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

IMPORTANT notice about this article: Please note that the Feed-in Tariff (FiT) scheme closed to new applications on March 31st 2019.

water tap with hot water steam

Would you like to take back control of your solar energy, maximize your savings and get free hot water? Read on to find out how.

Solar PV panels will often produce more energy than you can use in a day and, without a solar battery, your surplus will be sent to the National Grid.

However, you may not be aware that there is a gadget which enables you to keep this surplus energy, use it to power your immersion heater, reduce your electricity bills even further and still receive the same amount in Feed-in Tariff Export payments.

Here we introduce the solar power diverter and how it can maximise your solar savings.

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What is a Solar Power Diverter?

If you have a solar PV system there will be ‘export periods’ during the day when it is generating more energy than you can use. Unless you have a solar battery to store this extra energy for use later, it is automatically sent to the National Grid.

A solar power diverter 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, heating the water in your cylinder with free energy. Your diverter will prioritise the other appliances in your home. This means if your surplus energy is heating your immersion and then you turn on your kettle, for example, the diverter will automatically adjust and redirect the energy to the kettle.

This means you won’t export any of your surplus energy to the National Grid, you won’t need your boiler to work as hard and can buy less electricity from your supplier. Essentially, you will be maximising your savings.

Installation is usually very straightforward as a solar power diverter will work with your existing immersion heater (as long as it has a thermostat) and the supplier will include everything needed to make it work. The gadget itself looks a lot like a slightly larger heating thermostat.

What About the Feed-in Tariff?

Green home using solar panels

You’d be forgiven for thinking that keeping hold of your energy rather than exporting it would mean you’d receive less from the Feed-in Tariff. In reality it will have no effect whatsoever and you’ll continue to receive exactly the same payments.

How is that possible?

Well, while most solar panel installations include a generation meter to track how much energy is being produced, the majority of homes do not have a way of measuring how much is used vs exported to the National Grid. The result is that energy companies don’t actually know how much energy you’ve exported, so they pay you 50% of whatever your generated energy figure is. So it doesn’t matter if you send all, some or none of your solar energy back to the grid – you will still be paid the same.

In addition, the small number of homes which do have an export meter are paid around 5p for every unit they send back. When you consider they will have to pay nearly 3 times that to buy the same unit back from a supplier, there’s not much incentive to export.

The logical conclusion is that you should keep hold of and use as much of your solar energy as possible. You’ll still receive exactly the same Feed-in Tariff payments and you’ll need to buy less energy from a supplier.

Savings with a Solar Powered Immersion Heater

According to the Energy Saving Trust the average cost of electricity is about 13.33p per kWh in the UK (based on March 2018 figures). This means it costs around 40p to power a 3kW immersion heater for an hour. Based on this figure we’ve estimated some immersion heater running costs.

Immersion Heater Size Running Cost Per Hour (Approximate) Running Cost Per Week (Approximate, Running 2 hours a day) Running Cost Per Month (Approximate, Running 2 hours a day over 28 days) Running Cost Per Year (Approximate, Running 2 hours a day over 52 weeks)
3kW 40p £5.60 £22.40 £291.20
6kW 80p £11.20 £44.80 £582.40

Clearly there are significant costs associated with running an immersion heater on standard electricity. If you can supplement your supply with free solar energy it’s likely you’ll see a significant reduction in your energy bills.

Several of the power diverters on the market include a user friendly display which tells you when it is diverting power to the immersion heater and how much this is saving you in energy. Some give you the option of viewing your savings for the past 7 days or month as well.


Here are some of the leading manufacturers of solar power diverters for immersion heaters in the UK. Solar iBoost are the most popular but as the technology becomes more and more well known other brands are releasing competitive products.

Manufacturer Product Wireless? Warranty Approximate Price
Marlec Ing Co Solar iBoost Y 2 years £300
SISEM Ltd Immersun Y 3 years £320
Apollo GEM Y 5 years £275
SolarImmersion SolarImmersion N 1 year £199
Earthwise Products Ltd SOLIC 200 N 10 years £199

Heatrae Sadia are the manufacturer behind the leading unvented hot water cylinder, the Megaflo. They produce unvented cylinders which are solar PV ready and also offer the Megaflo Buddy which is their own model of solar power diverter.

What About a Solar Battery?

Installing a solar storage system is another solution to consider as it stores any surplus energy you’ve generated for use when the solar panels aren’t generating e.g. at night. Find out more about solar batteries.

What About Solar Thermal?

A solar thermal system is another way of heating water with solar energy but is a separate technology and process to that of solar PV panels. It also requires a solar compatible hot water tank. Find out more about solar thermal.

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