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Window Air Conditioner: Compact Cooling For Small Spaces

Window air conditioners are units that sit in an open window and pull hot air out of your home. Window air conditioners are popular among apartment-dwellers because they don’t take up indoor space and do not require a bulky floor-mounted outdoor unit.

Window Air Conditioner
Window air conditioners contain the internal and external units in the same housing and get installed in a home’s window frame

Traditional air conditioners comprise two separate units: the evaporator inside the property and the condenser outside it. However, window air conditioners are made up of a single unit that’s internally split into the evaporator and condenser sections. The evaporator section absorbs heat from the air inside your home and the condenser section deposits that excess heat outside.

You should consider a window air conditioner if you have limited storage space but need something more powerful than a portable air conditioner. Window air conditioners are effective at cooling small spaces and most models cool a living room or even a studio flat.

However, a single window air conditioner is unable to cool an entire house as it can’t distribute cold air outside of the room in which it’s installed. Another factor to consider is that window air conditioners need a specific type of window in order to be installed properly and you need to make sure that you buy a model that matches your window frame.

The best alternatives to window air conditioners are portable air conditioners and evaporative coolers. Portable air conditioners are less powerful than window air conditioners but are more affordable and easier to move from location to location. Evaporative coolers aren’t air conditioners but are a step up from a normal fan and are much more affordable than any air conditioner. A wet pad is an evaporative cooler’s only way to reduce the temperature in your home as the evaporation uses up the heat.

The ideal window air conditioner has a cooling capacity that suits your property and a price that matches your budget. Be aware that window air-cons become a permanent fixture inside the home, so it’s best to find a model that’s aesthetically pleasing to at least some degree.

The UK online market has limited window AC options, so you’re only able to choose between the two models below.

  • TOSOT Window AC: The TOSOT comes with a 10,000 and 12,000 BTU capacity and is more affordable.
  • Soleus Over-The-Windowsill AC: The Soleus costs more than the TOSOT but offers a sleeker design and comes in 6,000 BTU, 8,000 BTU, and 10,000 BTU options.

A window AC’s energy consumption depends on the unit’s capacity and energy efficiency. Below is a rough guide to window air conditioner power usage based on cooling capacity.

  • 5,000 BTUs: 400 – 600W, or 0.4-0.6kWh
  • 8,000 BTUs: 600 – 800W, or 0.6-0.8kWh
  • 10,000 BTUs: 800 – 1,000W, or 0.8-1.2kWh
  • 12,000 BTUs: 900 – 1,200W, or 0.9-1.2kWh

Most window ACs cost between £30 – £100 to run for a month. How much you pay depends heavily on the capacity of your window AC, how long you use it daily, and your electric tariff. You could spend as much as £200/month if you run a 12,000 BTU AC 12 hours a day and pay 50p/kWh, or as little as £20 if you run a 5,000 BTU unit 4 hours a day and pay 35p/kWh.

There are lots of ways to save money running a window AC if these prices are making your eyes water. For example, you’re able to run your window AC for a few hours before bedtime to cool the bedroom before you sleep and the outdoor temperatures drop. Other tips include getting a properly sized AC for your space, installing your unit out of the sun, and only using the AC to cool certain rooms. Finally, you could consider air-con alternatives that are cheaper to buy and operate, such as fans and evaporative coolers.

Proper installation is key with a window AC. Adequately supporting your unit with wood blocks and brackets and installing a weatherstrip are essential to preventing damage to your property. Leaky window ACs encourage mould growth, while unsupported units damage your window frame over time and may fall into the street below.

It’s a good idea to consider a window AC if you live in a part of the UK that gets hot during the summer, especially as heat waves get longer and more intense. However, air conditioning is not for everyone and you should only pick cooling solutions that fit your budget. Evaporative coolers and fans remain good options.

What is a window air conditioner?

A window air conditioner is a type of air conditioner that you install in an open window. Window air conditioners are a popular choice for apartments that don’t have the outdoor space for the external condenser unit required by central and ductless ACs. Window air conditioners contain the internal evaporator unit and the external condenser unit in the same housing. An internal divider separates the two units, with the condenser facing the outdoors and the evaporator facing the interior space.

