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ImmerSun - £345 or Fitted uk wide For £545 inc VAT

Unfortunately we have suspended Immersun sales until further notice as the manufacturer appears to have has gone into administration.  Please contact us if you would need advice on an alternative system. Sorry for the inconvenience.

ImmerSUN for £345 inc VAT or ImmerSUN fully installed UK wide for £545 inc VAT.  ImmerSUN solar immersion heats your hot water for free using the spare energy that you would normally export to the national grid.  You will typically see a money-back return on the price of the unit in about 3 years or less; with oil-fired heating ImmerSUN 2 has a payback of less than 2 years.  An ImmerSUN is simple to install and uses your existing immersion heater. We can supply an ImmerSUN for you to fit or we can arrange an ImmerSUN installation using our national network of local qualified Part P electricians. The ImmerSUN solar immerison heater has a 3-year manufacturer's guarantee and with our Green Tick installation warranty you have complete peace of mind. Shipping is £4.99.  Click here for the ImmerSUN datasheet





Buy ImmerSUN NOW for £345 inc VAT - click here

UK Wide ImmerSUN Installation from £545 inc VAT - click here




ImmerSUN Benefits

  • Heats water from your PV system or Wind Turbine
  • Increases your return on investment
  • Payback of between 2 - 5 years
  • Uses 100% of your generated electricity - you still get your export FiT
  • You no longer needs to be at home to use the free electricity
  • Automatically adjusts power levels to the immersion heater
  • Keeps exported power levels to virtually zero
  • Uses your exisiting 3kW immersion heater
  • Simple to install
  • Built-in hot water boost timer
  • LCD display - shows energy saved, output power and mode
  • Always exports a small amount of electricity so you never import from the grid
  • British product with a 3 year manufacturer's product warranty
  • Click here for the Immersun Datasheet


Immersun 2 Wireless Transmitter


Add the ImmerSUN wireless transmitter if you need to operate the immerSUN remotely, or if your immersion heater is taken of an existing circuit and it does not have a dedicated cable going back to your consumer unit.  The ImmerSUN wireless unit will allow you to operate muliple ImmerSUN devices at the same time, cascading power to separate immersion heaters or other appliances.


Buy the ImmerSUN wireless transmitter for £49.95 inc VAT - click here





Add the myimmerSUN remote bridge to your purchase to enable use of remote monitoring for your immerSUN and solar PV system.  Now information about your energy generation, energy usage, money saved and energy diverted to heat your water can be viewed on your tablet, smartphone or via your web browser - historical or in real-time.


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Add the myimmerSUN data logger for £120.00 inc VAT - click here


Immersun warranty

The ImmerSUN carries a 3 year manufacturer's warranty and is matched with our Green Tick guarantee, which gives you complete peace of mind.  Click here to review our Green Tick installation warranty and guarantee.




ImmerSUN Review - Mr Harper in Aberdeen: The great thing about compulsive record-keeping is that you know to the last penny how much you save on power bills after you install various energy-saving measures. In the calendar month of June 2012, before we installed our 4kW solar-PV array, we used 604 units of electricity. In June 2013, after we had installed the array, our electricity consumption dropped to 308 units.

Halving our electricity bill was excellent, but we felt we could still do more, so we asked the Energy Saving Trust (EST) to survey our house. The consultant spotted that our electricity bills soared in summer months. We explained that that was because we always switched off our central-heating boiler from April 1 to September 30 each year. There is so much glass in the house that even the slightest sunshine heats it comfortably without a boiler. The downside to switching off the boiler was that all our water from April to September had to be heated by an electric immersion heater: an eye-watering expense.

The EST man suggested that since we had a solar PV array already, we should install an ImmerSUN 2. He said this would take our surplus solar-generated electricity which we were exporting straight to the national grid and would divert it via our hot-water tank. Only once our tank was up to temperature would exports to the grid resume.

We took his advice and asked Big Green Technology to install an ImmerSUN 2 for us. The result? In June 2014, our electricity use dropped even further ... to 199 units. In other words, our monthly electricity costs have dropped from £92 (at current prices) in June 2012, to £47 in June 2013, to just £31 in June 2014. We have to hit the ImmerSUN's "boost" button on dull days, using grid electricity as usual to heat our water, but the water is being heated from a higher ambient temperature so, even on dull days, electricity use is lower.

