Air Conditioner Power Calculation

1Room dimensions and exterior walls

Room length, m

Room width, m

Ceiling height, m

Number of exterior walls

Select the exterior walls

2Construction and solar heat gain

Exterior wall insulation

Window area, m²

Sun exposure

What is above the room

3Temperature and internal heat gains

Outdoor air temperature, °C

Desired indoor temperature, °C

Number of people in the room

Household appliance power, W

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About Air Conditioner Power Calculation

The results are approximate. Before use, verify the calculations against the applicable standards and consult a specialist. The developer is not responsible for the consequences of use without project verification.

The Air Conditioner Power Calculator determines the required cooling capacity for a room based on its dimensions, external walls, thermal insulation, glazing, solar exposure, temperatures, roof conditions, number of occupants, and heat released by household appliances. The result is calculated in kW and BTU/h, making it suitable for estimating air conditioner power in kW per sq m (BTU/h) and selecting a split system with sufficient cooling capacity.

The calculation starts with a specific cooling capacity based on floor area and then applies successive corrections for the actual room conditions. As a result, rooms with the same floor area may require different air conditioner capacities.

Guidelines and recommendations

Floor area, room volume, and base cooling capacity

Floor area. First, the floor area A in m² is calculated from the room length L and width W, entered in metres.

A = L × W

Room volume. The room volume V in m³ is obtained by multiplying the floor area by the ceiling height H.

V = A × H

Base cooling capacity. For preliminary sizing of residential air conditioners, a commonly used guideline is 100 W per 1 m² of floor area under typical conditions. The calculator uses this value as the starting point.

Q_area = 100 × A

For example, a room with a floor area of 20 m² has an initial cooling capacity of 2000 W, or 2.0 kW. This value is then adjusted by all subsequent correction factors.

Ceiling height correction

Reference ceiling height. A ceiling height of 2.4 m is used as the reference value. If the ceiling is higher or lower, the base cooling capacity changes in proportion to the room volume.

Q_height = Q_area × H / 2.4

At a height of 2.4 m, the factor is 1.00. For example, at a ceiling height of 3.0 m, the initial capacity is multiplied by 3.0 / 2.4 = 1.25, which means an increase of 25%.

Outdoor and indoor temperature correction

Temperature difference. The calculator first determines the positive difference between the outdoor temperature t_out and the desired indoor temperature t_in. If the outdoor temperature is lower than the selected indoor temperature, a value of 0°C is used for this correction.

ΔT = max(0, t_out - t_in)

Temperature factor. A temperature difference of 8°C is used as the reference condition. For every 1°C deviation, the factor changes by 2.5%.

k_T = 1 + 0.025 × (ΔT - 8)

To prevent the temperature correction from becoming excessive, the factor is limited to the range from 0.85 to 1.20. Therefore, when ΔT is 2°C or less, a factor of 0.85 is used; at ΔT = 8°C the factor is 1.00; and at ΔT of 16°C or more, the maximum value of 1.20 is applied.

Reference cooling capacity. After the ceiling height and temperature corrections are applied, a reference value is obtained. All subsequent corrections are calculated relative to this value.

Q_ref = Q_area × H / 2.4 × k_T

External walls and thermal insulation

Share of external walls. For a rectangular room, the calculator determines the total length of the selected external sides. This length is multiplied by the ceiling height, and the resulting external wall area is compared with the total area of all four room walls.

f_wall = A_external / A_{all walls}

The selected external wall area is calculated from the full geometric wall area. Window openings are not subtracted because glazing is accounted for separately.

External wall exposure correction. A room where approximately 50% of the total wall area is external is used as the reference case. For every 50 percentage points of deviation from this value, the cooling capacity changes by 10% of the reference capacity.

Q_exposure = Q_ref × 0.20 × (f_wall - 0.50)

For example, if all four walls are external and the external wall share is 100%, the correction is +10%. If there are no external walls, the correction is -10%.

External wall insulation. An additional factor is applied according to the selected insulation level:

poor insulation or an older building: +0.08;
average insulation: 0;
good insulation or a modern building: -0.08.

The effect of insulation increases with the share of external walls. At an external wall share of 50%, the full listed factor is applied; at 100%, its effect is doubled. The multiplier for this correction is limited to a maximum value of 2.

Q_ins = Q_ref × k_ins × min(f_wall / 0.50, 2)

Total external wall correction. The final wall correction combines the effect of the external wall area and its thermal insulation.

Q_walls = Q_exposure + Q_ins

Glazing area

Window-to-floor area ratio. The window area A_win is compared with the floor area. A glazing area equal to 15% of the floor area is used as the reference value.

r_win = A_win / A

At a window area equal to 15% of the floor area, the glazing correction is zero. A smaller window area reduces the calculated cooling capacity, while a larger area increases it.

k_win = 0.08 × (r_win - 0.15) / 0.15

The factor is limited to the range from -0.08 to +0.12. Therefore, with no external glazing the correction is -8%; at 15% glazing it is 0%; at 30% it is +8%; and from approximately 37.5% glazing upward it reaches the maximum value of +12%.

