A 3 ton air conditioner is enough

Which air conditioner for which room size?

Outside, the sun literally glows on your skin, the heat makes it difficult to breathe - but in your own four walls you can breathe deeply and relaxed: An air conditioning system ensures fresh coolness. But only if their performance is sufficient for the space. This UPDATED guide will tell you what power your air conditioning system needs to have so that the bedroom, children's room or winter garden have a pleasant temperature.

BTU and watt: These are the units of measurement for an air conditioning system

In order to choose the right air conditioner for your own four walls or for the office, two units of measurement are important for the cooling capacity: BTU and watt. BTU stands for "British Thermal Unit" and describes the energy it takes to heat a pound of water by one degree Fahrenheit. In the area of ​​air conditioning, however, BTU stands for BTU / h, i.e. the cooling capacity per hour.

Watt (with the unit symbol W) also denotes a certain amount of energy used per unit of time. 1,000 BTU / h corresponds to 293,071 watts. In the following, the required power of the air conditioning system is calculated and specified in watts. If you come across BTU while looking for the right device, you simply take the watt number roughly times three and round it up.

The thermal energy that has to be transported out of the room, so to speak, is called the “cooling load”. The cooling capacity, which is indicated by watt for the air conditioning system, shows how much cooling load the device can handle.

3 tips to keep the air conditioning running smoothly

Help the air conditioner cool the rooms by making sure it can run smoothly. To do this, you should keep the following three points in mind:

  1. Close windows and doors.
    Heat flows in through open windows and doors, which puts additional strain on the air conditioning. In addition, many systems have sensors that detect open windows and then switch off the device.
  2. Keep the openings clear.
    The intake and exhaust air ducts should always be free from blockages, for example from leaves or dust.
  3. Clean the facility regularly.
    Regularly replacing the filter and cleaning and disinfecting the air conditioning system will help to maintain optimal performance.
    You can find out how to keep your air conditioning clean in the guide
    How, when and why should the air conditioning system be cleaned and disinfected?

How to calculate the cooling load depending on the size of the room

You can obtain an initial value for the required cooling capacity per room using the following rule of thumb: Room area in square meters multiplied by 60 (with good insulation and little solar radiation) to 100 watts (with little insulation and high solar radiation).

For a 20 square meter room this means: 20 square meters x 100 W = 2,000 W. Often the cooling load is given in kilowatts (kW) instead of watts. The following applies: 1 kW = 1,000 W. In our calculation example, you therefore need an air conditioning system for a cooling load of 2 kW.

Another rule of thumb is: Cubic meters of space times 40 watts. If the 20 square meter room from the previous example is about 2.5 meters high, calculate: 20 square meters x 2.5 m x 40 W = 2,000 W.

Depending on the rule of thumb, room height and insulation or window area, you get the same cooling load: 2,000 watts or 2.0 kW or around 6,800 BTU.

By the way: The required cooling capacity does not have to be provided by an air conditioning system alone. Instead of one device with 2 kW cooling capacity, you can also choose two devices with 1 kW capacity each. The advantage: The cool air is distributed more evenly in the room.

The cooling of the rooms does not necessarily have to be done by a single device. You can also divide the required power between two air conditioners.

These factors affect the cooling load

The calculation of how much power the air conditioning system needs to effectively cool down a room does not only depend on the size of the room. There are other sources of heat that come into consideration, for example:

  • Which rooms are next to, above or below the room (air-conditioning, basement, attic)?
  • How much window area with what solar radiation does the room have?
  • How is the room insulated? Are there many gaps and gaps such as doors or ventilation grilles through which cold can get out and heat can get in?
  • How many people are there in the room?
  • Which technical devices (computer, TV, but also lighting) give off how much heat?

With air-conditioned adjoining rooms or good insulation, you can set the cooling capacity correspondingly lower, with many heat sources and poor insulation you should choose a stronger device.

Add about 250 watts for each person in the room. For electronic devices, use the watt consumption as a yardstick: If your PC is running at 200 watts of power, add 200 watts to the required cooling capacity. You can find out how many watts a device consumes on the glued-on nameplate or in the operating instructions.

Whether bedroom or living room, whether attic or mezzanine floor makes a difference when calculating the cooling load.

Online calculator for calculating the cooling capacity per room

Regardless of whether it is an attic, children's room or bedroom: every room has its own individual requirements for cooling and thus for air conditioning. Online tools will help you with the detailed calculation.


The tool from Thermoflux, a manufacturer of alternative heating systems, is easy to use and shows you the required cooling capacity in watts after just a few details.

Here you can find the climate calculator from Klimaworld

Stiebel-Eltron cooling load calculator

The manufacturer Stiebel Eltron also offers a tool. The size and alignment of the walls are shown as a sketch.

Here you can find the cooling load calculator from Stiebel Eltron

Find out what else you can do to keep your home cool in this guide from Roombeez.