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23 September 2021

How to buy the best air conditioner

The UK might only have a few truly sweltering days a year, but on those days a decent air conditioner can keep your home blessedly cool. We explain how to choose a top-notch air conditioner.
Christina Woodger

The best air conditioners will cool any room quickly and evenly, which is great news if you're struggling with the heat. However, poorer air conditioners make a racket, use more energy than others and leave warmer and colder patches. Pick the perfect air conditioner using our expert buying guide.

For most people in the UK, particularly those with small homes or with reduced strength to heave an air con unit about, an electric fan will be a sensible buy. But if you're shopping for a room-cooling air conditioning unit, terms like 'standalone', 'BTU' and 'refrigerant' can get confusing.

To help you shop savvy, we'll explain the difference between types of air conditioner, plus offer some tips on maintaining your air conditioner once you've got it set up. Once you know what you're looking for, head over to our air conditioner reviews and bag a Best Buy.

Types of portable air conditioners

If you own a chalet-style house, an attic conversion bedroom or a conservatory, an air conditioner could be a worthwhile investment – you'll probably find these rooms hard to cool without a reliable air con unit.

There are two main types of domestic air conditioner, each offering their own unique set of features. At a glance, this is what differentiates them:

  • Portable air conditioners – to use a portable air conditioner, you plug the unit into a mains power socket and dangle the hose out of a window or door.
  • Split-unit air conditioners – these have an indoor and outdoor unit. The indoor unit is fixed to the inside of an exterior wall, and the outdoor unit is positioned on the other side of the wall or installed on the ground outside.

How much does an air conditioner cost?

At the time of writing, our cheapest on-test air conditioner will set you back by around £300. But at the other end of the scale is a premium model that costs more than £700. Consider how much use you'll get from your air conditioner – you might not want to spend hundreds of pounds if you're only going to roll the machine out of storage a couple of times a year.

How much energy do air conditioners use?

Our data shows that portable air conditioners use as much energy in one hour as a typical fridge freezer uses in one day. You might not leave them running all day, but the costs can easily rack up. 

The energy efficiency of an air conditioner is usually measured by its energy efficiency ratio (EER). This is the ratio between the cooling capacity, in British thermal units per hour (BTU), and the power input, in watts. Generally, the higher the EER, the more energy efficient the air conditioner, although the actual BTU delivered by a machine can vary.

Best portable air conditioner

What is a portable air conditioner?

Portable air conditioners are also known as single-unit air conditioners, standalone air conditioners and mono-block air conditioners. They usually have wheels on the bottom so that you can roll them from room to room.

These systems are ideal for cooling a single room, and they stand on the floor with the attached hose dangling out of a nearby window. If you can't install a permanent window air conditioner because of a lack of space or building restrictions, a portable model could be a sound investment.

How does a portable air conditioner work?

Portable air conditioners extract hot air from your home and push it outside through the attached hose. A reliable model will cool the room you're in, but don't expect it to push cold air down the hallway or into another room.

Due to condensation, water droplets are collected by the portable air conditioner – you'll need to manually remove the water from the tank every so often and clean the filter while you're at it.

Are portable air conditioners worth the money?

Portable air conditioners: pros

  • Tend to be cheaper than split-unit air conditioners.
  • Cool your home without needing to install anything permanently.
  • Can be unplugged and placed in storage at the end of the season.
  • Premium models often double as dehumidifiers, electric heaters, or occasionally as air purifiers.

Portable air conditioners: cons

  • Not as efficient at cooling as split-unit models.
  • If you run the hose through an open window, this will allow some warm air back in through the gap. Most (but not all) models come with a window sealing kit for blocking off this gap, but these are only usable with certain types of window (such as sash windows).
  • Generally heavy appliances. If you have reduced strength and live alone, you might need enlist the help of someone else to help you set up your air conditioner.

Best split-unit air conditioner

What is a split-unit air conditioner?

Split-unit air conditioners, also known as fixed air conditioners, are comprised of two parts: a condensing unit (mounted outside) and an evaporator (mounted inside). Both parts of a split-unit air conditioner need to be connected, so installation is far more complex compared with a portable air conditioner.

These machines are quieter and more efficient than portable air conditioners, but need to be permanently positioned in just one room. If you've settled on a split-unit air conditioner, you'll probably need to get it set up by a professional – note that installers tend to be at their busiest during summer.

How does a split-unit air conditioner work?

A split-unit air conditioner features a compressor in its outdoor unit that circulates the refrigerant and turns it from a gas to a liquid.