Obviously the savings will be less in winter months than in summer, but we think the ImmerSUN 2 will save us an average £16 a month from April to September, and an average £5 a month from October to March. We reckon our annual saving on hot water will be £126. This means we can lower our monthly direct debit to the power company by £10, and the ImmerSUN will have paid for itself in five years. Note that these figures apply in the gloom of northern Scotland. In sunnier climes, which is just about everywhere else, gains will be greater and the break-even point sooner.

I should also explain that even although we are exporting less to the grid, the export part of our solar payment from the power company is not reduced. We still get the same money. That's because any solar PV array that does not have an export meter (as ours does not) has its export payback fixed at a nominal 50% of the standard export rate (currently 3.39p per exported unit). So, it's a win-win for us.

One final note of praise to Big Green Technology (BGT). Most normal ImmerSUN installations go smoothly first time, but our installation was not simple or standard because of the way our house is configured and its electrics were installed when it was built. Time after time, the ImmerSUN installation refused to work. We had three visits from the electrician who tried various things in conjunction with the manufacturer's helpline and BGT's people and still it refused to play. Eventually, a 10-second power cut in our village seemed to kick-start it for some mysterious reason and it has performed beautifully ever since.

Plenty of companies would have pocketed the money after the first abortive visit and left the customer to sink or swim. Big Green Technology stuck with it, even when everyone seemed defeated, and promised they would get it working whatever it took and whatever it cost. That is exactly what they did and we can't sing their praises highly enough. This is the sort of company you want to do business with again and again. And, no, we are not related to anyone at Big Green Technology or Immersun.

ImmerSUN Review - Fitted in Sheffield by Mr Robert Rawson:  Thank you for supplying and fitting an ImmerSUN for me. It's brilliant and I'm really pleased with it. It's only been in 16 days, but has already chucked 73 kilowatts in to warm my water, which would otherwise have gone to the grid. The electricians you sent to fit it were spot on; very confident with their work, polite, witty and generally very like able. Their work was neat and tidy and they gave me an air of confidence that it would be safe and work properly. I would recommend them to anyone having an ImmerSUN fitted in the north derbyshire/south yorkshire area, or indeed any type of electrical work. One was called Wayne Allott and the other I only knew as Carl. First class electricians - and they cleaned up as well! Once again, thank you and please feel free to use this letter as a testimony both for you as a company and the calibre of your workforce.

ImmerSUN Review - Fitted in Suffolk by Mr John Herring: ImmerSUN is an amazing little box of tricks, I must admit I was sceptical that the ImmerSUN was going to be as good as you said but I was wrong, ImmerSUN really is brilliant.  We have had it 3 weeks now and it has done all of my hot water heating, it puts in about 4KwH into the hot water a day, absolutely amazing and as I am on oil fired heating I recon I will get my money back really quickly, we have literally turned the boiler off, fantastic. The fitters that arrived were excellent, very good communication, tidy and fast.  ImmerSUN is great and your company was first class, we are so pleased we did this.

ImmerSUN Review - Fitted in Shetland Isles Scotland - our most northerly installation by far by Jane Cosegrove: We live pretty far from any regular power supply so it was really important to get the most out of our solar PV system.  We have a generator and an oil supply to heat water, but this is really expensive in Shetland. The immersun was recommended as it can heat 2 water supplies, so we had a second tank installed - with our 32 Kw system - we have enough power to run the house, charge the batteries and all the spare goes into the hot water, which is fantastic.  Since we have had it fitted we have not had the oil boiler on once. We were so impressed we have had another put in since and it now runs in cascade, which means it now heats underfloor heating as well.  The immersun is fantastic, I would recommend it to anyone.

