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Review of ImmerSUN: using solar electricity to heat water

Posted by Chris Rudge on 27 November 2012 at 9:28 am

This relevant newcomer to the market of power optimisation of solar PV systems seems to have taken off in a big way due to facilities, design and marketing. Certainly this little unit is well packaged and documented. 

Unlike most of the other PV power optimisation units currently on the market, the ImmerSUN unit does not need you to install a new  immersion element or power reducing device. It detects how much power is being exported and sends that power to your immersion heater. 

As can be seen from the photos the ImmerSUN is quite straightforward to fit and install. It simply puts a CT clip on your incoming power cable and  diverts the immersion heater circuit through it. This, along with all the other units, is critical. Your immersion heater needs to have its own dedicated circuit. Our electricians have found immersion heaters connected to the boiler or socket circuits, which while it may not be dangerous, does mean they will need to be re-wired to function correctly.

The trade off for a simple electrical installation is the set up! There are a number of calibration processes to proceed through, one of which involves running off some of your hot water to ensure the immersion heater will switch on, though it is all straightforward stuff, it is a little time consuming.

Another feature includes the ability to ‘daisy chain’ up to 4 ImmerSUN units

While the cost of the unit is probably higher than most immersion switch units, its ease of installation and function makes it far more economic. It will cost between £500 and £700 fully installed (depending on the complexity of your installation).

The ideal location for installation is right next to your consumer unit (fuse box) as you’ll need to divert the immersion heater circuit through the ImmerSUN. Your electrician will open your consumer unit and modify the wiring.

The supplied CT (current transducer) clip is simply clipped over one of the ‘tails’ coming in from your electricity meter. This clip will measure the amount of power being exported from the house to enable the ImmerSUN unit to divert the right amount of power to your immersion heater. The supplied CT cable is around a metre long, which will be adequate for most purposes, but can be extended with suitable cable to around 50mtrs if needed.


Could not be simpler, once it's switched on leave it alone and it will constantly measure the amount of power being exported from the house, and divert this to your immersion heater. This could be a few hundred watts to start pre heating the water, right up to a full 3kw, depending on the size of your PV system.We have found from our customers that during sunny weather,  they have not needed to switch on their gas/oil boiler at all. The ImmerSUN has provides all their hot water needs.

There are a few features that are available in addition to the basic installation.  For electricity only users, there is the option to set up a timer to switch the immersion on, just like the old timer, plus a 1 hour boost function.  For other fuel users there are such functions as de strat pump function which will operate the aux relay for operating a pump to circulate water eliminating stratospheric layers in the cylinder.. All clever stuff!

Summing up

The ImmerSUN is so much easier to install than the other types we have installed, including our own Immersion Switch unit.  Though it has a higher purchase cost, you can use your existing 3kW immersion heater, and it ONLY uses excess power.

Being picky, I would suggest the manufacturers could look at options of being able to flip the case around to be able to bring the cables out the top as well. Sometimes it just works better to take cables out the top of a housing.

For those who like a bit more detail:

a standard installation you will have 3 cables to the unit.

1. 16A feed from consumer unit

2. 16A cable to Immersion Heater

3. The CT clip cable.

As you will leave the standard 3kw immersion heater installed in your hot water cylinder, once the above three cables are connected, you are ready to switch on and calibrate the unit for your installation.


Referring to the instructions, you are able to carry out the following one time system calibration:

1. Turn On Heater. The unit is now looking to measure your immersion heater current. You need to ensure the thermostat is on, ideally run off a bucket load or more of hot water to ensure the immersion heater is switching on.

2. Turn Off Inverter. The ImmerSun is now looking to see the direction of current flow to ensure it switches on only when exporting power. This aspect of the setup can only be carried out in full daylight, preferably sunlight, so the ImmerSun can detect a good current flow.

3. If no errors come back, then that’s it!. 

There are quite a number of alternative switching arrangements using the auxiliary relay that’s built into the ImmerSUN, but nearly all the installations we have carried out have been basic immersion switching.

More information about solar switching devices from YouGen

Readers tips of the month disagree about solar switching devices

Review of the Amelec intelligent solar switch

Solar PV Guide

Blogs on energy storage

About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.

