Air-con calculator

What it works out

This estimates how much heat each room takes in on a hot day, its cooling load, in watts and BTU/h, and the split air-con unit size that covers it. Add one room or several: you get each room's figure and a unit for it, which is what you choose air-con by.

It also checks whether the unit's heating would keep the room warm in winter, since most split units are heat pumps too. It's an estimate to compare units with, not an air-conditioning design. A split unit's refrigerant circuit is for a certified installer, and its power supply is electrical work (see our electrical guidance).

How it works

Heat comes into a room several ways. Sun through the windows is usually the most: each window's area × the peak sun on glass facing its way × the share its glazing lets through (g) × its shading. A west window gets 500 W/m² at its peak, a north one 100, a roof window 650.

Through the fabric: the external walls and doors take U-value × area × (the outside temperature less the inside, plus 4 °C for the sun on them), a ceiling under a roof plus 12 °C, the windows the plain difference. Ground floors and walls to other rooms are left out.

From inside: each person gives about 70 W of heat and 50 W of moisture, and appliances and lights their own watts. And in air: 0.33 × the room's air changes an hour × its volume × the temperature difference. A margin goes on top. One watt is 3.412 BTU/h, and one kilowatt 860 frigorías/h.

Each room gets the next standard unit size up: 7,000, 9,000, 12,000, 18,000 or 24,000 BTU/h (2.0 to 7.0 kW). Then its heat loss on a cold day, worked out as the radiator calculator does, is set against the unit's heating, which drops by about 1.5% for each degree below 7 °C outside.

A worked example

A living room 4.5 m by 3.6 m, 2.4 m high, with two external walls, a 1.26 m² window facing south and another facing west, no shading, 3 people and 200 W of appliances and lights, another room above, and 28 °C outside in summer (-3 °C in winter).

1. Room 1, living room: 4.5 m × 3.6 m, 2.4 m high: floor 16.2 m², volume 38.88 m³
2. 28 °C outside, 24 °C inside: a difference (ΔT) of 4 °C
3. 2 external walls: 8.1 m × 2.4 m = 19.44 m², less 2.52 m² of windows and doors = 16.92 m²
4. Walls: 16.92 m² × U 0.5 W/m²K × (4 + 4 for the sun on it) °C = 68 W
5. Windows: 2.52 m² × U 2.8 W/m²K × 4 °C = 28 W
6. Sun through a south-facing window: 1.26 m² × 350 W/m² × g 0.75 = 331 W
7. Sun through a west-facing window: 1.26 m² × 500 W/m² × g 0.75 = 473 W
8. 3 people: 3 × 70 W = 210 W of heat, and 3 × 50 W = 150 W of moisture (latent)
9. Appliances and lights: 200 W
10. Air: 0.33 × 1.5 air changes an hour × 38.88 m³ × 4 °C = 77 W
11. Room 1, living room: 1,386 W of heat + 150 W of moisture = 1,536 W, plus a 10% margin = 1,690 W (5,765 BTU/h)
12. 1,690 W → a 7,000 BTU/h (2 kW) unit, the next standard size up
13. Heating check: in winter, at 21 °C inside and -3 °C outside, the room loses 645 W through its fabric + 462 W in air, plus the 10% margin = 1,217 W, by the radiator calculator's method
14. The unit heats about 2,600 W at 7 °C outside; at -3 °C, 1.5% less for each degree colder (at least half kept): × 0.85 = 2,210 W
15. That covers the room's heat loss.
16. Every window is taken at its own peak sun. The sun is never at its peak on two sides at once, so for a room with windows on opposite sides this errs high.
17. Moisture from outside air isn't counted, only people's. On humid days, by the sea especially, a unit spends more of its capacity drying the air.
18. The room needs 1,690 W × 3.412 = 5,765 BTU/h. A multi-split's outdoor unit serves several rooms and isn't their sum: the installer sizes it.
19. An estimate from typical figures, not an air-conditioning design. A split unit's refrigerant circuit is for a certified installer: F-gas certified in the UK, holding an attestation de capacité in France, and in Spain with a certificado de gases fluorados and registered as a RITE installer.

Common mistakes

• Going by floor area alone. Rules of thumb per square metre miss the windows. A west-facing room with a big window can need twice the unit of a north-facing one the same size.
• Leaving out the shading. Shutters or an awning outside stop about three quarters of the sun; blinds inside only about a third, since the heat is already in.
• Adding the rooms up into one outdoor unit. The rooms don't all peak at once. A multi-split's outdoor unit is chosen by the installer, not by summing these.
• Buying the biggest unit to be safe. An oversized unit cools in short bursts and leaves the air damp. The next size up from the load is the right one.
• Counting on it for heating without checking. A unit's heating drops on cold days. If the check says it falls short, the room needs a bigger unit or another heat source.

Questions people ask