The “internal” half of a window air conditioner contains an evaporator coil, a filter, and a fan. The purpose of the evaporator coil is to circulate cooled, liquid refrigerant slowly so that it can absorb heat from the interior environment, which causes the refrigerant to evaporate. The fan blows indoor air over the refrigerant and circulates it around your room while the filter removes any harmful pollutants.

The “external” half of a window air conditioner contains a fan, a condenser coil, and a compressor. Heated, gaseous refrigerant travels from the interior side of the air conditioner and passes through the condenser coil. A fan then blows outside air over the condenser coil, which absorbs some of the excess heat. Refrigerant returns to a liquid as it cools.

The compressor mediates the refrigerant pressure between the evaporator and the condenser coils. The compressor forces the gaseous refrigerant into a denser, liquid form. The temperature of the refrigerant rises in the process, thus facilitating the transfer of heat to outdoor air.

An expansion valve sits between the condenser coil and the evaporator coil and controls the amount of refrigerant that is allowed to pass into the evaporator coil. Maintaining a lower pressure in the evaporator coil allows the refrigerant to expand into a gas, causing its temperature to drop below room temperature.

window air conditioner diagram
Window air conditioners transfer indoor heat outside using a single unit that comprises both the evaporator and condenser coils

Should I get a window air conditioner?

Consider the four factors below when deciding whether you should get a window air conditioner.

  • Cooling needs: Window air conditioners are able to cool large spaces of up to 50 square metres. However, one air-con unit is only able to serve a single space, so you can’t use one window air conditioner to cool a flat with more than one room. You need a window air conditioner for every space in your home that you wish to cool.
  • Budget: Window air conditioners sit at the lower end of the air-con price range. Likewise, these window units cost less to operate than full-home solutions, such as central and ductless air conditioners.
  • Available space: Window air conditioners have the smallest footprint of all air conditioner types. You need neither interior wall or floor space for the evaporator, nor the outdoor space for the condenser. All you need is the right window type to accommodate the unit.
  • Window type: Single or double-hung windows are best-suited for window ACs. Sliding windows work, but you need to have panels to cover up the space above the air conditioner. Casement windows are not ideal, though some larger casement windows may accommodate a window AC if you secure the unit properly to the frame and seal the opening above.

What are the advantages of window air conditioners?

There are two advantages of window air conditioners. Firstly, window ACs have no separate internal or external units to be mounted to the interior or exterior walls. Instead, window air conditioners comprise a single unit with both the external and internal components, and this unit rests on your windowsill. The absence of individual internal and external units means a window air conditioner is a good choice for apartments with little interior or exterior wall space. Secondly, window ACs have relatively low purchase and running costs. Window air conditioners are less expensive than central and ductless systems because they do not require the installation of two separate units, ductwork, or refrigerant lines.

What are the disadvantages of window air conditioners?

There are three disadvantages of window air conditioners. Firstly, window ACs have a relatively low cooling capacity of 5,000-18,000 BTUs, which allows them to serve spaces up to 50-60 square metres. In contrast, central, floor-mounted, and ductless air-cons are often as powerful as 40,000-60,000 BTUs. This high cooling power allows the larger air-con systems to cool entire houses and spacious apartments with ease. Secondly, you need a separate window AC unit for every space you wish to cool. A window air-con has a single set of evaporator coils via which chills the air used to bring temperatures down in a space. In contrast, central, floor-mounted, and ductless air conditioners are designed to target multiple zones within a home. Likewise, portable air conditioners may be used to cool more than one space since it’s easy to transport them from one room to another. Thirdly, not all windows are able to accommodate an AC unit. For example, casement windows are not designed for ACs. You must screw the unit to the frame and seal the space above the unit if you’re planning to fit a window AC into a casement window.

The disadvantages of window air conditioners are mostly tied to whether this type of air conditioner fits your property. Small apartments are well-suited to portable air conditioners but larger properties may need something more powerful. Similarly, window air conditioners work well if you have the right type of window and are looking to save on floor space.

What is the best alternative to a window air conditioner?