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

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35 comments - read them below or add one

Gilly Jones

Gilly JonesComment left on: 13 October 2014 at 5:33 pm

Question from Mark: Can a immersun work in a house that has a hot water hot tank and a combi boiler that work in conjunction together? Would we need to turn off the hot water on the combi boiler and there for in theory could we run out of hot during heavy demand periods ie child's bath time etc etc

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MuppetKeeperComment left on: 29 April 2014 at 5:30 pm

@Gary.  I too find the carbon cost debate a little strange. 

As I see it, natural gas is a premium fuel, it is easily tranportable (now the infrastructure is in), it is very flexible but most importantly it is irreplacable.  

Electricity on the other hand can be produced by at least half a dozen methods, of which this site explains them all, and it's just the UK's position at the moment that only 34% of electricity comes from low carbon fuels.

Aside from the carbon debate, electricity is generated from less flexible fuels, such as coal & nuclear, neither of which do well at a domestic level, I'm pleased to see that in 2013, the UK burned less gas to generate electricity than in any year since 2006.

I'm all for doing the best we can for the environment, and I like to believe I'm thinking long term when I use an Immersun to heat my water from my solar panels.

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GaryComment left on: 22 April 2014 at 9:08 pm

I agree that whatever I export will be used in my locality but if one runs that all the way back to the grid generators then surely they will have excess production at carbon cost that will be lost as they cannot generate the exact amounts needed to supply the nation's needs.  The question is where does one stop the carbon footprint discussion? In ones home, the immediate neighbourhood or the country as a whole??

So, if I use what I generate as a subsitute for gas I can be sure that a fossil fuel is not used. Once it goes outside my house the comparison becomes increasingly unclear the further out one goes...

Time to duck again???


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Solstice Energy Ltd

Solstice Energy LtdComment left on: 29 March 2014 at 9:12 am


Regarding your last paragraph about reducing carbon - remember that if you don't use that electricity it is not 'wasted'. Exported power will be used by your neighbours (it will just take the shortest route to a load), and will make a contribution to someone else's carbon footprint, without them knowing of course!

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Cathy Debenham

Cathy DebenhamComment left on: 5 December 2013 at 4:20 pm

I put your question to ImmerSUN and this is its answer:

'It is normally good practice to have an immersion heater on it's own circuit for isolation and if it fails it therefore will not interrupt normal appliances etc. Situations where the immersion heater is not on a dedicated circuit can still have an immerSUN unit retro fitted providing the electrical safety is not compromised in any way. If you have any more questions please do not hesitate to contact me

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Gilly Jones

Gilly JonesComment left on: 4 December 2013 at 2:21 pm

Via email

Our current Immersion heater into our cylinder doesn't have its own circuit, and goes back to the fuse box. An electrician we have lined up is saying that he will put the Immersun near the immersion heater and use a wireless unit at extra costs. This didn't sound right. I thought the Immersun HAD to be connected to a Immersion heater with its OWN CIRCUIT. Is it ever possible to install an Immersun when the Immersion heater doesn't have its own circuit? Thanks for replying asap, before we fork out on something that may not be wired correctly! Kind regards, Paul

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GaryComment left on: 4 November 2013 at 8:25 pm

I thought I would add my experience to this thread as I have now had my Immersun on a 3.5kWp system on a west-facing roof since the end of June.

The unit cost me £350 including VAT (installed) and has used about 220kWh to date and allowed me to use no gas from the date of installation until part way through October.  Since then it has been a question of how much sun one gets each day and the Immersun benefit ranges from nothing to pre-heating or fully heating the tank (as happened today)

What I don't think has been mentioned here is the fact that using gas to heat hot water only, in the summer, is pretty inefficient as one must heat all of the pipework to and from the tank and that heat is simply lost to the floor or walls. 

While not scientific by any means, my experience is that I was using spending about 30p on gas each day to heat water in the summer. 

What I see with the Immersun is that on days when I am sure the water has been fully heated (i.e. I see the green light burning continuously rather than flashing on the Immersun) I use between 1.5 and 2.5kWh of electricity out of the 15-20kWh I have generated.  This suggests that if my use of hot water was greater, or the tank less well insulated, I was generating enough to power the immersion heater for longer, in which case payback would be quicker!