Q_win = Q_ref × k_win

Solar exposure

Solar correction. The effect of sunlight is calculated separately from the glazing area, but it is scaled according to the window area relative to the 15% reference glazing ratio.

little sunlight or shade for most of the day: factor -0.05;
moderate solar exposure: factor 0;
strong direct sunlight, especially from the south or west: factor +0.10.

k_sun,total = k_sun × min(r_win / 0.15, 2)

The final solar correction is limited to the range from -10% to +20% of the reference cooling capacity. With glazing equal to 15% of the floor area, shade gives -5%, moderate exposure gives 0%, and strong direct sunlight gives +10%. With glazing equal to 30% or more, the corresponding limiting values become -10%, 0%, and +20%.

Q_sun = Q_ref × k_sun,total

Roof and attic

Heat gain from above. If another room is located above the calculated room, no additional correction is applied. For rooms directly below an attic or roof, the following values are used:

well-insulated attic or roof: +5%;
average attic or roof insulation: +10%;
poorly insulated roof or hot attic: +15%.

Q_roof = Q_ref × k_roof

Occupants and household appliances

Occupants. The base calculation includes normal occupancy by up to two people. Starting with the third person, 180 W is added for each additional occupant.

Q_people = max(0, N - 2) × 180

Household appliances. The electrical power of appliances operating at the same time as the air conditioner is added directly to the cooling load. For example, appliances with a combined operating power of 300 W increase the required cooling capacity by the same 300 W.

Q_equipment = P_equipment

Total cooling capacity

Combination of corrections. The final cooling capacity is obtained from the reference capacity after all positive and negative corrections are added.

Q_total = Q_ref + Q_walls + Q_win + Q_sun + Q_roof + Q_people + Q_equipment

The final value cannot be lower than 0 W. The calculator also determines the specific load in W/m² and the volumetric load in W/m³.

q_A = Q_total / A

q_V = Q_total / V

Conversion to BTU/h and air conditioner size selection

Unit conversion. The calculated cooling capacity in watts is converted to BTU/h using the relationship 1 W ≈ 3.412 BTU/h.

Q_BTU = Q_total × 3.412

Air conditioner size selection. The calculator selects the smallest capacity not lower than the calculated requirement from the series 7000, 9000, 12000, 15000, 18000, 21000, 24000, 28000, 30000, 36000, 42000, 48000, and 60000 BTU/h. If the calculation exceeds 60000 BTU/h, the value is rounded upward to the next 1000 BTU/h.

Capacity reserve. The percentage indicates how much the selected nominal capacity exceeds the calculated cooling requirement.

Reserve = (Q_size / Q_BTU - 1) × 100%

European standards reference

EN 14511 “Air conditioners, liquid chilling packages and heat pumps for space heating and cooling and process chillers, with electrically driven compressors”. This standard establishes conditions and methods for determining and testing the rated cooling capacity of air-conditioning equipment. Therefore, when selecting a specific model, the calculated value should be compared with the cooling capacity declared by the manufacturer rather than with the electrical input power of the air conditioner.

EN 16798-1 “Energy performance of buildings. Ventilation for buildings. Part 1: Indoor environmental input parameters for design and assessment of energy performance of buildings”. This standard provides European approaches to indoor environmental parameters, including thermal conditions in buildings. The 100 W/m² guideline and the percentage corrections used by this calculator are calculation assumptions of the algorithm and are not coefficients directly specified by these standards.

FAQs

Why is air conditioner power calculated using more than just the room area?

The same floor area does not necessarily mean the same cooling load. A high ceiling, external walls, large glazing areas, direct sunlight, a hot roof, occupants, and operating appliances can significantly increase the required air conditioner power. Therefore, the 100 W/m² guideline is used only as the initial basis for further corrections.

Why is it important to select the actual external walls?

In a rectangular room, long and short walls have different areas. The calculator uses the actual total length of the selected external sides, so one long external wall affects the result differently from one short wall. In a square room, all sides have the same length, so selecting a particular side does not change the result.

How are glazing and solar exposure taken into account together?

First, the window area is compared with the floor area and produces a separate glazing correction. The solar correction is then calculated using the same window area: the larger the glazing, the stronger the effect of direct sunlight or shade. With moderate solar exposure, the solar correction is zero.

Why do the first two occupants not increase the required air conditioner capacity?

The base guideline of 100 W/m² assumes normal residential use of the room, including the presence of one or two people. Additional heat gain is added starting with the third occupant at 180 W for each person above two.

Why is the selected air conditioner size higher than the calculated cooling capacity?

Air conditioners are manufactured in standard nominal capacity sizes, so an exact match with the calculated value is uncommon. The calculator selects the nearest size that is not lower than the required cooling capacity and shows the difference as the capacity reserve. For example, if the result falls between 9000 and 12000 BTU/h, the 12000 BTU/h size is selected.