Frequently Asked Questions


Thanks to overwhelming demand we have a significant backlog of orders and are now operating a waiting list. We don’t like to keep customers waiting, so are working as fast as we can to scale up production.

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. This is easy for us to handle with lifting equipment, so we take care of putting down a concrete base and delivering the unit.

Each Warmstone will store 100kWh of usable heat. 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.

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

Yes, to enable Warmstone controls in the myenergi app.

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. Using any excess generation from your own solar PV will be zero carbon.

Using a Warmstone

It would take 5 hours to charge at full power, which assumes you are not using anything else in the home at the same time. 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 would take around 8 hours to reach 500 degrees, but this would only happen after installation or being switched off for weeks.

Yes. We install sensors provided by our partner myenergi to monitor the amount of electricity being used by other devices, and will adjust the speed at which your Warmstone charges if necessary.

The Warmstone is normally set to charge overnight when electricity is cheapest and greenest, but it can be boosted during the day to make sure you never get cold or to absorb electricity generated by your own solar PV.

As a Warmstone can produce hot water at the same temperature as a boiler there will be no change. Our approach is to help homeowners decarbonise their heat and hot water needs with the minimal amount of disruption and no change in experience.

Heating System

Yes, we call this a hybrid install. One reason for doing this would be in case your Warmstone runs out of heat during a cold winter’s day. Instead of topping up with more expensive daytime electricity you could instead use oil or gas.

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.

If you generate your own electricity from solar PV, the Warmstone can be used to store excess electricity that would otherwise be exported to the grid. The more solar you have installed the more likely you are to have free hot water over the summer.

We promote Warmstone as an alternative to heating oil and LPG as these are particularly expensive and high carbon. Heat batteries are also a good alternative to mains gas, but depending on current electricity tariffs and how much solar you have you may find your bills go up.


We have not yet announced pricing for future sales.

At the time of writing energy prices are changing rapidly, so we have removed the price comparison tables from our website. We will update the site when the market settles.

Warranty & maintenance

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.

Much of the effort we put in to designing the Warmstone went towards making it simple. The parts that may fail over the years, such as the pump and heat exchanger, are intended to be commonplace and simple for any heat engineer to swap out.

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.


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 at 20 years it should last around twice as long as an electric 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 a well-insulated house with underfloor heating, you may be better off with a heat pump. Every home is unique, so this just a rough guide.

A normal domestic hot water cylinder is around 150 litres. A Warmstone stores as much energy as 14 of these. This is based on some assumptions, for example that the cold water is at 10 degrees.