Two numbers get quoted when someone asks what size air conditioning a room needs: a capacity in kilowatts, and a capacity in BTU per hour. Neither answers the question alone. The size a room needs is the output of a calculation, and there is a recognised way of doing that calculation in England, set out in the Government’s guidance on meeting the Building Regulations.
Kilowatts, BTUs and which number actually matters
Kilowatts and BTUs are two ways of writing the same number. One kilowatt is 3,412 BTU per hour on the International Table BTU, which is why the familiar retail sizes look the way they do.
| Nominal capacity | BTU/hr | Usually sold as | The nominal it came from |
|---|---|---|---|
| 2.5 kW | 8,530 | 9,000 BTU | Three quarters of a ton |
| 3.5 kW | 11,942 | 12,000 BTU | One ton |
| 5.0 kW | 17,061 | 18,000 BTU | One and a half tons |
| 7.1 kW | 24,226 | 24,000 BTU | Two tons |
The BTU figures came first, and not from air conditioning. Twelve thousand BTU per hour is definitionally one ton of refrigeration — the rate that melts one short ton of ice in twenty-four hours — and the metric kilowatt nominals were marketed against those older sizes, not the other way round. The kW figure is the one worth reading: it is the number the design calculation and the Building Regulations guidance both work in.
Why a floor-area rule of thumb is not a sizing method
Rules of thumb are everywhere: so many watts per square metre of floor, or a size picked off a room’s dimensions. They are a sanity check and nothing more, not a recognised sizing method. Two rooms with identical footprints in the same street can need meaningfully different capacities, because a heat gain calculation asks a different set of questions from a tape measure:
- Glazing area, orientation and shading. A west-facing room peaks in the late afternoon on top of a whole day’s accumulated fabric heat, which is why west-facing living rooms and bedrooms are so often the worst in the house.
- Fabric and build age. A solid-wall Victorian terrace and a new-build behave very differently.
- Room volume, not floor area. It is the air that has to be cooled, and a three-metre ceiling holds 25% more of it than a 2.4-metre ceiling over the same footprint.
- Airtightness, occupancy and equipment gains. A home office with two monitors and a docking station is a small heater running all day.
- Roof gain. A converted loft takes heat through the roof plane all afternoon, often with rooflights too, and is often the hardest room in the house to cool.
The recognised method, and the 120% ceiling
Approved Document L (Volume 1: Dwellings) is the Government’s guidance on meeting the Building Regulations in England. On sizing it says the specification of a comfort cooling system should be based on a heat gain calculation for the dwelling, worked out following both CIBSE Guide A and the manufacturer’s guidance, and that systems should not be significantly oversized: in most circumstances the cooling appliance should not be sized for more than 120% of the design cooling load.
The wording matters: an Approved Document is statutory guidance on one way of complying, not a mandatory rule. But it is the published benchmark, and a size arrived at without a calculated heat gain cannot know whether it sits inside the 120% figure, because it never established the design cooling load.
Approved Document L also says reversible heat pump systems, those providing both heating and cooling, should be designed such that they are optimised for heating — one reason a system sized for a British winter usually has cooling capacity to spare in a British summer, as air conditioning or air-to-air heat pump explains.
The temperatures a design calculation has to assume
Where a system is designed as an air-to-air heat pump rather than as comfort cooling, there is a separate national design standard — MCS MIS 3005-D, the MCS Heat Pump Design Standard, amended in December 2025 to cover air-to-air heat pumps. It governs installations certified under that scheme, sizes on the building’s heat loss, and fixes the internal design temperatures, taken from CIBSE guidance:
| Room | Design temperature, degrees C |
|---|---|
| Living room, dining room, bedsitting room | 21 |
| Bedroom, hall and landing, kitchen, toilet | 18 |
| Bathroom | 22 |
External design temperatures come from CIBSE Guide A, and the standard’s own external table lists eight UK locations, not one of them in Yorkshire; the choice of location and column is left to the designer, so it is fair to ask which design temperature was used for a particular address and why. The two calculations described so far answer different questions — one sizes on summer heat gain, the other on winter heat loss — and this guide does not merge them into a single number.
Minimum efficiency: the floor today, and the change in March 2027
Approved Document L, the England guidance, sets a floor on efficiency as well as on sizing: an air conditioner working in cooling mode should have a seasonal energy efficiency ratio — SEER — of at least 4.0, or four units of heat moved out of the house per unit of electricity over a cooling season.
Product law in Great Britain sets a higher bar for the machine itself. Under assimilated Commission Regulation (EU) No 206/2012, a new air conditioner other than a single-duct or double-duct unit — the portable type — rated under 6 kW and using a refrigerant with a global warming potential above 150 must reach at least SEER 4.60 in cooling and SCOP 3.80 in heating to be placed on the market at all. The energy label’s SEER scale excludes those same portable machines, and its top band starts at SEER 8.50.