ImmerSUN Review - Fitted in London: by Vic Walsh: To be honest I was a little sceptical, but after reading the reviews and chatting to the team at Big Green, several times, I decided to go for it.  After buying the install online the ImmerSUN arrived the next day and true to their word I had a call from a local electrician that afternoon.  John who only lived 2 miles away admited that he had never fitted one before but true to his word he had it in and running within the hour.  He gave me an installation certificate and 2 days later I got the warranty and VAT invoice through from Big Green.  I have to admit the ImmerSUN is absolutley fantastic and it has exceeded my expectations.  As I have a rather large array (4Kw) I got John back and he connected my undefloor heating up to it as well, so when it has finished heating my hot water the spare power goes into the kitchen floor making it nice and warm - really impressed - Vic Walsh

ImmerSUN Review - Fitted in Norwich - by Andrew Anderson: Amazing technology.  Simple to install, really effective and a brilliant way to use the spare power from your solar PV system.  I have a 4kw system and a 300 ltr hot water tank.  It has been heating the tank without fail since March this year and its now October.  Really impressed with the results.  We are on oil so I recon it will pay for itself in 13 months.  If you have solar PV - get one of these now. 

ImmerSUN Review - Installed in Northern Ireland by Terry Adams:  The ImmerSUN arrived within 3 days and the guys from Big Green Technology had it in and installed within the week.  Our Solar PV system is 8kw and facing straight south.  The immersun got straight to work and has already clocked up 32 KWh in 6 days, and its winter, so really pleased with the hot water it is producing, the immersun is going to save us a fortune.

ImmerSUN Review - Fitted in Aberdeen - by Kendal Jones: ImmerSUN 2 is fantastic, we wanted the latest technology that would make the most of the 12 kw PV system we have.  The ImmerSUN 2 heats 2 immersion heaters and our underfloor heating.  We got it with the wireless transmitter and it transmitts to 2 ImmerSUN modules, one our normal hot water tank and another spare tank we put in to run the washing machine and dishwasher, it does both really well so we are now saving energy heating hot water and saving power on our appliances.  Its great, get one now - I really recommend it. - Kandal and Jim Jones

ImmerSUN Review - ImmerSUN running my hot water and underfloor heating - By Andy Jones Aldershot: We chatted to the Big Green team and they recommended the ImmerSUN 2 beacuse we wanted to use the power from the 2 solar PV system we have to heat the water and 2 underfloor heating areas.  We ordered online and within 2 days the unit arrived really quickly.  The local installer got in touch within the week and we arranged the install date direct with him.  Ted was really professional and he left a really neat and clean job.  The ImmerSUN 2 now heats our hot water then sends spare power to the underfloor heating in the kitchen and then the bathroom.  What is great is that I can change the priority when I like with the press of a button and send the spare power to any place I want.  The ImmerSUN 2 is a fantastic box of tricks; I thoroughly recommend it.

ImmerSUN Review - Fitted in Bury - by Adam Shepherd:  We liked the ImmerSUN beacuse of its simple design and display.  The unit tells you how much energy you are diverting and how much money you have saved.  The ImmerSUN just sits in our cuboard and I am constantly amazed at how much hot water it produces, free of charge.  The Big Green team were excellent and the local Electrician 'John' was friendly, tidy and very quick.  Im an nothing but impressed and I would recommend the ImmerSUN to anyone with solar PV.

ImmerSUN 2 Review - Fitted in Saltash - Irene Skittle:  Our neighbours had an ImmerSUN fitted by these guys, it was a really professional job and as we are all on oil heating it was an obvious thing to do. We talked to Big Green Technology over the phone then ordered the unit, which arrived within 3 days.  Dan, a local electrician turned up the follwoing week and installed the ImmerSUN.  It worked immediately and we have now turned off our boiler.  We are saving an absolute fortune.  The ImmerSUN and service is highly recommended - Yours Irene.

ImmerSUN Review - Fitted in Southwald - by Andrew Baynes: The ImmerSUN version 2 arrived really quickly and the electrician came the following week to install the ImmerSUN.  The job was really quick and the immersun was producing hot water almost immediately.  I have since turned the boiler off and the ImmerSUN is now providing all our hot water.  Awesome piece of equipment.  I would recommend the ImmerSUN and I would recommend Big Green technology.  Really good service - Thank you.




