The formula
cooling load = volume × 40 × aspect factor × insulation factor + internal gains
Where it comes from
The quick rule of 400 BTU per square metre ignores two things that matter a great deal: which way the room faces and how it is insulated. A south-facing room needs around 30 % more capacity than the same room facing north, and single glazing adds another 20 %.
How to work it out by hand
- Multiply the floor area by the height for the volume
- Multiply by 40 kcal/h per cubic metre
- Apply the aspect factor and the insulation factor
- Add 100 kcal/h per person and 200 per heat-producing appliance
What is worth knowing
Undersizing is worse than slightly oversizing: the unit runs flat out without reaching temperature and uses more. But oversizing costs too, because the unit cools fast, cuts out and never dehumidifies, leaving an unpleasant clammy feel. To convert between the units a manufacturer might use: 1 kcal/h is 3.968 BTU/h and 1.163 watts.