Cooling load calculator

Cooling load calculator for sizing air conditioning

What goes into a cooling load

1. Solar gain through glazing — window area × solar irradiance for the orientation × the glass's g-value × a shading coefficient for blinds, overhangs or louvres. West and south-west glass is the worst case.
2. Glass conduction — window area × U-value × the outdoor–indoor temperature difference.
3. Fabric — only walls that face outside, using a sol-air temperature to allow for sun on the surface; the roof only on a top floor.
4. People — sensible and latent heat per person for the activity (for example 65 W + 50 W seated, 75 W + 65 W office work).
5. Lighting and equipment — W/m² by type, with diversity factors (0.85 lighting, 0.8 equipment).
6. Fresh air — infiltration plus outdoor air per person; offices at 10 l/s per person (Approved Document F 2021). Includes the latent load from humid outside air.

AeroCalc adds a 10% design margin and recommends the next standard unit size — from a 2.5 kW wall split up to commercial cassettes, VRF and packaged units. Portable units are sized 15% up because their exhaust hose draws in warm air.

Why rules of thumb get it wrong

"100 W per square metre" ignores the things that matter most. A north-facing bedroom with small windows may need well under that; a west-facing office with floor-to-ceiling glass, eight people and computers can need twice as much. The unit type doesn't change the load — only the room does.

Design conditions

Choose a city preset (London 29.5°C, Manchester 26.4°C, Madrid 38°C, Dubai 46°C and more) or set the outdoor design temperature and humidity yourself, plus the indoor setpoint and humidity.

From calculation to quote

Save each room to a location, share it with your team, and — if you use LilaCRM — send the rooms and their cooling loads to the job as a survey, ready to price.