A 2026 edition of Approved Document L has been published to support the Future Homes and Buildings Standards, but The Building Regulations etc. (Amendment) (England) Regulations 2026 do not bring it into force until 24 March 2027, and 24 September 2027 for higher-risk building work; work today is judged against the 2021 edition incorporating the 2023 amendments. Its sizing guidance is unchanged, and the minimum SEER rises to 4.6 for new dwellings while staying at 4.0 for existing ones. Running costs are covered in air conditioning running costs in Yorkshire.
Why oversizing is not a safety margin
Buying extra capacity feels like insurance. Building Regulations guidance says systems should not be significantly oversized, and there are good reasons for that.
An oversized machine reaches temperature quickly and then stops, so it lives in short bursts rather than long steady runs. The published SEER figure is a seasonal, bin-weighted number that legally includes part-load operation, thermostat-off periods, standby and crankcase heater energy, which is why a unit’s seasonal figure is typically around double its flat-out instant efficiency. But it is measured against the machine’s own declared design load, so it describes a unit matched to its duty, and says nothing about the same unit once chosen for a load well below the one it was rated against.
Short bursts also handle humidity badly: a cooling coil strips moisture from the air only while it is running, so a machine that satisfies the thermostat in minutes leaves a room that reads cool and still feels clammy.
Adding capacity later is not free either. For an existing dwelling, Approved Document L says that extending or expanding the capacity of a comfort cooling system by over 25% of its previous capacity is work for which the system’s energy performance should be assessed, the result recorded and given to the owner. How many indoor units are genuinely needed is therefore a design-stage question, bound up with the types of air conditioning available.
The electrical side: a dedicated circuit and a local isolator
Sizing a machine and powering it are two different jobs, and the second is where a lot of sizing advice stops. A fixed system normally needs its own dedicated circuit back to the consumer unit rather than a spur off something that already exists.
Regulation 12(6A) of the Building Regulations 2010 makes three things notifiable in a home in England: installing a new circuit, replacing a consumer unit, and any addition or alteration to an existing circuit in a special location. Part P notification is triggered by the circuit, not by the appliance. Approved Document P names the appliance anyway, giving as its own examples of the parts of an installation outside the dwelling "fixed lighting and air conditioning units attached to outside walls". Notifiable or not, Approved Document P says that all electrical installation work carried out in a dwelling is subject to the Part P requirement. Who may lawfully carry out which parts of the job is covered in who can legally install air conditioning.
Alongside the circuit, a means of isolation local to the outdoor unit is normal practice, so the machine can be made dead in sight of whoever is working on it. The specific unit’s installation instructions are the reference for what it needs.
Cable, protective device and the residual-current question
Cable and protective device selection follows the specific unit, not a table in a guide. At least one manufacturer publishes, for each model, a rated running current and a separate, higher maximum running current — the ceiling the machine can modulate up to, roughly double the rated figure on that published data, and the figure the circuit has to be designed around. A variable-speed compressor ramps up from zero rather than being thrown straight onto the supply, so there is no direct-on-line starting surge — which is a different point from the steady maximum.
An inverter does not leak current in a tidy 50 Hz sine wave; it produces composite, mixed-frequency residual current, with components both above and below 50 Hz. That changes which type of residual current device suits the circuit, because not every type is built to detect that waveform. Fit the wrong one and the device can be blinded — still in the board, still looking correct on inspection, and no longer reliable. Which device is appropriate is decided by the equipment manufacturer’s stated protection requirements and by ordinary selection practice, because an installer cannot see inside the inverter to work it out independently.
The board, the supply and the network operator
For a Yorkshire household that already has solar panels, battery storage and an EV charger, this is the real constraint: adding air conditioning is a whole-board question, not a spare-way question. The supply is limited by the service fuse at the meter, and every new load has to be added into a maximum demand assessment against it. The charge point already has its own residual current device and, in practice, its own way in the board; solar and battery storage have taken further dedicated ways. Where a charger uses load management it backs off as household current rises, so a hot evening with cooling running will slow the car charge unless the design accounts for it.
Nor is a significant new load purely a matter between a household and its electrician. Energy Networks Association guidance states that installers have a responsibility to inform network operators when making modifications to a service, through the Connect Direct portal, and that where a residential property’s new maximum demand falls between 60 A and 100 A inclusive the installer must apply for a connection before installing, with the operator confirming within ten working days whether the equipment can be connected.
Which operator that is depends on the address: Northern Powergrid covers Hemsworth and much of West and South Yorkshire, including Wakefield, Leeds, Barnsley, Doncaster, Sheffield, York and Huddersfield, but the ground to the south and south-east, into north Nottinghamshire and Derbyshire, falls to National Grid Electricity Distribution in the East Midlands, and the far west approaches Electricity North West.
That is the difference between choosing a box off a floor area and designing an installation: the size is an output of a calculation, not a number picked before the calculation is done.