If I see that I have used less than 1.5kWh I will boost the immersun for 30 minutes (max 1.5kWh or until the water is hot) to ensure the water is hot enough.  Over the period of no gas use I had to do that only 2 or 3 times.

So, on the basis that I have saved about 30p per day over the July-Sept summer period, or about £27, I might expect to save about £70 per year.  Ignoring inflation that would repay the unit in about 5 years, which seems fine to me.

But however the actual maths works out, using the Immersun means my carbon footprint is lower than it would have been and for an additional payment of just £350, that really should be a consideration for anyone thinking about green energy...

Probably time to duck now, when the responses fly in...


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Rudge Energy

Rudge EnergyComment left on: 15 July 2013 at 9:54 pm

For followers of the Smart Meter saga, try here  to get the ball rolling. The project was handed over to DECC in April 2011 where it seems not a lot has been decided on direction yet.

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girishgraoComment left on: 15 July 2013 at 8:38 pm

Smart meter effect:

It seems to me that this issue of what happens when Smart meters are installed to the 50% deemed export benefit of 4.5p/kWh.  If that payment is going to be done only for you actually export, after smart meters are installed, then as commented by Solastic Energy, it would not be really worth to invest £ 400-500 on a proportionate controler like Immersun or Sola controla. Does anyone know for sure what is going to be the official policy of the Govt on export payment after smart meter roll out?

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arwooldridgeComment left on: 18 June 2013 at 10:02 pm

I woud like to clarify some points from this thread.

Several commenters mention the reduced cost of several simple switchers.

This is however more than offset by the extra power that the true proportional controllers recover. I estimate this as some 2 to 3 times as much power recovered by a good proportional controller.

There are only two true proportional controllers in the market at reasonable price.

They are the Sola Controla PV immersion controller and the Immersun.

The Sola Controla was first to market and the latest v3.0 has typically only 40 watts export power (deliberately biased to export to avoid any import) without hunting or flickering. With over 18 months fault free continuous working in the field.

The Immersun will export around 120watts or more and as mentioned by one commenter has severe flicker. This is clearly visible if you feed a standard "naked" heater element the element glow will flicker wildly.

Another  main differences is that  the Sola Controla is all analogue and very high accuracy it works out of the box with no setup or calibration required.Once the CT is fitted the correct way round, which instructions clearly specify,The unit is ready to go to work.

The  competitor is a digital controller and needs a setup procedure.

Analogue systems cannot suffer processor lockup or need rebooting.

Another commenter mentioned the very tight wiring and small terminal sizes. 

The Sola Controla has large terminals which will take 4mm wire and has adequate creepage distances unike its competitor.

The Sola Controla has full IP splash rating and is fully insulated.

The competitor had IMO a potentially lethal unearthed front cover, lets hope they fixed that by now.

The Sola Controla has published MTBF figures ( the designer spent years in military high accuracy systems where accuracy and nonfailure are a matter of life and death) which the competitor could not match as it still uses a relay for auxilary output.

The Sola Controla is rated at a conservative 4kwatts, deliberately as its the most popular PV system power agains the competitor's 3kwatts.

It means you can overload a Sola Controla even with two 3 kwatt loads in parrallel as the power is limited to the peak PV power available.

Slave units can be added to the Sola Controla immersion heating controller at very low cost. to achieve the lowest cost per proportional kwatt controller in the market. The best auxiliary load for this is a storage heater.

Simple visible led bar graph display of output can be read from a distance.

Systems designs up to 27 kwatt three phase are available.

Declared interest as designer of the Sola Controla.

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Rudge Energy

Rudge EnergyComment left on: 9 June 2013 at 3:24 pm

Hi Graham

Though the Immersun (and similar type) units come with their own manufacturers warranty, getting an extended insurance backed warranty such as the type that is already in existence for PV systems may be difficult as PV insurance is a package put together by insurance companies for that market. Would you not buy a TV if it did not have an extended 10 year warranty. Plus the TV does absolutely nothing for your house and living conditions. Purchasing anything is about perspective. We have found the perspective of our customers are based of effective use of generated power and a real interest in renewables and how they can be integrated into our lives.

