Air Conditioning Efficiency: SEER, SCOP, EER and Energy Labels

Air conditioner datasheets are full of acronyms, and the two that matter most are SEER and SCOP. SEER, the seasonal energy efficiency ratio, describes cooling: how much cooling a unit delivers per unit of electricity across a whole season rather than at a single moment. SCOP, the seasonal coefficient of performance, describes heating on the same seasonal basis. UK guidance is consistent on the floor for cooling: the seasonal energy efficiency ratio of an air conditioner working in cooling mode should be a minimum of 4.0¹, a figure that appears both in the Approved Document L guidance for England² and in the Welsh review consultation¹. Independent advice goes further and suggests looking for ratings of 7 or above to keep costs down³.

The reason these seasonal figures exist is that a single test-point number flatters an air conditioner. Real cooling and heating happen at part load, in varying outdoor temperatures, with the unit cycling on and off. Seasonal ratings are designed to capture that, which is why planning guidance for large developments asks for details of the Seasonal Coefficient of Performance, the Seasonal Performance Factor and the Seasonal Energy Efficiency Ratio for proposed heat pumps⁴. This page explains what each measure means, how to compare them like for like, what the energy label tells you, and what the rules require once a system is installed in a UK home.

Seasonal efficiency explained: what SEER and SCOP tell you

SEER and SCOP are attempts to describe efficiency the way a household actually experiences it: across a season, not in a laboratory at full load. The seasonal energy efficiency ratio of an air conditioner working in cooling mode should be a minimum of 4.0 according to the Welsh Government's Approved Document L consultation¹, and the same 4.0 figure appears in the Approved Document L for dwellings in England². Northern Ireland's building regulations discussion document proposes the same direction of travel, to increase the seasonal energy efficiency ratio to 4.0 for cooling systems⁸.

SCOP, the seasonal coefficient of performance, is the heating counterpart. It is measured in accordance with the procedures in BS EN 14825:2013, the standard for air conditioners, liquid chilling packages and heat pumps with electrically driven compressors, which tests and rates them at part load conditions and calculates seasonal performance⁶. The standard temperatures for calculating SCOP in "average" and "colder" climates are -7 and 15°C respectively⁵, so a SCOP figure is anchored to defined outdoor conditions rather than to a maker's choice of a flattering test day.

The four measures a household will meet on a datasheet line up like this:

For a household, the practical point is that seasonal figures are comparable between products in a way single test-point figures are not. Two units with the same headline cooling capacity can behave very differently across a British summer of mild days and short hot spells, and the seasonal rating is the one that reflects that. It is also the figure that planning and building control guidance asks to see: London's energy assessment guidance requires details of SCOP, the Seasonal Performance Factor and SEER for proposed heat pumps⁴.

EER and COP: the instant readings that underpin seasonal scores

Beneath the seasonal figures sit the older snapshot measures: EER, the energy efficiency ratio, and COP, the coefficient of performance. COP is the ratio of useful heat delivered to electricity consumed at a given moment. A COP of 4 tells you that for every 1 kW of electricity the unit uses, it produces 4 kW of usable energy as heat. The same logic applies to EER in cooling mode. These are the numbers that describe a unit at one operating point, and they are the building blocks from which the seasonal figures are calculated under BS EN 14825⁶.

The two kinds of figure answer different questions:

• A snapshot COP or EER describes the unit at a stated outdoor temperature and load, which is useful for comparing behaviour on the hottest or coldest days.
• A seasonal figure describes what to expect across a whole season of mixed conditions, averaging part-load operation, cycling and standby.
• For running costs, the seasonal figure is usually the more honest guide, and it is the one that appears in compliance documents⁴.

One caution when reading published figures: check which measure is being quoted and at what conditions. A manufacturer's seasonal COP is not the same thing as a COP at a single test point, and the two can differ substantially. Daikin, for example, publishes air-conditioning unit specifications stating a seasonal COP of 5.2⁷, a figure measured under the seasonal standard rather than at one moment. Comparing a seasonal figure from one product with a snapshot figure from another is the most common way households end up comparing unlike with like.

Real world performance: roughly 4 kW delivered per 1 kW consumed

What do these ratings mean in a real home? Analysis by Ember, drawing on a DESNZ literature review of air-to-air heat pumps, assumed an average heating output of 7.65 kW and an average COP of 2.85 when modelling typical air conditioning demand⁷. At that average COP, a unit delivers roughly 2.85 kW of heat for every 1 kW of electricity it draws, so the electricity consumed is a fraction of the heat delivered. A COP of 4, the figure used in maker guidance to explain the concept, corresponds to 4 kW of output for every 1 kW used⁷.

The size of the unit matters as much as its rating. A single-split system typically has a power rating of 0.8 to 2.5 kW³, which sets the scale of electricity the system draws when running. How much of that ends up on a bill depends on:

• how many hours the system runs;
• how well the room is insulated;
• how hot or cold it is outside, since efficiency falls when the unit works against a bigger temperature difference.

