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Free hot water from your Solar PV system


It can be hard to find ways to make the most of your home-grown electricity consumption. Optimmersion® is an automatic controller that diverts your surplus generated electricity to heat water in your immersion tank. Investing in an Optimmersion® system for use with solar panels will pay for itself in as little as 2 ½ years, and will bring you savings on your energy bills year after year.

For those with a 3.68kW PV system, allowing Optimmersion® to divert surplus generation means water is heated during sunlight hours; this minimises the cost of energy used to heat your water, or even make it totally free.

The Optimmersion® is the most responsive and efficient system on the market, ensuring that never import power during operation and use as much of your generated power as possible.

Once the cost of system has been paid back, you will see years of profit ahead of you in the form of lower energy bills. The Optismart bonus refers to additional savings made simply by being aware of and controlling your home’s electricity consumption, using the Wattson energy monitor.

What people are saying about Optimmersion®

“Anything that can take the edge off gas usage is a good idea. It’s amazing how Optimmersion Wireless with a 4kw PV system can heat a DHW cylinder by lunchtime, even in winter on a really good day”

Chris Cowap, Sustainable Energy Engineer, ZLC Energy

“I knew the system would be efficient when I chose to install it, but I never guessed how fantastic it would be in terms of saving energy and money”

Steve Davies, Newport

“Today, Tuesday 8th April 2014, I didn’t have the boiler fire on at all during the day and I had loads of hot water! Following an injury whilst serving in the Fire Brigade, I like to ease my aches and pains in a deep hot bath, this is going to be great and I get to lower my gas bill too– thank you so much Optimmersion Wireless.”

Stephen Wareing, Weston Super Mare

“Since we installed our Optimmersion® we can’t believe how much we have already saved! In November alone we saved £20 in oil bills because we used our own generated surplus electricity to heat our water tank. We were able to check the figures using our Wattson.”

Jill & Tony, Cambridgeshire

Product information

Optimmersion is a proportional controller that diverts surplus electricity to heat water in the immersion tank. It only uses surplus electricity – electricity that is not being used in the house, so it is effectively giving you free hot water.

Optimmersion graph

Optimmersion® versions

All versions are supplied with a controller unit and a power throttle, part of the modular design. The controller unit needs to be accessible as not only does it become your controller, but it shows useful information about the performance of your system.

Optimmersion® OIMM-NLEN-UK

Optimmersion® makes use of surplus generated electricity to give free hot water. It diverts power to the immersion heater proportionally, starting at a surplus of just 50W – much lower than the 200W+ required by its competitors, making it much more effective during the winter when needed most.

It now has a large 5” touch-screen display showing Generation, Usage, Import/Export and Diverted Power. A 1hr boost can be set with a single touch. Multiple timers for use with off-peak tariffs and to ensure water is hot when required can also be turned on/off with a single touch.

Optimmersion® has a Wattson transmitter built-in, giving wireless connectivity between the Controller and the Power Throttle so there’s no need to do any re-wiring if other things are wired onto the immersion heater spur – installs are standardised, repeatable and fast.

Optimmersion® is now simpler to install as it self-calibrates – the sensor clips can be placed in any orientation around the cables. It also calculates the size of the attached load to display accurate diverted power data automatically and displays historical data.

Optimmersion® can send accurate data to all other Energeno products, including Wattson, Optiplugs and the new Optismart Gateway for real-time online data, offering a complete solution to make the best use of surplus generation in a home – it can do much more than just heat water.

Why choose Optimmersion®?

Optimmersion® has replaced the Optimmersion® Wireless OIMM-WIEN-UK in the new-look Optimmersion range.

Optismart® Hot Water OSHW-NLEN-UK

Optismart Hot Water comprises an Optimmersion® and a Wattson Solar Plus display in one cost effective solution that is cheaper and quicker to install than buying them separately. The dedicated Wattson display included has an additional mode showing diverted power.

The transmitter in the Optimmersion can also send data to the Wattson Anywhere portal via the Optismart Gateway and can control Optiplugs to make best use of generation with other appliances.

Wattson Solar Plus is an energy monitor that makes it very easy for everyone to see when they are generating more power than they are using by glowing green and shows how much more power they could be using for free in big bright numbers. When they are importing a small amount of power from the grid it glows blue and gets redder and redder the more they are buying in. It also shows them the cost of the power they buy so they can make better informed decisions about usage to bring their bills down.

Why choose Optismart® Hot Water:

Optismart® Hot Water has replaced the Optismart® Hot Water OSHW-WIEN-UK in the new-look Optimmersion range.

Optimmersion® Classic OIMM-CLEN-UK

Optimmersion® Classic is the entry-level model in the Optimmersion range of intelligent immersion controllers that give free hot water from surplus renewable electricity. It proportionally diverts surplus electricity to the immersion heater, stating at a surplus of just 50W, much lower than the 200W+ required by its competitors, making it much more effective during the winter when needed most.

Optimmersion® Classic has a 1hr boost plus multiple timers allowing for use of off-peak tariffs and to ensure water is hot when required. The colour display shows how much electricity is being generated, used and how much surplus is being diverted to the tank. Optimmersion® Classic can cater for multiple tanks in either a shared or cascaded formats.

Optimmersion® Classic has two main components, a Power Throttle and Controller. These need to be hard-wired together, and both are installed near the consumer unit as the controller needs to attach its 2 sensors to single core wires from the inverter and from the grid, both of which can easily be accessed here. Optimmersion® Classic is suitable for immersion heaters on dedicated circuits to be certain no re-wiring is required; the alternative Optimmersion® Wireless links the Controller and the Power Throttle wirelessly for easy install when the immersion heater is not on a dedicated circuit.

Why choose to sell Optimmersion® Classic?

Optimmersion® Classic has replaced the Optimmersion® Simple OIMM-BAEN-UK in the new-look Optimmersion range.

How it works

Optimmersion® is a proportional controller to add to a Solar Power installation. It diverts free electricity to the immersion heater when surplus power is available.

Simple to install, it supplies variable power depending on the excess electricity being generated. It automatically harvests surplus electricity and all of it is used to heat water and maintain temperature level.

It is installed between the immersion tank element and the mains, no modification to the existing immersion element, hot water cylinder or pipework needed.

Optimmersion® uses both directional current transformers and voltage sensors to constantly measure the power being generated by Solar PV and also power in use in the home. The clever Optimmersion® system regularly calculates the excess electricity which varies the power to the heating element. The advanced electronics provides a true variable output.


More info

Additional Products

Where to buy

Benefit from the renewables reduced VAT rate of just 5% when Optimmersion® is bought & installed by a qualified installer.


Optimmersion® is available from your usual Wattson supplier.

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