Modern heat batteries

Modern thermal batteries use electricity to store heat in natural materials (stone or 'salty' water) that can be released slowly to supply heating or hot water for the home. There are currently two types of heat battery for domestic use: Sunamp’s hot water unit and Tepeo’s ZEB boiler (stands for Zero Emissions Boiler).

Sunamp uses a heat exchanger submerged into a 'phase change' liquid that releases energy as it freezes. NB Sunamp can only supply hot water, not heating. The unit is available in various sizes supplying between 150-300 litres of hot water. The primary benefit is its compact size compared with an equivalent volume hot water cylinder, often needing just quarter of the space.

Tepeo's ZEB is a form of electric boiler that will work with existing radiators. It is physically large compared to a gas boiler - about the size of a washing machine - and floor standing. The ZEB uses raw materials to create a dense brick in the middle of the boiler that can hold around 40kWh of energy for heating. The unit can supply a separate hot water cylinder, however this does stretch its 40kWh capacity during winter. It is better to use a direct electric hot water cylinder, or Sunamp, that can charge during the same off-peak window.

Both Tepeo ZEB and Sunamp use smart technology to determine the best time to charge from the grid or from solar PV if you have it.

Charge times

Both will charge up over a four hour off-peak window and this will usually provide enough hot water or heating for 24 hours, although this is totally dependent on the property and heating demand (more on suitability below).

Suitability

Sunamp’s hot water unit is a great option for any property, particularly if tight on space as they are box shaped and compact compared to a cylinder. Sunamp works with all forms of heat source, including heat pumps and gas boilers. 

The ZEB is limited by its storage capacity of around 40kWh, which means they are best suited to smaller homes with a low heating demand. 

How are they different to heat pumps?

A heat pump uses heat from the air, ground or water (aka a renewable energy) to convert a small amount of electricity into three-four times as much energy for heating and hot water. For example a household currently buying 10,000kWh of gas for heating, will only need to buy ~3,000kWh of electricity for a heat pump to convert into 10,000kWh of heating. Heat pumps are the only technology to significantly cut how much energy you need to buy.

Heat batteries do not use a renewable energy to multiply how much heat they provide. Like all electric heating systems however, they do cut your energy demand a little, but not as much as a heat pump. This is because electricity is 100% efficient whereas gas boilers are around 85% efficient. 

Other significant differences are: no external unit and no need to change radiators (although this can often be the case with heat pumps as well). Heat pumps offer much less scope to 'shift' energy use to off-peak periods, but still tend to cost less to run because they produce 65-75% of your heating supply for free.

Running costs

The main feature of heat batteries is moving most of your heating demand to low cost off-peak tariffs, so whilst it does not reduce how much energy you need to buy as much as a heat pump, it does reduce how much you pay for electricity.

This means in the right circumstances running costs between a heat pump and a heat battery can be pretty similar, but in the wrong circumstances a heat battery will be much more expensive. 

Off-peak tariffs are not new. Economy 7 tariffs provide lower and higher tariffs for electricity use overnight, although the tariffs are rarely that generous (currently 16-18p per kWh for off-peak). Modern ‘time of use’ tariffs from Octopus offer much lower rates between 12.30-4.30am or 11.30pm-5.30am for electric vehicle owners, but this rate is extended to all other electricity uses during that time. Their tariffs at the time of writing are 7.5p/9.5 per kWh. A gas boiler and efficient air source heat pump will cost about the same to run, i.e. 7.5-8.5p per kWh.

Environmental benefits

As the UK brings more renewable power sources online, so the times when electricity is generated becomes less predictable. Windfarms for example can produce electricity at night when household electricity demand is low. Batteries of all kinds, including heat batteries and electric vehicles, allow for grid flexibility, whereby surplus green electricity generated at night can be distributed to homes with storage capacity. 

Whilst we continue to use gas to generate electricity, storing windfarm and solar powered electricity as it is generated has an important role to play in decarbonising our homes over the coming decade. Ramping up our grid flexibility with home batteries means that more of the wind and solar energy is used, meaning less comes from gas fired power stations.