to have a capital cost lower than the cost of the fuel which would otherwise be used to heat water. The first problem is to identify the likely cost of that fuel but there are so many variables this could only ever be an intelligent guess - unless someone out there has come up with a clever calculator.

We live in Scotland where the heating season is probably around eight months and the cloud cover is such that the availability of power from the PV array is significantly restricted. During the four summer months we will only require the hot water for domestic use.

In our first month of use (January) we have only produced more power than we use on two days and then only for an hour or two. My guess, therefore, is that, generously, betwen November and February we might generate an average of 10 kWh of energy which could be diverted to water heating, a total of some 40 kWh representing a saving of about £1.20.

Jayne's figures for electricity produced over three years are very interesting since she lives in a brighter part of the country than us and her house is oriented better than ours so her production figures counld be said to represent a maximum likely production for us.

Based on her figures we could expect to produce 1090 kWh per annum. Our base day time load is around 250 Wh which means we can expect to use around 1.5 kW per day or 550 Kw per annum maximum. In addition we use power for cooking and laundry which probably consume around 5 kWh per week or around 250 kWh per annum. In total, therefore we could be consuming as much as 800 kWh for general domestic purposes leaving about 300 kWh for water heating.

If we dod not use the PV power we would use gas instead at a cost of approximately £9 per annum I cannot see, therefore, how I can justify the £700 cost of an immersun controller as I would be about 120 years old before it paid for itself.

No doubt I have made some mistakes in my calculation which I hope simeone will correct.

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