Efficiency and ratings
The numbers that tell you how much cooling or heating you get, and what it costs to run.
BTU (British Thermal Unit)
An older imperial measure of cooling or heating capacity, still used in marketing (you will see "9,000 BTU" or "12,000 BTU"). One kilowatt is roughly 3,412 BTU per hour, so a 9,000 BTU unit is about 2.6 kW and a 12,000 BTU unit about 3.5 kW. It is easier to size a room in kW, so treat BTU as the same figure in different units. See what size air conditioning you need.
kW (kilowatt)
The metric measure of an air conditioner's output, or capacity: how much cooling (or heating) it can deliver. A typical single room needs around 2.5 to 3.5 kW. Do not confuse output capacity with the smaller amount of electricity the unit actually draws, which is why an efficient unit gives several kW of cooling for one kW of power.
kWh (kilowatt-hour)
A unit of energy: one kilowatt drawn for one hour. Your electricity bill is charged per kWh, so it is the figure behind running costs. Under the Ofgem price cap of 26.11p/kWh (1 July to 30 September 2026), you multiply a unit's power draw in kW by the hours used by 26.11p to estimate a running cost. See air conditioning running costs.
SEER (Seasonal Energy Efficiency Ratio)
A measure of cooling efficiency across a whole cooling season, at a range of temperatures and part-loads, rather than at one fixed test point. A SEER of 6.0 means the unit gives about 6 kW of cooling for every 1 kW of electricity, averaged over the season. The higher the SEER, the cheaper it is to run in summer, and the better its cooling energy rating.
SCOP (Seasonal Coefficient of Performance)
The heating version of SEER, for reverse-cycle units that also warm a room. It is the average heat output per unit of electricity over a heating season, so a SCOP of 4.0 means about 4 kW of heat for every 1 kW of electricity. A high SCOP is what makes air conditioning a genuinely cheap way to heat.
EER (Energy Efficiency Ratio)
The older, single-point cooling ratio: efficiency measured at one fixed test condition, not across a season. SEER replaced it on energy labels because a seasonal figure reflects real-world use, standby and part-load running far better. You may still see EER on spec sheets alongside SEER.
COP (Coefficient of Performance)
The heating equivalent of EER: efficiency at one fixed condition rather than across a season. SCOP is the seasonal version that now appears on labels. A COP or SCOP of 4 simply means four units of heat for every one unit of electricity, which is why heat pumps beat direct electric heating.
Energy rating (A+++ to D)
The efficiency label on a reversible air conditioner, rated separately for cooling (from its SEER) and heating (from its SCOP). It runs from A+++ at the top down to D, so A+++ is the rating to aim for. As a rough guide, A+++ means a SEER of 8.50 or more and a SCOP of 5.10 or more. Note this is not the plain A to G scale used on fridges and washing machines.
Rates correct as of July 2026. The Ofgem price cap changes quarterly, so check the current rate before budgeting.
ErP and Ecodesign
The EU-derived rules, retained in the UK, that set minimum efficiency standards and the energy label for air conditioners. In practice they are why modern units must hit a minimum SEER and SCOP and carry a rating. You rarely need the term itself, but it is the reason the label exists.
Refrigerants and regulations
The gases inside the system, and the UK rules on who can handle them and where a unit can go.
Refrigerant
The working fluid that carries heat around the system, absorbing it indoors and releasing it outdoors as it changes between liquid and gas. Its type matters for efficiency, environmental impact and, crucially, who is legally allowed to work on it. See how air conditioning works.
R32
The refrigerant used in almost all new UK residential splits, with a global warming potential (GWP) of 675, well below the older R410A it replaced. It is classed as mildly flammable, which is one reason a split running on R32 must be installed by a certified engineer. See how air conditioning is installed.
R290 (propane)
A very low-GWP refrigerant (around 3) used in many portables and monobloc units. It is highly flammable, so it is only used in sealed, factory-charged appliances and depends on a correct installation for safety. It is a good environmental choice but not something to treat casually.
R410A
The previous generation of split-system refrigerant, with a much higher GWP of about 2,088. New single-split units under 3kg of R410A have been banned from sale in the UK since 1 January 2025, so new systems use R32. Existing R410A units are fine to keep running and service.
GWP (Global Warming Potential)
A measure of how strongly a gas warms the planet compared with carbon dioxide, which is set at 1. It is why R32 (675) replaced R410A (2,088), and why propane (around 3) is favoured on portables. A lower GWP means a smaller climate impact if the refrigerant ever leaks.
F-Gas
Short for the UK Fluorinated Greenhouse Gases Regulations, which control who may handle refrigerants. Any work on a split system's sealed refrigerant circuit must be done by an F-Gas-certified installer, and doing it yourself is a criminal offence. Sealed, pre-charged portables and monoblocs are exempt because nobody opens the circuit. See how air conditioning is installed.
REFCOM
The main UK register of F-Gas-certified companies, recognised by the Environment Agency. Checking a company is on REFCOM (or an equivalent scheme) is a quick way to confirm an installer is qualified to handle refrigerant legally.
MCS (Microgeneration Certification Scheme)
The certification an installer needs to fit a reverse-cycle unit that qualifies for the government grant, and the scheme behind the noise standard used in planning. If you want the £2,500 air-to-air grant, the installer must be MCS-certified.
Part P
The part of the Building Regulations covering fixed electrical work. The electrical connection for a fixed air conditioner is notifiable work and must be done by a competent person, which is another reason split installation is a job for a professional.
