What is a Warmstone Heat Battery made from?

Warmstone is our patented solid storage material that is made from a combination of recycled and natural materials. The material combines low cost, high thermal conductivity and high density. It is non-toxic and non-flammable.

How does it work?

We use electrical energy (supplied either from the grid or local sources, such as solar PV) to heat the solid core with electric elements and then use the stored heat to provide a supply of hot water to your home. It does not store hot water.

How do I work out whether it is better to fit a heat pump or a heat battery?

If you live in a house that uses heating oil or LPG, has solid walls (built pre-1930) and normal radiators, then you will probably be better off with a heat battery. If you have an insulated house with underfloor heating then you will probably be better off with a heat pump. As a rough rule of thumb, if your boiler water temperature is set to 65 degrees Celsius or above you will be better off with a heat battery.

What sizes does a Warmstone Heat Battery come in? How heavy is it?

One size. It is 1.1m in diameter and 1.8m high. Given the high density of the solid-state storage material used, the unit weighs 1.85 tonnes.

How long will a unit last? How often does it need to be serviced? What happens if something goes wrong?

We are designing our units to last a minimum of 20 years and are offering a three-year full parts and labour warranty. If something goes wrong within the warranty period, we will replace the defective unit with a new one free of charge.

With no moving parts beyond standard pumps, there is minimal maintenance required. A few minutes of annual maintenance may be needed, and we will provide more guidance on this when we start shipping commercial units.

Why does it need to be installed outside my house?

The unit weighs 1.85 tonnes and needs to rest on a solid base. This can be located outside a house or inside a garage with a concrete floor. We do not recommend installation of units inside homes. The ideal location is close to the existing boiler. We will connect the Heat Battery piping to the existing plumbing system. If long pipe runs are required, this will add to the cost of the installation and may increase the heat losses from the piping.

When you buy a Warmstone heat battery, we include delivery to the install location. This requires specialist handling equipment that we supply.

Why would I not buy an electric battery instead of a heat battery?

A home might use 4,000 kWh of electricity and 16,000 kWh for heating and hot water, so 80% of the home energy usage is heat. Given this and the price and reliability of electric batteries, heat storage with a Warmstone is significantly more affordable and will last around twice as long as an electric battery.

How efficient is the Warmstone heat battery?

More than 90% efficient due to the high-performance vacuum insulation we use to minimise our losses. Heat can be stored for 20 days or more.

How much energy does it hold?

100kWh. This is enough capacity to provide a day’s hot water for almost all homes, and for many homes will also be enough for a day’s winter heating.

How long does it take to charge? 

It would normally take 5 hours to charge. The minimum temperature of the core is normally 200 degrees Celsius, and when the unit is full this rises to 500 degrees. From cold the Warmstone will take approximately 8 hours to reach 500 degrees, though aside from installation and a power cut lasting weeks, this is highly unlikely.


The Warmstone will intelligently turn down charging, prioritising other needs such as your EV. We install sensors provided by our partner myenergi to monitor the amount of electricity being used by other devices and adjust accordingly.

What happens in a power cut? do I have any hot water or heating?

Like a normal boiler, the circulation pump will not work if mains electricity is off. The heat will remain in the core for weeks if not used.

Can I use the heat quicker than it can charge? 

Yes you can. It is designed to discharge at the same rate as a boiler (30kW). It charges at 20kW, so if you left all your hot water taps running then the unit would eventually discharge.


The Warmstone is normally set to charge overnight when electricity is cheapest and greenest. Through our partner myenergi we provide an iOS and Android app and this can be used to ‘boost’ the Warmstone during the day.

Do I need to tell my energy supplier that I am installing a Warmstone Heat Battery?

No, you do not. But we strongly recommend that you ensure you have an energy supplier that has off peak tariffs so you can charge your unit when the price of electricity is at its lowest.

You will, however, need to contact your local electricity distribution company (often referred to as the DNO) to ensure that you can draw a continuous high load to your home. We will do this for you.