That liquid passes through coolant lines to the indoor evaporator, where it transforms from a liquid to vapour. Heat is removed from the surrounding air and cooled air is blown into your home.

The outdoor unit will then turn the refrigerant vapour back into a liquid. This process will repeat over and over until the air conditioner unit reaches the temperature you've programmed.

Are split-unit air conditioners worth the money?

Split-unit air conditioners: pros

  • Ideal if you have one room that regularly gets very hot.
  • A secure way of air conditioning your home – no need to leave windows open.
  • Quieter and usually more efficient than portable air conditioners.

Split-unit air conditioners: cons

  • Usually more expensive than standalone units – expect to pay from around £500, excluding installation fees.
  • Fixed in place, so can't be moved between rooms.
  • Need to be permanently mounted on an outside wall by a certified professional.

Air conditioner features to look out for

Dehumidifier mode

Some air conditioners double up as a dehumidifier, meaning they serve a purpose throughout the year, not only in summer. If you're primarily after a dehumidifier, though, we recommend buying a dedicated dehumidifier instead.

With some air conditioner-come-dehumidifiers, you'll need to connect up a hose (which might not be supplied) to drain the water. With others, you'll need to set the machine up as you would in air conditioning mode, with the hose out of the open window.

Tempted by a dehumidifier? Before you hand over your money, check in with our dehumidifier reviews to ensure you're choosing wisely. 

Fan speeds

Having a range of fan speeds is useful, as it affects the rate at which your room cools down (and the noise the air conditioner makes). Most models have three fan speeds and some also have a fan-only mode, which has no cooling effect and simply circulates the air within the room. 

We don't recommend buying an air conditioner primarily for the fan function – you can save significantly on floor space and money by buying a fan instead.

Remote control or smartphone control

Useful if you want to adjust the settings without moving from your seat. When we get our hands on an air conditioner with smart features, we run it through rigorous privacy and security tests.

Heating mode

Some air conditioners can be used as an electric heater, which could prove useful in winter.

Sleep or night mode

This reduces noise by running the compressor and fan more slowly. Often, the target temperature will increase automatically across the night so that you don't wake up freezing cold.

But no air conditioner is completely silent, even on sleep mode. In fact, you're better off pre-cooling your room before you go to bed, rather than attempting to sleep with an air conditioner running. 


This allows you to set the machine to automatically switch on and off – useful if you want to come home to a cool house or save energy by having the unit switch off once you’ve fallen asleep. Pick an air conditioner with a clock and 24-hour setting. Some only have countdown or delay timers that need to be reset daily.

What size air conditioner do I need?

Air conditioners come in various shapes and sizes but are often described in terms of their BTU (British thermal unit) output.

In theory, the higher the BTU claimed, the more efficiently it can cool a room. As a general rule, 5,000 to 8,000 BTUs is adequate for most living rooms or bedrooms. Use this calculation to work out what BTU is right for you:

Multiply the dimensions (in feet) of the room by five. So for a room measuring 15 x 10 x 8ft: 15 x 10 x 8 x 5 = an air conditioner of 6,000 BTUs.

If you're concerned about how much energy an air conditioner will use, look out for a model with a good energy efficiency ratio (EER). This is the ratio between an air conditioner's BTU and its power input (in watts). Generally, the higher the EER rating, the more efficient the air conditioner.

You could also look at the claimed energy class. Air conditioner manufacturers are obliged to self-certify their energy class, from A to G. However, we conduct our own expert tests according to the European Standard BS EN 14511:2013. Often, we find the claimed and actual energy class to be different.

Where to buy air conditioners

When shopping online for an air conditioner, make sure you’re dealing with a reputable seller. Before you part with your money, check the retailer's returns policy and have a look at some customer reviews. Demand for air conditioners is lower in winter, so you may be able to bag a bargain in an end of year sale.

Popular retailers that stock air conditioners include:

  • Amazon air conditioners – hundreds of air conditioners from known brands including DeLonghi, Inventor and Russell Hobbs.
  • Argos air conditioners – portable air con units from Challenge, Meaco and several other brands. Expect to spend between £100 and £530.
  • B&Q air conditioners – B&Q only stocks two air conditioners at the time of writing, both of which are available for less than £400.
  • Currys air conditioners – Currys offers portable air conditioners ranging from around £200 to £600. Listed brands include Dyson, Logik and Staycool.
  • Homebase air conditioners – Homebase sells a small selection of Arlec air conditioners, with prices landing between £300 and £500.
  • John Lewis air conditioners – the retailer stocks portable air conditioners from AEG and Meaco – expect to spend £300-600.

