Heat Battery or Heat Pump?

A heat battery stores heat; a heat pump gathers it. That difference decides which one a household is actually choosing between. For most UK homes the realistic alternative to a heat battery is not a cylinder but an air source heat pump, which independent guidance describes as the most common type of domestic heat pump in the UK and suitable for most types of homes¹. Parliamentary research goes further: heat pumps are technically suitable for most UK homes if installed appropriately².

The money is the first thing most households look at. Independent guidance puts air-to-water heat pump installation at around £11,000, with a Boiler Upgrade Scheme grant of £7,500 against it³. The wider range quoted for air source installation is £7,000 to £13,000, while another source gives around £14,000 as a typical cost, so the two figures for the upper end do not agree¹. Heat batteries attract a smaller grant of £2,500, and that grant becomes available only once legislation and product standards are in place⁴.

The efficiency case is not close. Official guidance states that heat pumps are three times more energy efficient than traditional boilers⁶, and the Climate Change Committee puts them at around three to four times more efficient than gas boilers⁷. What a heat pump does not do is remove dependence on electricity, a supplier and a tariff. What it does do is cut the gas connection out of the heating and hot water load, which is the largest single step most homes can take.

The short answer: an air source heat pump for most homes

Air source heat pumps are the most common type of domestic heat pump in the UK and are suitable for most types of homes¹. The same wording appears in the Energy Saving Trust's in-depth guide¹⁰, and its heat pump overview states plainly that heat pumps are suitable for most houses in the UK¹¹. Parliamentary briefing material agrees that they are technically suitable for most UK homes if installed appropriately².

Two types dominate the UK market: air source and ground source¹¹. Ground source is less commonly used in homes because it usually requires lots of outdoor space and carries higher installation costs than air source¹². Which of the two suits a given property depends largely on budget and how much outdoor space is available¹¹. That is the same trade-off a heat battery presents, in a different form: a heat battery is most suitable for smaller homes that do not have space for an outdoor heat pump unit³.

So the comparison is not really heat battery against heat pump as rivals on equal footing. It is a choice between a store that needs no outdoor unit and a generator of heat that needs one, with the outdoor unit being the price of the higher efficiency. Where a household has the wall, the garden or the roof space for an outdoor unit, the heat pump is the mainstream option. Where it does not, the heat battery is the alternative that the grant system now recognises, at a lower level of support.

How an air source heat pump works: heat moved, not generated

An air source heat pump is a device that transfers heat from outside your home into your home using electricity, rather than generating the heat itself⁸. It works like a fridge in reverse: it absorbs heat from the outside atmosphere and transfers it at low temperature to the pump compressor⁸. Official guidance describes the same principle as absorbing heat from outside air to provide heating¹³, and as moving heat from the air to a hot water supply¹⁴.

The consequence is that the amount of heat produced by the system is more than the electricity needed to power it⁸. That is the whole basis of the efficiency claim, and it is why a heat pump is not simply an electric heater with a fan. An air-to-water system transfers heat drawn from surrounding air to water in a wet central heating system, which is what allows it to serve radiators and a hot water store¹⁵.

Air source heat pumps heat homes by taking warmth from the outside air and using it for heating and hot water¹. They can also heat your water as well as your home⁸. That matters to the heat battery question directly: a heat pump and a heat battery are not mutually exclusive, because the pump can charge the store. The household that installs both is using the pump for efficiency and the battery for timing.

Efficiency: three to four times a gas boiler

Official guidance states that heat pumps are three times more energy efficient than traditional boilers⁶, and the clean energy campaign repeats the figure as 3x more energy efficient than traditional boilers¹⁶. The Climate Change Committee gives a range of around three to four times more efficient than gas boilers, and notes that this should lead to lower household energy bills⁷. Bristol's warm homes plan puts heat pumps at typically over three times more efficient than the most efficient gas boilers or electric heaters¹⁷.

The highest figure in the official statistics is wider still: current available heat pump technologies can be up to 3 to 5 times more efficient than a natural gas boiler¹⁸. Welsh Government guidance settles on up to 3 times more efficient than other heating systems¹⁹. The spread between three and five reflects different measurement conditions and different system designs rather than a disagreement about the principle.

Two documents give coefficient of performance figures of 4.3 and 3.9 for air source heat pumps, and that conflict is not ruled on¹. A household comparing quotes should treat any single efficiency number as conditional on the design of the specific system, the flow temperature it runs at, and the condition of the radiators and pipework it feeds.

Efficiency is also where the insulation question bites. Heat pumps are more efficient in well-insulated homes, and draughts or poor roof insulation can cause substantial heat loss and wasted energy¹². Official guidance states that heat pumps are best suited to buildings with good insulation levels, and that insulation may need to be added internally or externally where homes do not already have good levels²⁰. That is not a reason to rule a heat pump out; it is a reason to expect the installer's survey to include the fabric of the building.

Cost: £7,000 to £13,000 installed, with £5,000 of Boiler Upgrade Scheme support

The headline range for an installed air source heat pump is £7,000 to £13,000¹. Independent guidance gives around £11,000 for an air-to-water system, with the Boiler Upgrade Scheme providing £7,500 against it³. A separate source gives around £12,000 for the same category, and that conflict is not ruled on¹. One further figure of around £14,000 as a typical cost appears in the documents and is also unresolved against the £7,000 to £13,000 range¹.

The grant structure is where the heat battery comparison becomes concrete. Official scheme rules give £7,500 for air-to-water heat pumps and ground source heat pumps, and £2,500 for air-to-air heat pumps and heat batteries⁵. Ofgem's guidance for property owners states £7,500 for air-to-water and ground source heat pumps⁴. The £2,500 heat battery grant will be available once the legislation and appropriate product standards are in place⁴.