I think it is a bit pessimistic to assume the unit will only work for its warranty period. Most white goods are designed for a long life, but the statutary 12 month warranty is only applied and we still buy them. The Immersun guys have taken it a stage further as a sign of product support.

Realistically, if cash is the motivator, it all comes down to how much hot water do you use? We are currently getting calls from people telling us their Immersun has heated the whole tank by lunchtime and are keen to install more facilities for hot water storage. Most households only have hot water for hand wash and baths, but commercial applications can use a number of Immersun units to provide hot water for washdown and other options too.



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Graham.SelseyComment left on: 9 June 2013 at 9:53 am

No one has mentioned total cost of ownership. If the unit is only guaranteed for 3 years, then there could be a hefty replacement cost equal to buying the unit again.

If the installed unit originally cost £600, then over the 3 years this equates to £200 per year cost against any savings.

At best you my have a repair bill for less than £600, baring in mind the box may have to be sent away for repair and an electrician would have to visit twice, once to diagnose that it is broken and send it away for repair, the the electrician would have to come back a second time to fit the repaired unit.

While you are without the unit, no free heating of water! Another cost...

And how long the repair will take and how long to get the electrician back is another question, not quick I suspect! 

Repaired units may only have a one year warranty, this would need to be checked out before buying a unit.

All in all you may lose any benefit in savings made.

Any warranty terms need to be read carefully to see if there any limitations which may mean you have to pay for part of the repair. The electrician's costs will not be included, and that alone for 2 visits could cost circa £300.

So check the warranty terms before signing up.

Also you may be able to negotiate an extension on the warranty to 10 years, which i did when I had my solar panels fitted, for the panels and inverter, at no extra cost!!

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Cathy Debenham

Cathy DebenhamComment left on: 26 April 2013 at 8:52 am

Thanks Birty. It's always really interesting to hear how other people are getting the best out of their systems. Do you have a mixer tap feeding into the hose so you can control the temperature. I have this vision of lots of shrunk washing from that lovely solar heated hot water!

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birtyComment left on: 22 April 2013 at 3:33 pm

My Immersun was installed (retrofitted) to a top mounted 3kw immersion coil. It usually dumps between 1 and 4 kwh per day into the tank, so the gas boiler has less work to do. I also now regularly use a hose from the kitchen tap to fill my washing machine through the drawer with the freely heated water and then select 'cold wash' so the power-hungry heating element in the washing machine never activates.  My central heating's 'hot water' is now set to come between 6 and 7pm in case the sun was not strong enough to heat the whole tank, but most days it doesn't need to do anything.  I had to do some research and call the installers back to also install a destratification pump on the cylinder. This can also be powered from and configured in the immersun settings and it simply pulls hot water from the top of the tank and puts it back in at the bottom. This is because if you have a top-mounted coil, it will shut off after a relatively small amount of heated water rises to the top of the cylinder and opens the thermostat on the coil.  Now my immersun will cycle through heating / mixing / heating / mixing the water in the tank.  It works really well, and my savings are being maximised by filling the washing machine with pre-heated water via the drawer and then selecting 'cold wash' option.

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jccj18Comment left on: 19 March 2013 at 11:02 am

A friend of mine had his immerSUN installed and it caused a flickering in his entire neighbourhood?! Has anyone else had this problem?

I got a bit put off by this and went an ordered a Solarimmersion switch, the mark 3. Got a coupon code JC1803 and it gave me £10 or so off so was basically free delivery.

It came last week and is running perfectly, brilliant well built product. Going to monitor it for the next couple of weeks and hopefully get my mates one too.  Its a hell of a lot cheaper and seems to work better??

Any thoughts or advice welcome?

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Rudge Energy

Rudge EnergyComment left on: 7 March 2013 at 6:44 pm

Good to see your comments Heathtim  As with any of the many immersion heater power diversion devices out on the market right now, the amount of payback/usefulness really depends on what your hot water needs are.