Published performance figures vary widely, and the spread is worth understanding:

These differ because they measure different things. Neither is wrong, but a household comparing products should compare like with like, and treat any single number as a rating under stated conditions rather than a promise of what every installation will achieve.

Air conditioning versus portable electric heaters compared

This is where the efficiency ratings translate directly into money. A plug-in electric heater is a resistance appliance: every 1 kW of electricity it draws becomes at most 1 kW of heat. An air-to-air heat pump is different, because it moves heat rather than generating it, so its output is a multiple of its input. Which? reports that with an air-to-air heat pump you will see lower running costs than with other types of electric heating and plug-in air conditioning⁹.

That gap is the whole argument for the ratings on this page. A unit with a COP of 4 delivers four times the heat of a resistance heater drawing the same power⁷, and even Ember's more conservative modelled average of 2.85⁷ means the heat pump uses roughly a third of the electricity for the same heat. Over a heating season, that difference compounds, which is why the seasonal figures matter more than the snapshot ones.

For a fuller picture of running costs, see air conditioning running costs and electricity use.

Understanding energy labels, from A grades upwards

The energy label is the household-facing version of all this. Energy labels rate products from A to G in terms of energy use, with A being the most efficient and G being the least efficient¹¹. That A-to-G rescaled layout is the current basis for household products, replacing the older labels that ran to A+ and beyond, so a modern label's A is not directly comparable with an A+++ on an older brochure.

Behind the label sit minimum standards. Scottish draft guidance states that fixed air conditioners should have a minimum energy efficiency classification of Class C in Schedule 3 of the labelling scheme adopted under the Energy Information (Household Air Conditioners) (No 2) Regulations¹². The Welsh consultation document also sets out a seasonal primary energy efficiency ratio, SPEER, described as a measure of the use of primary energy for all types of heat pump, fossil fuel boiler and gas-driven cogeneration technologies, as well as hybrid systems where solar heating or a heat pump is backed up with electric heating or a fossil fuel boiler¹³.

For a household, the label is a screening tool rather than a full specification. It shows where a product sits in its class, but the underlying SEER and SCOP figures tell you more about running costs, and the two should be read together. Products with the same letter can have different seasonal figures, and the seasonal figures are what determine how much electricity the unit draws for the cooling and heat it delivers.

One unit for both heating and cooling: the heat pump principle

The single most useful thing to understand about a modern home air conditioner is that it is almost certainly a heat pump. HMRC's understanding is that most air conditioning units are air source heat pumps¹⁴, and the House of Commons evidence base makes the same point from the other direction: air-to-air heat pumps are often known as air conditioning units, and also provide an efficient heating option for homes¹⁵.

This matters for how you read the ratings. A unit that cools in summer and heats in winter has two seasonal figures, SEER for cooling and SCOP for heating, and both describe the same physical machine working in opposite directions. It also matters for cost. Because the unit heats as well as cools, its efficiency rating applies across the year, not just during a few hot weeks, and the lower running costs compared with other electric heating apply in winter too⁹.

For more on units that heat, see air conditioning that heats.

What Part L of the Building Regulations demands of installations

Once a system is installed, the ratings move from brochure to building control. Part L of the Building Regulations covers the conservation of fuel and power, and its requirements apply to buildings, or extensions of such buildings, or work to or in connection with such a building or extension where the building is a roofed construction having walls and uses energy to condition the indoor climate¹⁷. Air conditioning falls squarely within that, and the government's own guidance lists putting in a fixed air-conditioning system among the alterations covered by building regulations approval¹⁸.

In England, Part L is set out in four parts, L1A, L1B, L2A and L2B, for different building classifications, and comprises higher standards for windows and the efficiency of heating, domestic hot water systems, mechanical ventilation, air conditioning and lighting, as well as the building fabric¹⁹. Compliance is not just a paper exercise: BRE explains that it requires a combination of design calculations, like SAP calculations, and "as built" verification through airtightness testing and photographic evidence²⁰. SAP itself is a rating for ranking a dwelling's energy performance, calculated on a scale of 1 to 100+, complemented by a scale from A to G on an EPC²⁰.

Airtightness is part of the same picture. On a new build property, the maximum permeability rate to achieve is 8.0 m3 (h-m2) @ 50Pa, and on an existing property retrofitted with energy efficiency measures, a figure around 10 m3 (h-m2) @ 50Pa would be expected²¹. The current framework in England dates from 15 June 2022, when the new Building Regulations Part L for new and existing dwellings came into force²². Part L was initially introduced in 1995 for new buildings and revised in 2005 to include existing dwellings where works are carried out in homes over 25 m2²³.

The rules continue to evolve, and differently in each nation:

A household planning an installation should check the position in its own nation, and for the overheating rules that sit alongside Part L, see Part O: the overheating rules for new homes.

