Solar water heating systems make use of the heat of the sun to generate hot water for showers, taps, radiator, and more.

Installation of a solar thermal system effectively contributes to decreasing carbon emissions and heating bills. Your dependence on the usual heating system will be less, thus reducing the quantity of oil, electricity, and gas bought from suppliers.

Through the free energy generated via solar thermal, the average homeowner sources 40-70% of the required hot water. The remaining requirement is supplied via a boiler in the homeowner’s hot water cylinder.

What is a Solar PV and Immersion Heater?

In the UK, many homeowners have Solar PV installed to enjoy the benefits of greener electricity. You can also make use of your Solar PV to benefit free hot water.

An immersion heater is the electric heating part of the hot water cylinder. It implies that you will heat water in your house with home-generated free solar energy and will not have to export any surplus energy back to National Grid power.

Through this, you can purchase less electricity from the supplier and will not need the boiler to operate very hard. Thus, you will end up maximising your personal savings.

Installation is generally a very simple process as solar power diverters work with the existing immersion heater, and the suppliers include everything you might need to make it work.

How Solar PV powers Immersion heaters?

Solar PV PanelsWe know that solar panel generates power from the sun, and this can be combined with an immersion heater over a hot water tank so as to generate hot water with the use of a power diverter.

This diverter constantly measures the power that the solar PV generates and the amount of which is being utilised in a household.

Whenever there is excess power being produced, these particular units will divert the surplus to the immersion unit over the hot water tank that provides the house free electricity.

How does the Solar PV System Work?

To begin with, solar thermal panels contain tubes of fluid that heats upon exposure to the sunlight during daytime. Then the fluid travels downwards to the hot water cylinder, where the heat is passed onto the water present in the cylinder through a solar heating coil.

As most households use the majority of their hot water requirements when solar thermal panels are, in fact, not generating heat in the mornings and evenings, a hot water cylinder is essential to the solar thermal system.

This cylinder stores the solar hot water generated during daytime for when the need arises.

On the days when there is not adequate sunlight to get the water to the right temperature, the immersion heater will be required to top up the balance.

Most solar PV optimisers require clipping a sensor onto electrical cables that can accurately determine whether solar energy is being used or exported in real-time.

If it detects that energy is being exported, then it diverts the energy to the immersion heater to heat water.

Solar thermal with an immersion heater

Immersion heaters are generally included in hot water storage cylinders.

The immersion heater is a metallic element that is immersed in the water which is inside the tank. It heats the water when powered up by electricity.

To turn the immersion heater on and off, it is isolated with a controlling switch. Else, a timer can be fit to suit the household’s routine and heat the water or even when the energy tariff is at its cheapest like nighttime.

Solar PV & Immersion Heaters: How to Get Free Hot Water?

Immersion heaters are generally powered up by electricity from suppliers. But when a solar PV system is installed along with a solar thermal system, it can reduce the cost of heating.

Solar Panels on RoofSolar power diverters are constantly monitoring the electricity generated from your system, and it compares it with the energy being used by all your appliances. When these detect excess electricity, it diverts the surplus to an immersion heater.

Homeowners who have solar PV panels will have periods when the solar panels generate more energy than they can use. Thus leaving surplus energy exported back into the National Grid, unless they have a solar battery that can store this extra energy for later.

Advantages of Immersion Heaters with solar PV systems

  • An immersion diverter, like an add-on device, does not require to be installed simultaneously as a Solar PV Installation. It makes for a useful additional investment.
  • It makes heating water with solar energy convenient and reduces carbon footprint and energy bills!
  • They have a quick ROI of almost 2 years.
  • Once connected to the Solar PV system, the solar power diverter or immersion diverter works seamlessly in diverting the excess energy.
  • These ensure zero green energy waste or 100% usage of the solar generation.
  • Installation of a solar power diverter takes only about 30 minutes.
  • The diverters reduce the use of conventional boiler or gas boiler.
  • Feed-in tariff payments are not affected by installing an immersion diverter because the feed-in tariff pays for almost 50% of the energy generated. No matter what quantity you send back to the grid, you still earn the same.
  • A solar PV, when paired with an immersion heater and diverter, is cheaper and maintenance-free than solar thermal. It is an affordable add-on to heat the water and is reliable.

Solar PV and Immersion heaters – How do Solar PV optimizers link them?

To accurately determine if electricity is being exported or used in real-time, most solar PV optimisers require the clipping of a sensor on electrical cables. If it is detected that electricity is getting exported, it then diverts electricity for heating the water to the immersion heater.

This implies that you can lower your heating bills since you do not need to use supplier’s gas to heat water. Heating bills normally contribute 70% to the energy bill.

If you produce electricity of your own, then this is an ideal technology as it allows you to utilise the surplus energy generated during the days to heat up the water with an immersion heater.

The best part about this is that it works with the existing immersion equipment and there is no requirement for other equipment to work this out.

How to know if your home is suitable for a Solar Power Diverter?

It would be best if you met the following criteria to install a solar power diverter in your home:
Roof Solar Panels

  • Have a Solar PV or wind turbine- your own method for energy generation installed
  • Have a hot water cylinder system and a traditional boiler.
  • A less than 30m distance between the water tank and utility meter.
  • Energy used should not exceed the energy you generate because a diverter requires surplus energy to work.


How many kWh does it take to heat an immersion tank?

The Energy Saving Trust has researched that the average cost of outsourced electricity is around 13.33p per kWh in the UK. Implying that the cost to power a 3kw immersion heater for an hour is about 40p.

Is it best to leave the immersion heater on all the time?

Your heater will store the hot water after heating it. The tank must have a good insulating layer to keep water hot all day without the need to reheat it.


The installations of most solar panels include a generation meter that tracks how much energy is produced. While most households do not have a method to measure the energy used vs. the energy exported to the National Grid, which results in energy companies not knowing the exact figure of energy that is exported. Consequently, they pay 50% of whatever energy you have generated. Therefore, irrespective of the amount of excess solar energy you send back to the grid, you will still receive the same payment.

Installing a power diverter will help maximise solar energy usage.

It is noteworthy that you should not increase the quantity of electricity being consumed just because it is available. Ideally, you should use the quantity of electricity but alter usage patterns to use it during the day when the sun is bright and shiny and produced. To use more energy in the day, you can adopt some behavioural changes like running heavy electricals like dishwasher and washing machine in the morning as you leave for work so that they run during peak of electrical output from the solar system.