Start with a baseline design load
Every calculation starts from a baseline figure in watts per square metre of floor area, which varies by how the space is used: 120 W/m² for a home, 160 W/m² for a commercial space like an office or shop, and 200 W/m² for an industrial space such as a warehouse or plant room.
For a 20m² room, that means a domestic baseline of 20 × 120 = 2,400 W (2.4 kW), before any adjustments.
Four factors adjust the baseline
The baseline is then multiplied by four separate factors, each reflecting something real about the room:
| Factor | Values | What it captures |
|---|---|---|
| Insulation | Poor ×1.15 · Average ×1.00 · Good ×0.90 | UK insulation standards tightened sharply around 1980, and again in 2002 under Building Regulations Part L — so we use building age as a reliable proxy when you don't know your wall U-value. |
| Sun exposure | Low ×0.95 · Moderate ×1.00 · High ×1.15 | A shaded room needs less cooling than one with large south-facing windows. |
| Occupancy | Low ×0.95 · Typical ×1.00 · High ×1.10 | More people in a room means more body heat to remove. |
| Ceiling height | 0.85× to 1.60×, scaled from a 2.4m standard | A higher ceiling means more air volume to cool, clamped at the extremes so an unusual room doesn't produce an unrealistic result. |
Heat-generating equipment adds a flat amount
If the room has heat-emitting equipment — a home office setup, kitchen appliances, a server rack, a commercial kitchen — we add a flat amount on top: none adds nothing, moderate adds 0.5 kW, and high adds 1.5 kW.
Multi-room jobs apply a diversity factor
When you're cooling more than one room with a shared (multi-split) outdoor unit, we apply a diversity factor of 0.8 to the combined total — because not every room hits peak demand at exactly the same moment, the shared unit can be sized smaller than the sum of every room running flat out, without leaving anything underpowered. This only applies to multi-split systems; single-split rooms each have their own dedicated outdoor unit and are sized individually, at full duty.
A worked example
A 4m × 5m (20m²) domestic living room, average insulation, moderate sun, typical occupancy, standard 2.4m ceiling, no extra equipment: 20 × 120 W/m² = 2,400 W, and every factor is 1.0, so the required duty is exactly 2.4 kW (about 8,189 BTU). The same room as a commercial office would need 3.2 kW; as an industrial space, 4.0 kW.
FAQ
Why does my quote ask about building age instead of insulation directly?
Most people don't know their wall's exact U-value, but building age is a strong, easy-to-answer proxy — insulation standards changed materially around 1980 and again in 2002. If you're not sure, we assume a typical mid-range level rather than guessing an extreme.
Does a taller ceiling always mean a bigger system?
Yes, but the effect is capped — the height factor only scales between 0.85× and 1.6× of the standard 2.4m baseline, so an unusually tall or short ceiling doesn't produce an unrealistic result.