A portable air conditioner is the best alternative to a window air conditioner. Portable air conditioners are the most similar to window air conditioners in terms of price and output. Like window AC, portable air-cons comprise a single unit that houses both the evaporator and the condenser, so there’s no need for two separate units or ducts, wiring, and refrigerant lines between them. The advantage of choosing a portable air conditioner over a window air conditioner is that you’re able to move the former from room to room with ease. In contrast, moving a window unit from one room to another requires lots of effort for two key reasons. Firstly, a window air-con does not have wheels, which come with every portable AC. The lack of wheels means that you must carry the bulky and cumbersome appliance from one window to another. Secondly, most window ACs must be fastened to the opening in one way or another, so you must unfasten the unit and then reattach it properly every time you move it. The two challenges described above make moving window ACs from room to room impractical. Thus, portable ACs are a fitting alternative to window ACs as they don’t limit cooling to a single space.

Which is better, a window AC or a portable air conditioner?

There are two reasons why a portable air conditioner is better than a window AC. Firstly, portable air-cons are easy to move from one room to another, whereas moving a window AC is far more challenging. The mobility means that you’re not limited to using your portable AC in a single space. You’re able to run the portable air-con in the living room during the day, then move it to the bedroom when you go to sleep. Secondly, portable air conditioners do not require any installation. All you need to do is fit the exhaust hose through a window and set up the panel that seals the opening, a process that takes 5 minutes and requires no modifications to the window frame. In contrast, a window AC must be adequately secured to the window to prevent the unit from falling out.

On the other hand, window ACs may be better for you if you’re not a fan of portable air-cons’ unsightly exhaust hose. You may choose to go with a window unit if the poor aesthetics of a portable air conditioner outweigh the benefits its mobility brings to the table. However, remember that a portable AC easily fits into a closet or any other storage space when you don’t need it, whilst a window unit remains an eyesore year-round.

Which is better, a window AC or an evaporative cooler?

Whether a window AC or an evaporative cooler is better depends on your expectations and circumstances, since the two systems are substantially different. Consider the following three factors when deciding whether a Window AC or a cooler is better for you.

  • Cooling needs: Evaporative coolers are not air conditioners and are unable to change your home’s internal temperature meaningfully. Think of an evaporative cooler as a highly effective fan that produces a cooling breeze through evaporation. You should choose a window AC if you need to lower indoor temperatures substantially, especially if you’re concerned about preventing heat stroke during periods of hot weather. However, evaporative coolers are effective in exceptionally dry climates, where the cool, moist breeze is able to improve your comfort more effectively than a fan at close range.
  • Budget: Evaporative coolers are much cheaper than window ACs. A typical evaporative cooler costs £60 on average, whilst the costs of a window AC range between £500-£1,500. You may want to choose an evaporative cooler if a window AC is out of your budget.
  • Mobility: Evaporative coolers are easy to carry around with you from one room to another. In contrast, window ACs take a while to install properly, and moving them between different rooms is impractical.

How to choose a window air conditioner?

Choose a window air conditioner according to your budget, cooling needs, and property configuration. Consider the following five criteria when picking the ideal window air conditioner model.

  • Cooling capacity: The cooling capacity of a window air conditioner is measured in BTUs (British Thermal Units). A higher BTU rating means that a window air conditioner is capable of cooling more space. You should opt for a window air conditioner that has a BTU rating matching the size of the room you want to cool.
  • Physical dimensions: Size is a big factor in picking the perfect window air conditioner for you. Make sure the unit you choose fits into your window of choice. At the same time, make sure that the window configuration is able to accommodate the AC.
  • Design: Aesthetics are a matter of personal preference. A window AC unit is quite visible inside your home, so pick one that you won’t hate looking at.
  • Price: Pick a window air conditioner that balances your budget with your cooling needs. Lower-capacity window air conditioners are generally more affordable as are units with fewer features.
  • Operating costs: Calculate the hourly running costs by taking your electric unit rate and multiplying it by the hourly energy consumption of the unit you’re considering. Estimate how many hours per day and month you intend to use the unit to get a better idea of how much the window AC costs to run.

The above advice is a rough guide. The best way to know exactly the right window AC for your property is to consult the manufacturer’s guidelines on a specific window AC unit. This literature should give you a clear idea of what windows and properties a unit works best for.

Best window air conditioners in the UK

Finding the best window air conditioners in the UK is a challenge because of limited options. Below are the two window air conditioners that are readily available online in the UK.