We often get calls from people asking what the 'payback time' will be. No one can honestly answered that question, but Heathtim has given us some useful figures for guidance... Thanks


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PANTHERComment left on: 7 March 2013 at 4:57 pm

I use a 3.3Kw 110v transformer to supply voltage to the 3.5Kw Imersion heater, this heats the water in the cylinder and only use,s 670watts from my 2.7Kw PV supply. This unit is fed through a 13A timer and set it for 4hrs between 12 and 4pm. switched the gas boiler off and just use the electric.Our total year return paid for all the gas,Elec and the Water rates. I looking for a more accurate way to control the Transformer.Are any companies other than Immersun and Earthwise that do a control.

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Cathy Debenham

Cathy DebenhamComment left on: 6 March 2013 at 10:06 am

Thank you Tim. Good to have real life figures.

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heathtimComment left on: 6 March 2013 at 9:42 am

I was one of the very first Immersun customers and and have been delighted with the performance.

To date, it has used 460 kW of electricity from my 4kW PV system - that's in about  8 months - and with almost no sunshine in the last 3 months!!


The math is simple - not only have I saved the equivalent costs of 460 kW, but during last summer, our thermal store (450L) was usually either fully heated or heated enough so that no further energy input was needed to have hot water.


I have read the other posts with interest and can only summise that there are factual errors in a number of posts.  Two spring to mind - if you have an old meter and it spins backwards when the sun is out, then you will have a new electronic meter fitted as sson as the FIT team realise that is the case - their software can't cope with negative readings!

Talk of payback taking tens of years is just nonsense - clearly it depends on the size of your PV installation and how efficient it is, but I expect mine to payback within 5 years maximum, purely based on the amount of kW's it has diverted.

That doesn't take into account the increase in electricity costs from the suppliers in the next 5 years, or inflation, or , as many consumers will be finding out, standing charges that are being introduced to keep supply prices down!

Which ever way you look at it, it's a no-brainer!

To those that question the costs, I would ask if you've actually got one yourself, and ask the simple question:

Why would you prefer to lose / not use all the electricty you generate, rather than divert it (intelligently) to save using other energy sources to achieve the same end?


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Nick Hanna

Nick HannaComment left on: 24 January 2013 at 1:50 pm

Hi, I just called Earthwise to ask approx costs on the Solic 200 and they said about £550-600 installed, when I said that was more than the Immersun they said their product is designed to last the lifetime of your panels EG 25 years (altho they only give a 10 yr guarantee) and is very robustly built.

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Sims Solar Ltd

Sims Solar LtdComment left on: 24 January 2013 at 11:55 am

One aspect of these type of devices that I have been struggling with is the VAT status i.e., are they supplied and installed at the 5% or 20% rate.

There seams to be a general view that they can be charged at the reduced rate, citing Para 2.6 of VAT Notice 208/6.

I've written to the VAT man on this and they have discounted Clause 2.5 i.e., it is not an energy-saving material.They have not been very helpful and not prepared to answer specific questions definitively.They do suggest I refer to Para 2.6 but in the spirit of what these devices are intended to do are they electronic timers to heat hot water? Yes they have a timer function, but that is a back up function.

I would be interested in any definitive experience as to the VAT status when installing, has anyone been challenged by the VAT man and sucessfully defended a 5% rate?

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Helen Bird

Helen BirdComment left on: 7 January 2013 at 10:20 am

There seems to be loads of new products coming onto the market that are in direct competition with the Immersun. I've investigated a fair few, but feel drawn to the SOLiC 200 with a 10 year guarantee - sounds like it's been built to last. I'd be interested to hear about any other contenders.

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Cathy Debenham

Cathy DebenhamComment left on: 7 January 2013 at 9:38 am

Hi Mikey

It is the responsibility of your energy supplier to install a meter that is fit for purpose. If your meter is running backwards it will need to be replaced. I suggest that you inform EDF that it is running backwards, to avoid any arguments down the line when they (inevitably) work it out for themselves about how much you have benefited. Click on the link to read more about electricity meters running backwards - there's lots going on in the comment section too.

My advice about FIT suppliers would be to choose on customer service rather than on hiding the fact your meter goes backwards: read what we found here. This was confirmed by Which? here.

