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11 June 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 your room quickly and evenly. The worst will be slow to cool your room, make a racket and be such a hassle to set up that they leave you hot and bothered. 

For many people, an electric fan will be a better option than air conditioning. If you already know that you want an air conditioner, but you're puzzled by words like 'portable', 'standalone', 'BTU' and 'refrigerant', read on to find out all you need to know before buying. 

We've reviewed air conditioners for a range of budgets. Head over to our air conditioner reviews to see all those we've tested. 

What are the different types of air conditioner?

There are two main types of domestic air conditioner:

Portable air conditioners

Portable air conditioners are also known as single-unit air conditioners, standalone air conditioners and mono block air conditioners. You plug these into a mains power socket and dangle the hose out of a window or door, as pictured above.

Split-unit air conditioners

Split-unit air conditioners are also called fixed air conditioners. These have an indoor and outdoor unit – the indoor unit is fixed to inside of an exterior wall, and the outdoor unit is fixed on the other side of the wall or installed on the ground outside.

Portable air conditioners: pros and cons

Pros of portable air conditioners:

  • Tend to be cheaper than split-unit air conditioners – prices start at £380.
  • An effective way to cool your home without needing to install anything permanently, meaning you can change the room you have it in, if you want to. Hot air is vented out through a hose, via an open window. 
  • Can be unplugged and placed in storage at the end of the season. 
  • More expensive models can double as dehumidifiers, electric heaters or occasionally as air purifiers. If it's cleaner, rather than cooler, air you're after, head to our air purifier reviews. And, if your home is damp or cold, discover the best dehumidifiers and electric heaters.

Cons of portable air conditioners

  • Portable air conditioners aren't as efficient at cooling as split-unit models. 
  • If you run the hose out through an open window, this will allow some warm air back in through the gap. Some (but not all) come with a window sealing kit for blocking off the gap from the open window, but these are only usable with certain types of window (such as sash windows).
  • They're generally heavy appliances. Many have castors and handles to make them easier to shift, but these aren't always that useful. If you have reduced strength and live alone, enlist the help of someone else to help you set up your air conditioner at the start of the season and get it back into storage afterwards.

Split-unit air conditioners: pros and cons

Pros of split-unit air conditioners

  • 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 units.

Cons of split-unit air conditioners

  • Tend to be more expensive than standalone units. You can expect to pay from around £500 for a split-unit air conditioner, excluding possible installation fees.
  • Fixed in place, so can't be moved between rooms.
  • Needs to be permanently mounted on an outside wall by a certified professional.

What size air conditioner do you 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. There's a calculation you can use to work out what BTU is right for you. As a very rough guide, multiply the dimensions (in feet) of the room by five. 

So for a room measuring 15x10x8 feet: 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 one with a good energy efficiency ratio (EER). This is the ratio between an air conditioner's BTU and its the power input (in watts). 

In general, 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 tests, according to the European Standard BS EN 14511:2013, and we often find the claimed and actual energy class to be different. 

What extra features should I look out for when buying a portable air conditioner?

Dehumidifier mode

Some air conditioners, such as the DeLonghi PACAN98ECO Real Feel or the AEG Chillflex Pro AXP26U338CW, 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 (often not supplied) to drain away 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: something you wouldn't need to do with a standalone dehumidifier.

Check out our dehumidifier reviews to buy one that's tried and tested.

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). Three is a typical number of fan speeds. Many models also have a fan-only mode, which has no cooling effect and simply circulates the air within the room. 

As with the dehumidification function, we don't recommend buying an air conditioner purely for the fan function. You can save on floorspace and money (and the faff involved in setting everything up) by just buying a fan instead. 

Heating mode

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

Remote control or smartphone control

Useful if you want to adjust the settings without moving from your seat. We put all smart air conditioners through a privacy and security test. 

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. No air conditioner will be silent, even on sleep mode. 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 a model with a clock and 24-hour setting. Some only have countdown or delay timers that need to be reset daily.

Setting up an air conditioner

Here are a few pointers on getting your portable air conditioner set up correctly. 

  • Setup is easier with two people. Although technically 'portable', all of the models we tested were very heavy (up to 30kg) and awkward to lift out of the box. If you live alone, get someone else to come over and help you set it up, particularly if you're pregnant or elderly.
  • Don't expect to use it right away. Some need to be left to stand upright for as long as six hours before being used. Make sure to check the manual. 
  • Attaching the connectors to the hose takes patience. We had to grapple with all the models we 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.
  • Allow 50cm around the unit for air to circulate.
  • Never plug an air conditioner into an extension lead. Always plug it directly into the mains socket. 
  • Air conditioners often shouldn't be installed in laundry rooms, wet rooms or bathrooms. 
  • 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.  

Using a window seal

If your portable air conditioner comes with a window sealing kit, you'll only be able to use it if you have certain types 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 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). 

However, with some windows even this may not be possible – for example, if your window opens from the top inwards, and the hose isn't long enough to reach. Make sure you have a suitable window before investing in a portable air conditioner.

Which? air conditioner reviews

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

For each air conditioner we assess a range of factors, including:

  • How quickly it can cool a room by 10°C 
  • How easy it is to use, including assembly
  • How good the instruction manual is
  • How easily the filters can be removed and replaced
  • How easy it is to move around
  • How noisy it is
  • How energy efficient it is
  • How closely our measurements of its energy class match the manufacturer's claims. 

Check out our air conditioner reviews to see all those we've tested. 

Maintaining your air conditioner

Here's how to keep your portable air conditioner in peak condition for as long as possible:

  • 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. Before you buy, check in store that you can bend down and unplug the water pipes easily, as sometimes bungs can be fiddly and difficult to grip. 
  • Clean dust filters and fans regularly on both portable and split models. This will often involve vacuuming the filter or washing it in warm soapy water and cleaning the fan with a soft, dry cloth. However, this varies from model to model – always follow the instruction manual.

Not sure if you need an air con after all? Our Should I buy an air conditioner?explains the pros and cons of air conditioners, and alternative ways you can cool down your home.