Solar immersion controller review 2018-01-12T16:03:18+00:00

Solar immersion controller review

A solar immersion controller is a way of storing solar electricity in your hot water cylinder

There are a number of alternative solar immersion controllers available:

Apollo Gem

Apollo Gem Solar PV Immersion Heater Controller


Eddie Solar Immersion Heater Controller


Solar iBoost



Power Diverter

Power Diverter

Solic 200

Solic 200

Cost effective solution

  • Choose iBoost.

Choose the iBoost if you want a cheap solution and have a backup gas or oil boiler.

For all electric homes we do not recommend this controller due to a difficult to use interface.

All other systems

  • Choose Eddi or Immersun

The Immersun is a well proven immersion controller that can control up to 3 loads – very useful for swimming pools or for homes with electric heating as well as immersion elements.

The Eddi is a more recent entry to the market and has some important features:

– Heat sync cooled, removing the need for the moving components an noise of a fan.

– Bypass switch – great if you boiler stops working.

– Separate wireless transmitter which draws its energy from the cable it is clamped to, removing the need for batteries or a socket.

Both the Eddi and Immersun are compatible with Tesla Powerwall 2 battery systems.

Immersion controller Daily boost Warranty (years) Wireless Loads Batteries Price Comments
Apollo GEM-C Wireless Weekend 5 Y 2 N £275 A good controller with some very neat features to allow additional electricity savings to be made, especially if you have an all electric hot water system.
Eddi Y 3 Y 2 N £365 One of the best controllers on the market at the moment, especially if you have an all electric hot water system. The most easy to use interface.
iBoost Weekend 2 Y 2 Optional £300 Most popular controller in the UK.  iBoost buddy allows data to be viewed on desktop display.
Power Diverter N 3 N 1 Y £349 An expensive basic controller.
Immersun Y 3 Y 3 N £320 Great for multiple loads and very user-friendly interface.
SOLiC200 N 10 N 1 N £199 Excellent warranty but basic functionality.
SolarCache No longer trading
Intelligent Solar Mark III No longer trading
Optimmersion No longer trading

Table updated 12 January 2018

Daily Boost

Having a solar immersion controller which can “daily boost” i.e boost on an individual day or over a weekend is very important if your hot water system is electric only (you do not have a gas or oil boiler).

This is because the hot water cylinder needs to be heated to 60 degrees once a week to kill potentially dangerous bacteria in the hot water cylinder (legionella).  Having a programmer that comes on every day will reduce the benefit of solar, since the cylinder will already be hot and not give the solar a chance to heat it.


This is less important since history has shown many companies go out of business in the warranty period.  Quality and reliability of the solar immersion controller is far more important.


A typical system comprises of two components.  The immersion controller and and sensor clamp.  The sensor clamp is placed around the incoming electrical supply to check to see how much energy is leaving the property from the solar system.  This can be wired to the immersion controller or communicate using wireless.

A wireless sensor clamp makes installation much easier and less disruptive to the home owner.


Some solar immersion controllers can control multiple loads.  This is very useful for hot water cylinders with a upper and lower immersion element.  The controller can heat the top of the cylinder first, and once hot, switch to the bottom immersion.  Water is drawn from the top of the cylinder, so heating the top first allows hot water to be drawn off.  Once the top of the cylinder is hot, the whole tank can be heated if there is enough sun.

Many controllers can also control alternative loads such as underfloor heating, electric towel radiators and electric panel heaters.


Having a battery operated wireless sensor means the batteries need replacing and you may not notice immediately that the system is not working.

The Eddi is unique since it does not require batteries or a socket to power it – it harvests its energy from the wire it is clamped around.