Find out which retailers are rated highly by Which? members with our expert guide on the best and worst shops

Can you hire or rent an air conditioner?

Yes, there are lots of companies that offer this. Trying a portable model before you buy could save you from making a costly mistake. For instance, you may not realise how much noise these machines can make. At their quietest, they sound like a fridge on its cooling cycle, and some models will rattle and gurgle.

Rental prices typically start from about £60 per week for a compact, lower-powered model, all the way up to £200-plus for higher-powered units. 

How to install an air conditioner

When setting up your air conditioner, it's usually easier if you have a helper. All of the models we've tested are heavy (up to 30kg) and awkward to lift out of the box. If you live alone, we suggest you get someone else to come over and help you set it up, particularly if you're pregnant or elderly.

Don't expect to use your air conditioner right away. Some models need to be left to stand upright for as long as six hours before being used, so check the manual. 

Attaching the connectors to the hose takes patience. We've had to grapple with all the models we've tried, although some were worse than others. There's a knack to it, and you're likely to struggle unless you've had a similar appliance before.

Ideally, you need to allow 50cm around the unit for air to circulate. Never plug an air conditioner into an extension lead, either – always plug it directly into the mains socket.

When setting up wi-fi on your air conditioner (if applicable), place the air conditioner as close to the router as possible. Manufacturers often state that the unit requires a good signal during the setup process.

If you've purchased a split-unit air conditioner (as shown below), this will likely need to be installed by a professional.

Air conditioner window seal

Your portable air conditioner may come with a window sealing kit, but you'll only be able to use it if you have a certain type of window. Generally, the window kits provided are only suitable for sliding or sash windows or French windows. 

If you have a different type of window, you should still be able to use the air conditioner with the hose extended through an open window, but this will work less efficiently and use more energy. The better the room is sealed, the less hard the air conditioner has to work.

How to maintain your air conditioner

To keep your portable air conditioner in peak condition, remove bungs to drain water from portable units at the end of summer, before they are packed away for winter. If the summer has been particularly humid, you may need to drain the unit every few weeks. Water pipes are normally located at the bottom of the machine.

You should clean the dust filters and fans regularly on both portable and split-unit models. This will often involve vacuuming the filter or washing it in warm soapy water and cleaning the fan with a soft, dry cloth.

What other options are there for cooling my home?

Air coolers

Air coolers look a bit like dehumidifiers and are generally pretty portable. They draw in warm air and cool it using water that is stored in a tank. Air coolers don't require installation and do not need to be vented through a window like portable air conditioners. They are much less effective than air conditioners, though.

Air cooler features to look out for:

  • The size of the water tank – a larger tank means you'll need to refill it less often.
  • A remote control to change the settings from a distance.
  • Ice packs to add to the water tank to make cooling more effective.
  • Handles and castors that make the air cooler easier to move.

Portable electric fans

Electric fans come in many shapes and sizes. They range from basic models right up to the expensive and multi-functional Dyson Hot & Cool, which doubles as an electric heater. 

There are three main types. Desk fans, as their name suggests, are smaller fans designed to sit on a desk or table and cool you while you sit close by, while pedestal and tower fans are designed for a whole room.

Fans use just a fraction of the energy that air conditioners guzzle, as they only have to rotate lightweight blades. But while they may help your room feel less muggy, they only move the air around rather than actually cooling it.

Portable fan features to look out for:

  • A range of speed settings to control the flow of air.
  • Oscillation modes that move the fan from side to side.
  • A remote control so that you can control it without getting out of bed or getting up from the sofa.
Electric fan 475574

Explore our selection of Which? Best Buy electric fans, or see our tips on getting the most out of your electric fan.

Other ways to stay cool in the heat

There are a few useful things you can do to cool down in hot weather without having to invest in new equipment.

For starters, try closing the curtains when the sun is facing them. Closing curtains on an east-facing window before you go to bed can block the morning sun. For west-facing windows, you'll need to close your curtains in the late afternoon and early evening to block the sun.

Remember that opening the windows on either side of your house will allow air to flow through. If you own a conservatory, keep the door between it and your house closed, as a lot of heat may leak from the conservatory into other rooms.

Which? air conditioner reviews

We've tested portable air conditioners from a range of popular brands, including AEG, Argos own-brand Challenge, DeLonghi and ElectriQ. 

For each air conditioner that reaches the Which? test lab, we assess a range of important factors, including how quickly the product can cool a room by 10°C, how easy it is to use (including assembly) and how easily the filters can be removed and replaced.

Ready to buy an air conditioner? Browse through our full selection of air conditioner reviews and pick a model in line with your budget.