Will I notice any difference in my heating or hot water?

None whatsoever. The entire ethos of our approach is to help homeowners decarbonise their heat and hot water needs with the minimal amount of disruption and no change in experience.

How long will I have to wait for one? How long will it take to install?

We currently have a waiting list of approximately 9 months. We are looking to rapidly scale up production to meet this demand. Once delivered, the installation and commissioning takes two days.

Can a Warmstone Heat Battery act as a combi boiler?

The current version is not designed to work as a combi boiler. Our initial focus is on homes that have a hot water cylinder. We will release a version that does not need a hot water cylinder if there is sufficient interest.

Do I need a smart meter and if so, how do I get one?

Yes, you need a smart meter to take advantage of tariffs with cheaper off-peak electricity. If you do not have one, then you need to request one from your energy supplier or change to an energy supplier that will fit one.

What are the best Tariffs to use?

There are suitable tariffs available from a range of energy suppliers. The most widely used amongst our team is Octopus Go.

What temperature hot water can it supply?

The unit can supply water at up to 80 degrees Celsius. We would normally expect to run it between 65 and 70 degrees Celsius as this is the temperature used by most boilers.

Can my plumber install and service it?

Yes, but they will need to go through our accredited installer process. For early installs we will take full responsibility for installation and provide both plumber and electrician.

Will I qualify for the Renewable Heat Incentive? Will I be able to get a grant / interest free loan to support the cost of the Warmstone Heat Battery?

At the moment no. Heat batteries are not currently covered by the Renewable Heat Incentive. Nor do they meet the criteria for the Home Energy Scotland Loan. This is something we hope to address.

I have solar PV installed – can I store any excess I generate rather than returning it to the grid?

Yes you can. Depending upon the amount of panels you have and your usage, you may have free hot water for much of the summer!

How much carbon does it save?

That depends on how much energy you use, what fuel being replaced, and how often you top-up the Warmstone during the day. As a rough indicator, switching to off-peak grid electricity from heating oil will immediately reduce your carbon footprint by around 50%. This will increase as the electricity grid is decarbonised.

I’m switching from heating oil. Will my heating bills go down?

A 5p/kWh off-peak electricity price has the same cost as heating oil at 47p/litre. If oil is above this price, you will save money every year. There can also be savings from replacing an old and inefficient boiler.

We are clear that the primary driver of people who are looking to a heat battery for their hot water and heating is to achieve home heat decarbonisation in the fastest, most affordable and least disruptive way.

Why is this only being proposed as a replacement for homes heated by Oil or LPG?

Natural gas currently costs around 4p per kWh and even using the lowest off-peak tariff a Warmstone heat battery will be more expensive. We do not want to increase home owners annual bills and until the cost of natural gas increases we are focused on homes that use the more expensive fuels (oil or LPG) with higher carbon footprints. Given the UK governments commitment to net zero we expect the cost of fossil fuels to increase steadily and the cost of electricity, on a relative basis to decrease quite rapidly as more renewable capacity comes on stream.

My house is on mains gas, can I install a Warmstone Heat Battery?

Yes you can and you might choose this instead of a heat pump to reduce your carbon footprint. In the near term, your bills will be higher because the government still subsidises fossil fuels and taxes clean electricity.

Should I keep my existing system as a back-up?

You do not need to, but some of our customers are opting to in the short term to take advantage of differences in the price of electricity and heating oil/LPG. They intend to set their control system to charge the Warmstone heat battery whenever electricity it is cheaper than oil.

We would expect oil usage to decrease over 3-5 years as low off-peak pricing becomes more and more frequent and the costs associated with fossil fuels increase.

Does the unit require an internet connection?

Yes, to enable Warmstone controls in the myenergi app.

How much will it cost to buy?

£12,000 including VAT and standard installation. We will give final confirmation on whether your property requires a standard or complex installation after a site visit, but can give you an initial indication over the phone.