AC vs Fan vs Evaporative Cooler: What Actually Cools a Room

A fan cools you, not the room. An evaporative cooler only helps when the air is dry. The one device that actually brings the temperature down is an air conditioner. Here's the honest breakdown in degrees, plus what German law allows.

6 min read · Updated July 2026

When your flat stops cooling down at night, most people reach for a fan first. It's cheap, quiet enough and already in the cupboard. The catch: it lowers the room temperature by exactly zero degrees. An evaporative cooler buys you a few degrees, but only under the right conditions. An air conditioner does cool the room for real, and wants more electricity and a bigger upfront spend in return. Here's what each device genuinely does, and by how much.

The short answer: what actually cools a room?

The three devices do fundamentally different jobs. Mixing them up is why so many people spend money and stay hot.

• Fan: cools your body, not the air. Room temperature drop of about 0 °C.
• Evaporative cooler: cools the air stream by a few degrees in dry conditions, and adds humidity. Whole-room effect is small.
• Air conditioner: removes heat from the room and dumps it outside. A real drop of roughly 5 to 10 °C is normal.

The fan: it moves air, it does not lower temperature

A fan doesn't remove any heat. It moves air across your skin, which speeds up the evaporation of sweat and carries warmth away from your body. That can feel around 3 to 4 °C cooler while you sit in the draught. The air itself stays the same temperature, and the motor even adds a tiny bit of heat over time. One thing worth knowing: once the room climbs above roughly 35 °C, near skin temperature, a fan mostly blows hot air at you and the cooling feeling fades. Fans are cheap, use very little power and are good for taking the edge off. They are not a way to cool a room.

The evaporative cooler: a few degrees, but only in dry air

An evaporative cooler (also called an air cooler or swamp cooler) pulls warm air through a wet pad. Some water evaporates, and that process draws heat out of the air, so the stream leaving the device is cooler, often by 3 to 6 °C right in front of it. The physics only work when the air is dry enough to absorb more moisture. In a humid German summer, or in a small closed room where the humidity keeps rising, the effect shrinks fast and the air can start to feel muggy. It cools the stream, not the whole room, and you have to keep refilling water. Think of it as a fan with a modest bonus on dry days, not as an air conditioner.

The air conditioner: it actually lowers the room temperature

An air conditioner is the only one of the three that removes heat from the room and moves it outside. That's why it can genuinely drop the temperature, commonly by 5 to 10 °C depending on the room, insulation and the unit's cooling capacity (measured in kW or BTU). Not every AC is equal, though:

• Portable monoblock: a single box with a hose that vents hot air out of a window. Easy to move, no installation. Single-hose models are less efficient because they pull warm replacement air back into the room, so a good window seal matters a lot.
• Window unit: sits in the window opening, more efficient than a monoblock because the hot side is already outside.
• Mobile split with a sealed circuit: an indoor and a small outdoor part connected by a permanently filled, factory-sealed line. Quieter and more efficient, and it stays permission-free because the refrigerant circuit is never opened on site.

What German law actually allows

The legal line runs between permission-free devices and permit-required installations. Permission-free means no structural change to the building: mobile monoblocks, window units and permanently sealed mobile splits fall here. You can set them up in a rented flat without your landlord's consent, as long as you make no permanent alteration (sealing a tilted window does not count as one). A classic fixed split system is different. It needs a hole through the wall and an outdoor unit, so it is a structural change that requires landlord consent, and the refrigerant circuit may only be opened by a certified firm. Refrigerants are fluorinated greenhouse gases (F-gas), and under the F-gas rules only qualified technicians are allowed to handle them. Sealed mobile units sidestep all of that because they arrive filled and closed from the factory.

Which device fits your situation?

Match the tool to the problem instead of the price tag:

• You just want to feel less sticky and keep costs near zero: a fan.
• You have dry heat and only need to take a couple of degrees off one corner: an evaporative cooler, with realistic expectations.
• You want the room itself to be measurably cooler for sleeping or working: an air conditioner, and if you rent, a permission-free type.

So the honest ranking is simple. If you want to actually lower the temperature of a room, only an air conditioner does the job, by a real 5 to 10 °C. An evaporative cooler is a niche helper for dry days, and a fan is a comfort tool that never touches the thermometer. Pick based on whether you need to cool the air or just cool yourself, and if you are a tenant, stick to the permission-free devices so the only thing you have to think about is where to point the cold air.

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