  • TOSOT Window AC: The TOSOT is the more affordable of the two models and comes in two sizes – 10,000 BTUs and 12,000 BTUs, making it a better choice if you want a more powerful air conditioner.
  • Soleus Over-The-Windowsill AC: The Soleus is more expensive and comes with a sleek modern design that will fit in well in any home. The Soleus has three cooling capacity options – 6,000 BTUs, 8,000 BTUs, and 10,000 BTUs.

Soleus Over-the-Windowsill Air Conditioner

Soleus Over the Windowsill Air Conditioner
Soleus Over-the-Windowsill Air Conditioner

The Soleus Over-the-Windowsill Air Conditioner is an innovative air conditioner that fits beautifully into any home and doesn’t block your view. This window AC model is not cheap, but the premium cost gets you a unique, convenient design, high cooling power, and advanced features, like smart speaker integration.

Most customers are drawn to the Soleus Over-the-Windowsill because of its appealing design. The Soleus is shaped like an upside-down saddle that hangs over your windowsill so that the thinnest part of the unit passes through the window. The genius of this design is that your window offers an unobstructed view even with the AC unit installed. The AC’s design also means that installation is as simple as hanging the unit on the window sill, as the exterior unit does not require any additional supports.

Below are the four ways you’re able to control the Soleus wirelessly.

  • Hybrid AC smartphone app
  • Amazon Alexa
  • Google Home
  • Remote controller (included in the package)

The remote control that comes with the Soleus has a temperature sensor that allows the unit to target a specific temperature at your location. In contrast, most other window AC models take their temperature readings in the area surrounding the air conditioner itself. Basing its cooling output on the ambient temperature in the user’s proximity allows the air-con to meet the cooling needs with greater precision.

The Soleus features include a 24-hour timer, energy saver mode, and a sleep mode that reduces noise levels during the night. This setting is remarkably quiet with a noise level of 38 decibels, which is just above the volume of a whispered conversation.

Price is an obstacle for many customers as the Soleus costs £1,600 – £1,900. However, the operating costs are relatively modest. Soleus is slightly less expensive to run than the TOSOT with an hourly cost of 22p – 47p to run for an hour or £41.00 – £84.42 to run for 6 hours a day for a month. We calculated these figures by taking The Soleus’ .67 kWh power consumption and current UK energy prices of 34p – 70p per kilowatt hour.

The Soleus comes in three capacity sizes – 6,000 BTUs, 8,000 BTUs, and 10,000 BTUs. A 6,000 BTU air conditioner can cool a space of 150 – 250 square feet, which effectively cools a small room. An 8,000 BTU air conditioner can cool a space of 300 – 350 square feet and a 10,000 BTU air conditioner can cool a space of 400-450 square feet. The 10,000 BTU air conditioner would be able to cool a small apartment or a large living space.

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TOSOT Window Air Conditioner

TOSOT Window Air Conditioner
TOSOT Window Air Conditioner

The TOSOT Window Air Conditioner is a powerful unit that offers 10,000 or 12,000 BTUs of cooling power and is a more affordable option than the Soleus.

One of the advantages of choosing the TOSOT is that it comes with a built-in thermostat that senses the temperature in your room and automatically adjusts its output to hit your desired temperature. There’s a remote control and a light touch control panel if you want to tinker with the settings yourself.

The TOSOT is a 3-in-1 device like many other window air conditioners. The unit is able to operate as an air conditioner, a fan, and a dehumidifier. Brits may have more use for the dehumidifier function even when it’s too cool outside to need an air conditioner.

You are able to run the TOSOT in sleep mode if you need it on during the night. Sleep mode brings the unit down to 55 decibels of noise, around the volume of a normal conversation between two people.

The TOSOT costs £1,200 – £1,400. That’s a lot compared to the list prices for window air conditioners in the US, where the product is more common. You can buy window ACs on Amazon US for £400 – £1,000. However, the TOSOT is still more affordable than the Soleus, the other main window AC option in the UK.

The TOSOT is small compared to the Soleus with dimensions of 57.91 x 54.36 x 40.13 cm. The unit’s 29.48 kg weight means you should get help when installing it.