Maintaining performance through servicing and F-Gas registration

A rating on a datasheet describes a clean, correctly charged, well-installed system. Keeping it that way is a maintenance question. Independent guidance for heating systems recommends that the boiler and system are serviced and checked annually to ensure they are running at their best²⁶, and the same interval is the sensible working assumption for air conditioning, whose filters, coils and refrigerant charge all affect real-world efficiency.

Refrigerant work is legally restricted. Any engineer handling the refrigerant circuit must be certified under the F-Gas rules, which exist because the refrigerants used in air conditioning are potent greenhouse gases; the rules are covered in detail on F-Gas rules for home air conditioning. A household using a certified engineer for installation and servicing is not just meeting the law, it is protecting the efficiency figures the system was sold on, because a leaking or undercharged system delivers less cooling or heat for the same electricity.

The law also recognises that efficiency decays without attention. Scottish building standards guidance on air-conditioning inspections sets intervals ranging from three years for poorly maintained and inefficient systems up to five years for those that are well maintained²⁵. The message for a household is that the inspection regime itself is calibrated to maintenance quality: a system that is looked after is treated as one that holds its performance longer.

System lifespan and typical warranty cover

The legal framework gives a sense of a system's expected life. The Energy Performance of Buildings Regulations require that an air-conditioning system first put into service on or after 1 January 2008 is inspected by an energy assessor by the last day of the period of five years beginning with the date on which the system was first put into service, and thereafter at regular intervals not exceeding five years²⁷. The underlying duty comes from the Energy Performance of Buildings Directive, whose Article 9 requires that larger air-conditioning systems in buildings are inspected from an energy efficiency point of view²⁸.

Those inspection intervals, three to five years depending on maintenance²⁵, are a reasonable proxy for how the system's condition is expected to change over time. A domestic system is not subject to the mandatory inspection regime in the same way as a large commercial one, but the physics is the same, and each of the following drags the real efficiency away from the published SEER and SCOP:

• filters clog;
• coils foul;
• refrigerant escapes slowly.

The inspection regime itself is under review. The UK Government consulted in December 2024 on reforms to the Energy Performance of Buildings regime, covering changes to Energy Performance Certificates, Display Energy Certificates and air conditioning inspection reports²⁹. A household considering a system should treat warranty terms as installer-quoted and manufacturer-specific, and check what the warranty requires in the way of annual servicing, since a lapsed service record is a common reason for a warranty claim to be refused. For more on upkeep, see air conditioning maintenance.

Comfort and moisture control

Efficiency ratings describe heat, but comfort in hot weather is also about moisture. An air conditioner actively removes heat and reduces room temperature³⁰, and as room air passes over the cold indoor coil, water condenses out and drains away. That drying effect is a genuine part of the cooling a household feels: humid air can feel worse than dry air at the same temperature.

This is worth distinguishing from a dehumidifier, which does not lower the air temperature but can take out existing moisture in the air³⁰. The two appliances overlap in function but not in mechanism:

An air conditioner does both jobs at once as a by-product of its refrigeration cycle. The condensate has to go somewhere, which is why installation includes a drain route, and why a blocked condensate drain is one of the commonest faults in a domestic system.

For a household weighing options, the dehumidification that comes free with cooling is one of the practical arguments for a fixed system over simpler cooling methods, and it is covered in more depth on air conditioning, dry mode and humidity.

Efficiency ratings and their role in energy self sufficiency

Seen through the lens of this site, the efficiency ratings are the measure of how much energy independence a system buys. A unit with a SEER of 7 or better³ or a seasonal COP in the region of Daikin's published 5.2⁷ turns each kilowatt-hour of electricity into several kilowatt-hours of cooling or heat, which means the same solar array, the same battery or the same bill covers far more of the household's needs. That is why cooling with solar panels pairs naturally with high-efficiency equipment: the multiplier works in both directions.

The dependence that remains is real and should be stated plainly. A fixed system is grid-tied electricity with a manufacturer behind it, and each of these remains however high the rating is:

• the grid, or whatever generates the household's electricity;
• the manufacturer's continued existence, for spare parts and warranty support;
• certified engineers, for refrigerant work.

The efficiency rating does not remove any of that. What it does is shrink the electricity requirement for a given amount of comfort, and that is the foundation every other independence measure builds on.

The wider policy direction reinforces the point. Scotland is reforming Energy Performance Certificates to give clearer information on the fabric energy efficiency of a property, the emissions, efficiency and running costs of its heating system, and the cost of energy to run the home to standardised conditions³¹, and the mean energy efficiency rating of the Scottish housing stock under SAP 2012 was 68.1 in 2024³². Efficiency, at the dwelling level and the appliance level, is the currency in which all of this is measured, and SEER and SCOP are the appliance-level coins.