Permitted development
The planning route that lets you install some outdoor units without a full planning application, as long as you meet the conditions. A reverse-cycle (heating and cooling) unit can qualify in England, under rules in force since 29 May 2025; a cooling-only unit generally cannot and usually needs full planning permission. Conditions include a maximum unit size (1.5 m³ on a house, with up to two units on a detached house) and a noise limit of 37 dB(A) at 1 m from a neighbour's habitable-room window. Flats, listed buildings and conservation areas are stricter, and the rules differ across the UK. See planning permission for air conditioning.
Boiler Upgrade Scheme
The government grant that can pay £2,500 towards a reverse-cycle air-to-air heat pump in England and Wales, when it is fitted by an MCS-certified installer to replace a fossil-fuel or electric heating system. It is one per property and is not "free air con", since a typical install costs around £4,500. A separate 0% VAT rate on permanently-fixed air-to-air heat pumps runs to 31 March 2027; portables do not qualify. See air conditioning vs heat pumps.
Grant and VAT rules correct as of July 2026. Check GOV.UK and Ofgem for current availability before you rely on them.
System types
The main formats of home air conditioning you will be choosing between.
Split system
The most common fixed home system: an indoor unit on the wall connected by pipework to a separate outdoor condenser (hence "split" into two parts). It is efficient, quiet and usually reverse-cycle, and must be installed by an F-Gas-certified engineer. See wall-mounted split systems.
Multi-split
One outdoor unit serving several indoor units around the home, so you cool or heat multiple rooms without a condenser for each. It suits homes wanting two or more rooms covered with a single outdoor unit. See multi-split systems.
Monobloc
A self-contained unit with no separate outdoor condenser: all the works sit in one box, usually a fixed wall unit that vents through two cored holes in an outside wall. Because the refrigerant is sealed at the factory, it can be fitted without an F-Gas engineer. See air conditioning without an outdoor unit.
Portable
A free-standing plug-in unit on castors that vents hot air out of a window through a hose. It needs no installation and suits renters or one-off hot spells, but it is noisier and less efficient than a fixed system. See portable air conditioners.
Ducted (concealed)
A system where the indoor unit is hidden in a ceiling void or loft and cool air is delivered through ductwork to discreet grilles in each room. It gives the tidiest look with almost nothing on show, but it is the most involved to install. See ducted air conditioning.
Ceiling cassette
An indoor unit recessed into a ceiling, with a flush square grille that blows air in up to four directions. It is popular in rooms with suspended or high ceilings where a wall unit would not suit. See ceiling cassette air conditioning.
Floor-standing
An indoor unit that sits at floor level rather than high on the wall, useful where wall space is limited or under a window. It works like a wall split but in a console format. See floor-standing air conditioners.
Reverse-cycle (air-to-air heat pump)
A unit that runs in both directions, cooling in summer and heating in winter by moving heat rather than burning fuel. Most modern UK splits are reverse-cycle, which is what lets them qualify for the air-to-air heat pump grant and permitted development. "Reverse-cycle", "air-to-air heat pump" and "heat and cool" all mean the same thing. See air conditioning vs heat pumps.
Window unit
A single box that fits into an open window or a hole in the wall, common in the US but rare in the UK. It is worth knowing the term, but there are few models sold here. See window air conditioners in the UK.
VRF / VRV
Variable Refrigerant Flow (Daikin brands it VRV) is a large multi-unit refrigerant system for offices and other commercial buildings. It is not a home product, so it is out of scope for domestic buyers, but you may meet the acronym in listings.
Parts and components
The main pieces that make up a system, so a quote or a fault report makes sense.
Compressor
The pump at the heart of the system, usually in the outdoor unit, that compresses the refrigerant and drives it around the circuit. It is the part that does the hard work, and on modern units it is inverter-driven to vary its speed.
Condenser (outdoor unit)
The outdoor box, containing the compressor and a coil, that releases heat outside when cooling (and gathers heat from the air when heating). It is the unit that sits on a wall bracket or on the ground and is subject to the planning and noise rules.
Evaporator (indoor unit)
The indoor unit, or "head", containing the coil that absorbs heat from the room and a fan that blows the cooled air back in. On a wall split this is the slim unit you see mounted high on the wall.
Condensate and condensate drain
Condensate is the water that forms as the indoor unit cools warm, humid air, much like the drips on a cold glass. It runs away through a condensate drain (a small pipe to outside or to a drain), which is why a fixed install needs a route for that water.
Thermostat
The control that sets and holds your target temperature, whether built into the unit, a wall controller or an app. On an inverter unit the thermostat works with the variable-speed compressor to hold a steady temperature rather than swinging above and below it.
Comfort and operation
The settings and features you will actually use day to day.
Inverter
A variable-speed compressor that ramps up and down to match demand, instead of switching fully on and off like older units. It holds temperature steadily, runs more quietly and cuts running costs, and it is standard on modern fixed UK splits. If a unit is described as "inverter", that is a good sign. See how air conditioning works.
Dry mode (dehumidify)
A setting that removes moisture from the air with little or no cooling, useful on muggy days or for damp rooms. It runs the system gently to wring water out of the air rather than to drop the temperature, so it uses less energy than full cooling.
dB(A) and noise
Decibels on the A-weighted scale, the standard way to state how loud a unit is. A quiet indoor unit on its lowest setting can sit in the mid-20s dB(A), while the outdoor unit is capped for planning at 37 dB(A) measured 1 m from a neighbour's window. Lower numbers mean quieter running. See the quietest portable air conditioners.
Air changes per hour
How many times the air in a room could be circulated in an hour, a rough guide to a unit's airflow. Note that splits and portables mostly recirculate and cool the room's own air rather than bring in fresh air from outside, so this is about circulation, not ventilation.
Wi-Fi and smart control
The ability to run a unit from a phone app or a smart speaker, including scheduling, geofencing and remote on-off. It is convenient for pre-cooling a room before you get home and for keeping an eye on running time. See smart air conditioning.