The £5,000 difference between the two grant levels is the single largest financial fact in this comparison. It means the net cost gap between a heat pump and a heat battery is narrower than the headline installation figures suggest, and it means the heat battery route is supported at a level that reflects its smaller contribution to the home's heating load.

Running costs: more than gas on a standard tariff, less on a time-of-use tariff

On standard electricity tariffs, where electricity is nearly four times more expensive than gas, heat pumps can cost slightly more to run than new gas or oil boilers¹. That is the honest position and it is the one most often left out of sales material. The efficiency advantage does not automatically translate into a lower bill when the price of a unit of electricity is several times the price of a unit of gas.

The picture changes with the tariff. Independent research shows that by switching to a heat pump time-of-use tariff, running costs fall significantly¹. The in-depth guidance states that switching to a heat pump time-of-use tariff should reduce the cost of running the heat pump, making it more likely that a household will save money¹⁰. The fact-check page puts it as saving money on bills when switching to a heat pump time-of-use tariff²².

This is where a heat battery and a heat pump stop competing and start combining. A store lets a household buy electricity in a cheap window and release the heat later, which is exactly what a time-of-use tariff rewards. A heat pump on its own can be run flexibly to some degree, but a thermal store gives the flexibility somewhere to sit.

The risk case is a poorly insulated home. Official guidance warns that installing a heat pump in a home that is not well insulated may increase energy bills instead, because electricity is more expensive than gas⁹. That is a firm limit, not a caveat, and it applies regardless of how good the tariff is.

Space, noise and siting: the outdoor unit and the 42 decibel rule

An air source heat pump needs an outdoor unit²³. Ground source heat pumps do not need an outdoor unit, but they do need a large indoor unit, and they need outside space for trenches or boreholes plus enough room inside for a heat unit²³. So the space question does not disappear by choosing ground source; it moves indoors and underground.

Noise is governed by a fixed limit. Official guidance states that noise levels for an air source heat pump on its own must stay at or below 42 decibels, measured a metre from any habitable room⁹. Croydon's guidance states that in the UK the legal noise limit for heat pumps is 42 decibels²⁴. The typical range of a heat pump when it is on is between 40 and 60 decibels²², which puts the legal limit at the quiet end of the equipment's normal operating band.

Two official documents describe the 42 decibel rule differently: one frames it as measured a metre from any habitable room, the other as not exceeding 42dB at a distance of 1 metre from the centre of a neighbour's habitable room window, known as the assessment point⁹. That conflict is not ruled on, and the practical effect is that the assessment point matters as much as the number.

Permitted development: what can be installed without planning permission

Domestic heat pumps benefit from permitted development rights, which means that in most cases planning permission is not required, subject to certain limitations and conditions²⁵. Independent guidance agrees that in general planning permission should not be needed because most heat pump installations are a permitted development¹⁰. Ground source and water source heat pumps on domestic premises are usually considered permitted development, not needing a planning application²⁶.

The conditions are where the detail sits. Permitted development rights allow air source heat pumps to be installed on detached houses or blocks of flats without an application¹⁴. The rights apply to the installation, alteration or replacement of an air source heat pump on a house or block of flats, or within the curtilage, including on a building within that curtilage²⁷. Development is permitted only if the installation complies with the Microgeneration Certification Scheme Planning Standards, MCS 020a²⁷.

The volume of the outdoor compressor unit, including housing, must not exceed 1.5 cubic metres on a house or 0.6 cubic metres for a block of flats²⁷. Only the first installation is permitted development on a house that is not detached or on a block of flats, while for detached houses the first two air source heat pumps are considered permitted development²⁷.

All parts of the air source heat pump must be at least one metre from the property boundary⁹. Cotswold's guidance states that no part of the unit will be installed within 1 metre of the boundary, for dwellings excluding those that have had permitted development rights removed or that sit within an area of outstanding natural beauty or a conservation area²⁸. Northern Ireland has its own rule: no part may be installed within 1 metre of the boundary of the curtilage of another dwelling house, and no part may be less than 30 metres from another dwellinghouse²⁹.

Where a heat pump fits, and where it may not save money

The decision between an air source and a ground source heat pump comes down to space, budget and efficiency²³. Air source is the mainstream choice for most homes; ground source is less common because of the outdoor space and the higher installation cost¹². A heat battery sits outside that pair entirely, and is most suitable for smaller homes that do not have space for an outdoor heat pump unit³.

The limit case is insulation. Official guidance is direct: if a home is not well insulated, installing a heat pump may increase energy bills instead, because electricity is more expensive than gas⁹. Heat pumps are more efficient in well-insulated homes, and draughts or poor roof insulation cause substantial heat loss and wasted energy¹². A household in a hard-to-treat property should expect the survey to raise fabric measures before or alongside the heat pump.

Cold weather is less often the binding constraint than insulation. The greater risk is economic: official guidance notes that in a home that is not well insulated, a heat pump may increase energy bills instead of cutting them, because electricity is currently more expensive than gas¹⁹. Independent guidance makes the same point, that heat pumps may be no cheaper to run than a conventional gas or liquid fuel appliance, and are best suited to homes that already have plenty of insulation²².

On lifespan, an air source heat pump lasts up to 20 years on average⁸. On procurement, quotes should come from at least three different installers, both for air source specifically and for heat pumps generally¹. On independence, the honest summary is this: a heat pump cuts the gas connection from the heating and hot water load, which is the largest single reduction in fossil dependence available to most homes, but it leaves the household dependent on electricity, a supplier and a tariff. A heat battery does the same, at a smaller scale and with a smaller grant. Neither removes the grid.