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Mikey_mComment left on: 1 January 2013 at 9:53 am

My meter runs backwards when I am exporting. That would seem to nullify the benefits of the Immersun device that I have, I.e. I am being paid for my exports at the retail price of electricity. Have I been sold a pup, or will EDF soon get wise and replace my meter? My FIT supplier is British Gas, as advised by my PV installer, the reason being that EDF will be less likely to find out about my meter running backwards. But that is the same installer that sold me on the benefits of Immersun.

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Mikey_mComment left on: 19 December 2012 at 12:23 pm

I just had a 4kW system installed. I paid an extra £500 for the Immersun, but they haven't instlled it yet as I am informed that the manufacturer is having trouble keeping up with demand. The installer told me that he has installed lots of Immersuns lately. The supplier who didn't win my contract told me that he was skeptical about such devices but, having reviewed the latest Immersun spec, said he will be offering them to all his prospective customers in future.

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Rudge Energy

Rudge EnergyComment left on: 11 December 2012 at 9:47 am

One of the things to consider when thinking about getting an ImmerSun or similar device is quite how much hot water you do use. If your a very low user like myself... washing machine, dishwasher and electric shower.. this leaves only a bit of hot water being used for hand wash, etc, then financially the unit will take many years to pay back its installation cost. But there is a good feeling to simply be using the freely available power you are generating yourself rather than being beholden to volatile fossil fuel supply and cost.


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Sims Solar Ltd

Sims Solar LtdComment left on: 10 December 2012 at 11:07 pm

Yes, I understand the dilema, classic half full/ half empty. I suggest all installers are still learning and our opinions change with experience. All you can do is research and make a decision based on what you believe is the most appropriate solution.

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Mikey_mComment left on: 10 December 2012 at 7:15 pm

The problem with all this, to a consumer like me, is that the experts never agree on what is a reasonable set of assumptions. Thus, some experts will swear that an Immersun is a good investment whereas others say it isn't. There is a similar problem with the use of a voltage optimiser with a solar pv system.

one thing to bear in mind with all these payback calculations:- fr someone like me who wants to invest for income, the returns on an annuity are 3 to 5%, taxable, and the returns in savings accounts that provide monthly income are about 2% gross. Against that background, a p ay back period of 7 years on a system that continues to generate inflation proofed, tax-free income is pretty good.

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Solstice Energy Ltd

Solstice Energy LtdComment left on: 3 December 2012 at 11:27 am

2019 is the proposed year for Smart meters to be fully installed, and this is to take place from 2014 - 2019. I would expect PV owners to be first on the energy companies radar, and to calculate the financial returns of an immersion switch based on 7 years of deeming is risky.

I've crunched some numbers and found that an ideally situated 4kW PV system, with 31% of generation used by other appliances / lights etc, will cover the £500 installed cost of an Immersun Switch in just over 6 years. This is allowing for 5% increase on gas prices (as Chris @ Rudge rightly suggests).

The 2kW PV system comes nowhere close to covering the installed cost.

If you'd like to see these calculations in more detail please email me;

Otherwise, this is how I've worked it;

I have taken the monthly output predictions from a PVSYST report for two ideally situated PV systems,  a 4kW and a 2kW system. Total generation for the year is 3430 and 1716kWh respectively).

Then I deducted  3kWh / day use in the building  from electrical appliances on timers etc (which everyone still agrees is the best use of the power generated). This 3kWh/day (note that some Winter months don't even support that use) equates to 31% of total PV generation (of the 4kW system) being used in the building.

Cylinder requirement has been calculated at 7.7kWh / day (150 litres of water from 15deg to 60deg C). Add up the remaining available kWh which can be directed to the immersion heater each month, and there is an annual total of 1819kWh (with 4kW PV) or 764kWh (with 2kW PV) being saved  from the water heating bill.

For the 4kW system, first year savings are 4.3p x 1819kWh = £78.22. At 5% annual increase on gas prices this  is a saving of  £337 after 4 years, and £618 after 7 years.

For the 2kW system, these figures are: 4.3p x 764kWh = £33. After 4 years this is £142 and after 7 years this is £259.