Running the TOSOT consumes .84 kilowatt hours of energy per hour. That translates to an hourly cost of 28p – 58p to run for an hour or £51.40 – £105.84 to run for 6 hours a day for a month. We assume energy rates of 34p – 70p per kilowatt hour, which reflects current UK energy prices.

The TOSOT comes in two capacity sizes – 10,000 BTUs and 12,000 BTUs. A 10,000 BTU air conditioner is powerful enough to effectively cool a space of between 400 – 450 square feet while a 12,000 BTU air conditioner can cool a space of between 450 – 550 square feet. Those capacities make the TOSOT Window air conditioner suitable for cooling a small flat or large living space (like an open-plan kitchen/living room) but not powerful enough for a typical home.

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How much electricity does a window AC use per month?

How much electricity a window AC uses per month depends on how much power the unit draws and how much you use it. For example, a 1,000 watt unit consumes up to 1 kilowatt for every hour of operation. Running this 1,000-watt air-con for six hours per day would result in the daily usage of 6kWh of electricity and a monthly consumption of 183kWh. Use the formula below to determine how much electricity your air-con consumes monthly.

P (power consumption in kWh) x T (hours of daily use) x 30.42 (average number of days in a month) = monthly usage in kWh

Multiply the product of the formula above by your electric unit rate to get an estimate of your air-con’s monthly running costs.

Is it expensive to run a window AC?

Whether it’s expensive to run w a window AC depends on the unit’s wattage, how often you use it, and your electric tariff. For example, the TOSOT window air conditioner has a power of 1,000 watts, which means it uses roughly 1 kW for every hour of operation (the precise hourly power consumption varies based on the power setting you use). Assuming you use the air-con for 6 hours every day and your electric unit rate is 35p/kWh, use the formula below to calculate the daily running costs.

1kWh x 6h x £0.35 = £2.10/day

Daily running costs of £2.10 mean that it costs approximately £65 per month to operate the unit.

Is it cheaper to run a fan or a window air conditioner?

It is cheaper to run a fan than a window air conditioner. Fans consume much less electricity than air conditioners because they have a single motor with a single function: to revolve the fan blades. These revolutions do not require much electricity to produce, so the average power consumption of a fan is roughly 75 watts per hour. In contrast, air conditioners have two fans (or blowers), as well as a compressor, which alone draws at least 1,000 watts per hour as it changes the pressure and state of the refrigerant. Overall, it’s safe to say that fans consume roughly ten times less electricity than air conditioners.

What is the most efficient way to use a window AC?

The most efficient way to use a window AC is to cool your home in the morning. The indoor air is typically at its coolest temperature in the morning, so the air-con does not have to work hard to keep the space cool: it’s simply maintaining the current ambient temperature. In contrast, running a window AC during the hottest part of the day forces the compressor to work harder and consume more energy as the refrigerant has to move more heat outdoors.

Running an air conditioner during the summer may be a significant expense even if you only use it during the cool morning hours. Below are four strategies you should consider to reduce your air-con’s energy consumption — and the burden on your wallet.

  • Size your AC correctly: Choosing a window AC that’s not too big or small in terms of output is critical to keeping your energy bill in check. A small AC consumes more energy when struggling to cool a large space, while a unit that’s too powerful goes through frequent on-off cycles to maintain the desired temperature. On-off cycling is inefficient because your unit uses a great deal of energy during the start-up process.
  • Locate the AC away from direct sunlight: Position your window AC out of direct sunlight, if possible. Direct sunlight heats up your unit during the day, so it has to use additional energy to expel heat and supply sufficiently cold refrigerant to the evaporator coils. Keeping a window AC out of direct sunlight may improve its efficiency by as much as 10%.
  • Cool strategically: Install your window AC strategically in the space where you need conditioned air the most. For example, you may want to fit your air-con into the bedroom if you struggle to sleep well during heat waves. Conversely, consider installing the unit in the living room if that’s where you spend most of the day.

How long should you leave a window air conditioner on?

How long you leave a window air conditioner on depends on the following three factors.

