People all over the world are looking for a way to cool their apartment during the increasingly frequent heat wavesHowever, the cheapest air conditioning often spends twice as much electricity of better alternatives, and in some apartments it can even represent a security risk.
That's why the question arises: which device today cools the most efficiently, consumes the least amount of electricity and puts the least burden on the climate?
Mobile air conditioners: You buy cheap, you pay dearly
Mobile air conditioners are the cheapest and fastest solution, but they are often the least efficient.
They consist of one indoor unit that the user can install himself. The hot air exhaust hose is led through a window or door. In Germany, such devices can be purchased for around 270 euros.
The indoor unit is the size of a small suitcase and is generally on wheels, so it can be moved from room to room.
so-called monoblock devices they work according to the same principle as other climates. Using a refrigerant in a closed system, they produce cold on one side and heat on the other. The warm air from the room is then blown out through the hose. It cools the room.
However, compared to fixed-mounted devices, they are significantly noisier, less efficient and, according to the German organization Stiftnug Warentest, consume more than twice as much electricity for cooling.
There are several reasons for poor efficiency. The hot hose additionally heats the room, openings around windows or doors are often not well sealed, and the negative pressure created by the device in the room causes the warm outside air to enter again.
In apartments with gas boilers, mobile air conditioners can even be life-threatening. Due to the negative pressure, the odorless carbon monoxide from the gas appliance can imperceptibly enter the apartment and cause poisoning, as demonstrated by a recent case in Vienna.
That is why experts do not recommend using monobloc air conditioners in apartments with gas boilers without a prior expert check.
Electricity costs for a 70 square meter apartment in Germany when using a monobloc system are around 320 euros per year.
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Split unit efficiency
For cooling, a mobile split air conditioner is generally a better choice. With it, the indoor and outdoor units are separate, and they are connected by a cooling water about two meters long. And such devices can be installed without the help of a master.
Mobile split systems cost around 900 euros and have only been present on the European market for the last few years.
The outdoor unit can stand on a balcony or terrace, and it can also be attached to a window from the outside with a special bracket. However, the cooling line also requires an opening in the window or door, so a certain amount of heat still enters the room.
The advantage is that no negative pressure is created, which is why there is no danger in combination with a gas boiler.
Compared to mobile monobloc devices, split systems are significantly more efficient. According to the organization's tests Stiftung Warentest, annual electricity costs for one tested model were around 230 euros.
Fixed air conditioners are the best option for apartments
Fixed air-conditioning systems are even more efficient, quieter and more comfortable than mobile devices. They are composed of an external and an internal unit, and the price of the device ranges from around 400 to 2.500 euros. With installation, the total cost is from about 800 to over 5.000 euros.
The outdoor unit is usually placed on the roof, balcony or facade. The cooling line is permanently installed, and the system must be installed and checked by an authorized service.
In rented apartments, the consent of the landlord is required, while in residential buildings with owner-occupied apartments, installation is generally allowed.
Although they represent the best solution, many German landlords still avoid such an investment.
At the same time, the energy costs for cooling a 70 square meter apartment are only about 150 euros per year on average. The most efficient device in the test Stiftung Warentest he spent only 112 euros per year.
If the same device is also used for heating, annual electricity costs increase by around 500 euros. By way of comparison: natural gas heating alone for an apartment of that size costs approximately 1.200 euros per year, and prices tend to rise. This is why air conditioners can even cut energy costs in half.
For reasons of climate protection, air conditioners are currently subsidized in Germany with up to 80 percent of total costs.
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Electricity consumption for cooling is increasing
With more and more warm days around the world, electricity consumption is also increasing. In Europe and China, the number of days with temperatures above 30 degrees has increased by a quarter over the past five years. In Japan and South Korea, there are almost twice as many such days now as before 2020.
According to data from the International Energy Agency (IEA), air conditioners are owned by 93 percent of households in Japan, 90 percent in the USA, 79 percent in China, 29 percent in Southeast Asia, 23 percent in Europe and 16 percent in India.
In 2025, about 50 percent more electricity was used for cooling in the world than in 2015. The total consumption increased by about 1.000 terawatt-hours, to approximately 2.900 terawatt-hours per year. Currently, cooling accounts for about nine percent of global electricity consumption.
At the same time, the production of energy from renewable sources is growing, primarily from solar and wind power plants. During 2025, 4.300 terawatt-hours more renewable energy was produced than ten years earlier. Since solar panels produce the most electricity during the summer, when the need for cooling is greatest, this combination is considered particularly suitable.
Why refrigerant is important for climate
So-called refrigerants circulate in refrigerators, air conditioners and heat pumps. In a closed system, they evaporate and condense again, allowing the creation of cold.
However, some of these agents can cause great damage to the climate if they get into the atmosphere. Older appliances often use synthetic refrigerant R410A. If it leaks due to damage to the system or improper removal of the device, the climate effect corresponds to the emission of approximately two tons of carbon dioxide.
Therefore, from 2025, the sale of new air conditioners with this refrigerant is prohibited in the EU and the USA. It is most often replaced by R32, whose climatic impact during leakage is approximately three times smaller.
The European Union went one step further. From 2029, split air conditioners with one indoor unit will be able to use only specially climate-friendly refrigerants. Most often, it is natural gas propane, also known as R290. If propane leaks out of the system, it causes virtually no climate damage.
That is why more and more new air conditioners use propane as a refrigerant.
"We expect propane to establish itself as the standard for split systems in the long term, first in Europe and then in other parts of the world," says Michael Lehte, product marketing manager at the company. Mitsubishi Electric.
Air conditioning can further heat up cities
Where a large number of air conditioners are running, waste heat often makes the outdoor space even more uncomfortable, especially in densely built-up urban areas. In Japan, you can even see extreme heat warnings on certain particularly hot streets.
Research conducted in the Chinese metropolitan area of Nanjing, which has more than ten million inhabitants, showed that outdoor air-conditioning units in narrow streets can increase daytime temperatures by up to 1,9 degrees and nighttime temperatures by up to 3,3 degrees. This represents an additional burden for pedestrians.
Vertical greening of facades could lower temperatures by several degrees.
The International Energy Agency (IEA) recommends other measures against the so-called urban heat island effect. According to IEA estimates, new green areas, tree rows and the removal of impervious asphalt and concrete surfaces can lower the temperature in cities by as much as five degrees.
One of the solutions is remote cooling. Cities such as Dubai, Singapore, Berlin and the Chinese metropolitan area of Chongqing already use such systems to cool parts of buildings.
In these systems, the excess heat does not end up in the outside air, but is drained into rivers or underground water courses.
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