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MetgenComment left on: 30 November 2012 at 11:08 am

Don't forget the additional saving that you can get from switching off your traditional HWH altogether during summer months and thus extending the boiler lifetime

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Rudge Energy

Rudge EnergyComment left on: 29 November 2012 at 8:04 pm

@Ashtreex.. The timer system sort of works if you have a 4kWp PV and you only use the system during the summer. These last few weeks have been really dismal for solar, and you would have been buying all the power to heat the water at circa 14p a kW. Keeping to gas at 4.5p (currently), for this time of year will be more cost effective. Oil or LPG will still be cheaper, even though around 8p a Kw.

@Solstice Energy.. Your quite right on the economics with the current cost of gas, but there is a good probablilty that gas will be around 7p a Kw at least by the time Smart meters get rolled out in 2019 (source) which is far more likely than 3 years time as the final Smart meter spec is still yet to be decided by DECC. Oil & LPG will be far higher! The gov currently have far reaching plans to massively increase the UK's dependance on imported gas, guaranteeing future price hikes.

Don't forget that the majority of PV owners will actually be on a deemed 3.5p/kWh export. The 4.5p rate only applies for installations over the last couple of months!

Currently, using an ImmerSun during the peak solar months from a 4kwp PV will provide enough domestic hot water for a couple of people without having to burn gas. Currently, until Smart meters are installed in 2019, all small PV systems will simply have their exported power calculated from generated power, so it makes economic sense to use as much of your generated power as possible. The Immersun ensures this!

Look out for better innovations in the ImmerSun over the next few months, such as  wireless sensor clip, which will enable you to fit the ImmerSun near the hot water tank without extending cables..

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AshtreexComment left on: 29 November 2012 at 7:08 pm

I still think we've got the simplest answer, certainly the cheapest.

We had a timer fitted to our hot water tank to switch on the immersion heater (instead of the gas boiler) for 2 hours during the middle of the day, come rain come shine. There's always some pv electric being generated. The tank is very well insulated. Switched off the gas water heating, that's all we need for our hot water. 

How well does it work? Well, I can only say both gas and electricity consumption are less than pre this idea.

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Ian Smith

Ian SmithComment left on: 29 November 2012 at 2:53 pm

Is it reasonable to assume that the deemed export basis for PV FIT payments for systems less than 30kW will be flipped to a metered basis once a smart meter is fitted?  My understanding is that the smart meter programme is still scheduled to be complete by 2019 so people could be seeing these meters fitted in as little as two or three years' time.  If so, one could find that diverting physical exports with devices such as these will cause loss off export income - which they do not at the moment.  The payback equation on the investment may then look rather different to what it does today.

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Solstice Energy Ltd

Solstice Energy LtdComment left on: 27 November 2012 at 9:22 pm

Why would you want to save 4.3p / kWh gas at the expense of a 4.5p / kWh export rate for electricity? Immersion switches might save a little bit of money before smart meters are installed, but let's imagine you are fitted with a smart meter in 3 years time (1095 days). In that time you might save 15p / average day on your domestic gas bill, so you will save 1095 x £0.15 = £164 over three years.

So unless you don't have a gas supply (or have a very inefficient boiler) I'm not convinced by PV immersion switches, and still think that the best use of PV power is to try your best to offset other electrical appliances during the daytime.

The carbon footprint of the house might be reduced by decreasing gas dependence a little bit, but exported electricity is never wasted so there is no environmental argument for immersion switches either.

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Sims Solar Ltd

Sims Solar LtdComment left on: 27 November 2012 at 3:34 pm

We've installed several of these and to add to Chris's review they are straight forward to install accepting Chris's comments on the need for a dedicated supply to the immersion. The unit is compact and access for wiring is tight, particularly if you are running 2.5mm2 or larger cable and you are cascading to a second immersion or load which this device can do. 4EcoLtd the manufacturers have recognised this and a new model to be launched for March next year will give more space plus other functionality I'm told.

Because the calibration is looking at the direction flow of current because the CT clamp in bi-directional, it does mean set up can be done on a cloudy day. Some of the other units on the market need an exported power of >1.5kw to the grid during set up and to confirm correct operation which is a bit hit and miss at this time of year.

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