For properties with electric immersion tanks.

If a house has a solar PV system, there will be periods during the day when it is generating more electricity than the property is using. If there is a backup battery system also installed, then the excess power would be fed into that battery in order to fully charge it; thereafter the excess electricity would be exported to the grid in order to benefit from any payments applicable under government or private Company agreements.


However, if there is no battery or once the battery is full, the excess electricity would be exported back to the Grid; for which you would be paid between 0.01p to 5.3p per unit. You would then buy that same power back at anything up to around 21p per unit.

If you have a hot water storage tank (immersion tank) a more cost-effective way to deal with that spare electricity would be to have it ‘diverted’ into your tank to heat up the water which is stored there. In essence, your hot water tank can play the part of an energy store, just like a battery. Then once the temperature of the water has reached what was programmed into the system, the excess electricity would be exported to the Grid.


Ordinarily, most homes do not have the immersion switch turned on their hot water storage tank; it is far more expensive to heat hot water that way instead of using gas. However, if you have solar panels fitted then that function would be automatically controlled by the installation of A Hot Water Controller/Solar Power Diverter. This electronic unit constantly monitors the amount of electricity being generated by your system and compares it against how much energy is being used by your appliances. When it detects that there is an excess it diverts this electricity to your immersion heater thus heating the water in your cylinder with free energy.

The controller gives priority to the other appliances in your home so if your surplus energy is heating your immersion and then you turn on an appliance such as your kettle or a fan heater, the controller will automatically adjust and redirect the energy for use by that appliance. It means you buy less electricity from your supplier, therefore maximising the savings made from your solar panels.

Providing the existing immersion heater has a thermostat, installation is fairly straightforward as a hot water controller will work with the existing immersion heater.
The cost of a hot water controller can vary from £200 to £300; and the cost of installation will vary across companies and according to area of the country. 


A hot water controller is usually made up of 2 main parts; the ‘Controller’ which is usually installed in the airing cupboard and a ‘Sender’ which is attached to the electricity meter.

Operationally, what happens is that the Sender monitors the electricity being generated by your solar PV system and compares it against the amount of electricity being used in the property. It sends this information to the Controller wirelessly. Depending on make, Controllers can be preset to 100w or adjustable between 0-100w so that when it detects that you are generating that amount more energy than you are using, it will automatically send the excess to power your immersion heater.

Several hot water controllers also include a monitoring display, sometimes as an optional part of the system. It connects wirelessly showing when surplus energy is detected and is available to use; when the system is active; and how much is being saved. With some, you can read how much energy you have saved in a day, a week, a month and even the whole year by just pressing a few buttons.

Even though the warranty period can range from 1 year to 10 years depending on the manufacturer, Hot Water Controllers are designed to function for at least 25 years, are maintenance-free, have no moving parts and require no plumbing alterations.

Click here to get a free quote on hot water controllers

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