  • Indoor space: Using your window AC to cool a larger indoor space means you need to run the unit for longer in order to get that space to the desired temperature. You don’t need to run your AC for as long if your window AC is powerful relative to the size of the space you’re cooling.
  • Outdoor temperature: The hotter the outdoor temperature, the longer you’ll need to run your AC. A warmer outdoor environment means your home is letting more heat in every hour, so you must keep your AC running longer to maintain the same indoor temperature.
  • Budget: Running a window AC is expensive and how long you should run one for will depend on your budget. You may want to run your AC longer to maintaing a comfortable indoor climate, but doing so comes with a financial trade-off.

How to install a window air conditioner?

Install a window air conditioner by placing appropriate blocks and braces to support the window air conditioner, placing the window air conditioner in the open window, and effectively sealing the window opening. You may need additional materials like weather strips, blocks, and braces depending on the dimensions and design of your window. Follow the six steps below to install a window air conditioner.

  1. Set up blocking: A window air conditioner is only able to rest safely in your window opening if you create a level surface to support it. Check that the bottom rim of your window is level with a bubble level and add wood blocks where needed.
  2. Install bracing: It’s a good idea to install a brace on the outside of your window to support the weight of your air conditioning unit if your exterior windowsill is too small or if your air conditioning unit juts too far out of the window. You don’t want your unit to fall onto the street below.
  3. Install a weatherstrip: The next step is to install a weatherstrip around the window AC unit. The weatherstrip creates a seal between the air conditioning unit and the window that keeps moisture and hot air out of your home.
  4. Position the AC unit in the window: Next, install the air conditioner into the window opening. Air conditioning units are heavy, so ask someone to help you move the unit safely.
  5. Configure the wings: Most window air conditioners come with wings that extend out to the sides of your unit. Use either these wings or a special window kit to properly close the opening on either side of your air-con.
  6. Connect to the power source: The final step is to connect your window air conditioner to a suitable power source.

How to fit an air conditioner in a window?

You fit an air conditioner in a window using a window kit or the wings that come pre-attached to the unit. It is crucial to select a window AC unit that fits the windows you have available, so take accurate measurements of the window and check manufacturer size guidelines. Take the four following steps to ensure you fit an air conditioner in your window properly.

  1. Select the window: Select the most suitable window in your home to house the unit. Pick a window that slides vertically, creates as large an opening as possible, and has a sufficiently wide sill to support your unit properly.
  2. Measure the window: Measure from the windowsill to the highest point to which your window opens, then measure the width of your window from frame to frame.
  3. Select the AC unit: You now need to look for air conditioners that match your window measurements. The manufacturer should include a minimum and maximum window size in the unit details. Leave at least a centimetre of space on every side of the unit to leave space for a weatherstrip.
  4. Install the AC unit: Extend the wings to reach the sides of the window and seal the spaces. You should be able to close the window to the exact height of the window AC so that there is no clearance between the top of the air conditioner and the window. Use a weather strip on top of your windowsill and the top of the air conditioner to create an airtight seal that keeps moisture and hot air out of your home.

The size of the window isn’t the only consideration when fitting a Window AC. You also want to make sure that your window has a large enough windowsill to support the weight of a Window AC safely. You may want to consider installing external brackets if not.

Do window ACs damage windows?

Window ACs may damage windows, which is why some window assemblies come with warranties that prohibit the use of window air conditioners. There are three primary ways in which a window AC may damage a window if it’s installed incorrectly. Firstly, the air conditioner may weaken or crack the window frame. Window ACs are heavy and often rest directly on the bottom portion of the window frame. This excessive weight may cause cracks in the frame. However, this should not be too much of a risk provided you purchase a window AC properly designed for the frame’s material and configuration. Secondly, a window air-con unit may let in moisture around the edges of your window, leading to mould growth. This build-up of moisture occurs either because condensation leaks out of the air conditioner or rainfall gets in through the cracks around the unit. Thirdly, the AC installation process may leave scratches on your window’s paintwork.

That said, preventing air-con-related window damage is simple if you follow the two tips below.

  • Provide adequte supports: Protect your windowsill by adding wooden blocks or external braces that help support the weight of your air conditioner.
  • Install weatherstrips: Apply the weatherstripping to the top of your windowsill and the top of your AC unit in order to prevent rainfall from leaking through. Positioning your AC at a very slight angle (leading outside) during installation can help condensation drain effectively.
  • Take care during installation: Get someone to help you lift and mount the cumbersome unit onto the window frame to avoid losing control and scratching the window during installation.