An ImmerSUN unit takes any electricity not used by the house and sends it to the immersion heater in the hot water cylinder.  This is really smart as it will save you having to heat your hot water, which will save you money.  The time taken for payback of the purchase price varies on usage; it is approximately 3 years with gas central heating. If you are on oil or just electricity then this system will save you a lot more, therefore paying back much more quickly. If you have solar PV or a wind turbine then you need an ImmerSUN.  ImmerSUN monitors the electricity which is about to be exported to the grid and diverts it to your hot water immersion heater. It does this automatically so there is no need to turn appliances on and off to make use of your spare electricity. If the load from the house increases, for example the washing machine is turned on, the ImmerSUN will reduce the amount sent to heat water to help you maintain independence from the grid. The ImmerSUN measures and records energy applied to the immersion heater. As only the renewable energy is counted, this is effectively saved energy. You can view savings made for today, the week, the month, the year or total savings. Have a look at the ImmerSUN review 








Without ImmerSUN, most of the energy produced is wasted energy when a solar PV system is installed.  The ImmerSUN unit tracks the generated energy, ensuring that nearly all energy available is used. With ImmerSUN, we have been installing solar thermal hot water and solar photovoltaics since 2009. In our experience, a number of homes do not have the space to fit both - this product allows homes to use the maximum energy saving from their existing PV system at a fraction of the cost of a solar thermal installation.

Immersun 2 Trusine Technology




Buy an ImmerSUN NOW for £345 inc VAT - click here

UK- wide installation from £545 inc VAT - click here




James & Linda Hoult - Retired Couple living in Stirling Scotland


We have had an ImmerSUN installed by Big Green Technology.  It is fantastic, let us explain why.  Our 3.8kW Solar PV system was installed about 2 years ago and as we are both retired we were able to maximise its output, or so we thought.  About a year ago our electricity provider installed a Smart Meter and during 2012 it enabled us to establish that we generated 2548kw hours and of that we exported 1820 kW hours.  So, in fact we only utilised 728kw hours of the total amount we generated. 

This came as a shock as we always read the PV output display before we used the various domestic appliances in order to maximise the benefit and minimise the export of power. We even waited for sunny days before using particular appliances throughout the house and my very large power hungry woodworking machinery with which I can convert a tree into chairs.  As we had the time and opportunity to do this we were under the impression that we were indeed using most of the power our system generated.  The terms of our contract with our provider state that they allocate 50% of what we generate to be deemed as "Exported" and allocate a 0.03p tariff in payment of that amount. The smart meter results demonstrate that the 50% in our case is flawed, and the provider profited to the tune of 546kw hours in that year.  When I contacted them the provider initially stated that they could not accept the export reading from the Smart Meter as it was not accurate enough. It was only after contacting Offgen that we established that the reason was that the new meter was certainly accurate but had not yet been approved. I have little doubt that the present 50% arrangement seems at first hand to be rather generous, but we seem to have shown otherwise.

With that in mind the search was on for a means of addressing the leaching of power to the provider, and I have recently had installed an Immersun device by Big Green Technology. At the time of writing it has been in operation for 2 weeks or so and has successfully diverted 44kw hours into my Heat Store Tank. The domestic heating system we have takes heat from vacuum water heating tubes, a wood stove, a kero boiler and finally an immersion heater.

Prior to installing the Immersun, no mater how sunny a day it was, the boiler would come on during the evening to supplement the heating. These last 2 sunny days our Heat Store, with the assistance of the Immersun, attained a temperature of 82 degrees C and that level kept us warm throughout the evening without the need for the boiler having to activate.

After purchasing the Immersun from Big Green Technology it arrived 3 days later.  The installation was included in the cost.  They sourced a local qualified Part P electrician to installed the Immersun in a couple of hours.  The immersun has a 3 year warranty and the installation is guaranteed for a year. Big Green Technology did an excellent job, quick, clean and the Immersun is an excellent piece of technology.

For anyone with a Solar PV system, I would thoroughly recommend the Immersun as a means of maximising any return from a Photovoltaic system, or indeed any system where surplus electricity can be utilised. Well done 4Eco and Big Green Technology.

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