Quick calculation of the required cooling capacity for living and office spaces

How much power is required to cool a room?

The rule of thumb: Every cubic metre of room volume requires a cooling capacity of 30 watts.

According to this rule of thumb, the required cooling capacity can quickly and easily be determined, as can be seen from the following example calculation with a room’s floor space of 35 m² and a ceiling height of 2.5 m:

35 m² x 2.5 m ceiling height =
87.5 m³ cubature x 30 watts =
2,625 watts

This is only a rough calculation formula for living and office spaces with modern insulation (passive house standard), though. The required cooling capacity further depends on the room’s “thermal load”: For selecting an appropriate air conditioner, the factors of insolation, insulation, window dimensions, the number of persons as well as the heat sources play an equally important role.

No rule without exception

No one really has 1.47 children. And yet that is the German statistical average. Reality also does not usually offer an ideal-typical standard room on which the room-size-dependent 30 W-rule for the cooling capacity calculation is based. Still, it is a room that – according to the statistics – you are most likely to find and hence serves as basis of calculation.

This principle is well-known from the manufacturer’s specifications regarding the fuel consumption of your car. One will never fully attain the values in practice, but all manufacturers adhere to the same evaluation procedures regulated by law to permit a comparison of the different vehicles. The situation is not so very different for air conditioners.

The recommendations for room size suitability are based on ideal-typical conditions to be found indeed on a statistical average, but hardly ever in real life. We as individual producer cannot single-handedly adjust the equipment labelling for this would eradicate any chance of comparing the devices with competition models. One thing is certain: A device marked as suitable for 30 square metres has more or less the same cooling capacity regardless of the manufacturer. Possible room size recommendations are usually based on the rule of 30 watts per cubic metre.

Planning with practical aspects in mind and allowing for reserves as well

If you want to achieve a more distinct cooling effect, proceed with your capacity planning on the assumption that not all parts of your room will correspond to the statistical standard and therefore factor in some spare capacity for good measure. Not least because the number of occupants may vary and now and then there can be spells of particularly hot weather.

If nothing else, this is due to the individual requirements that go hand in hand with bringing about and maintaining an agreeable room climate under changeable circumstances.

As the figure below indicates, there can be various factors influencing the room size recommendation – as a result the basic value for the calculation is no longer 30 watts per cubic metre but 60 or more still. Accordingly, an air conditioner recommended for a room size of 40 m² can only effectively cool rooms of 20 m² in these changes conditions.