Air Conditioner Power Calculation
About Air Conditioner Power Calculation
The calculator estimates the required air conditioner power (cooling capacity) for a single room and shows the result in kW and BTU/h. The calculation is performed as a sum of heat gains from the air volume, occupants, household appliances, and selected adjustment factors for operating conditions.
Guidelines and recommendations
Final formula and calculation procedure
The final cooling capacity Q (kW) is obtained by adding the individual components. All components are calculated in kW. If an option is not selected, the corresponding addition is 0.
Q = Q1 + Q2 + Q3 + Qvent + Qtop
Room volume, temperature and base load
The room volume V (m³) is calculated from the floor area S (m²) and ceiling height h (m).
V = S × h
The design temperature difference ΔT is determined from the outdoor air temperature and the desired indoor temperature.
ΔT = max(0, tout - tin)
The base temperature load is calculated relative to a temperature difference of 8°C.
Qbase = (q / 1000) × V × (ΔT / 8)
The specific reference value is taken as q = 35 W/m³ at a design temperature difference of 8°C.
The solar gain is determined by the selected level of solar exposure.
Qsun = (q / 1000) × V × (ksun - 1)
The glazing addition is applied only when glazing area is included in the calculation.
Qglass = (q / 1000) × V × (ΔT / 8) × (kglass - 1)
Q1 = Qbase + Qsun + Qglass
Heat gain from occupants
The occupant load Q2 (kW) is calculated from the number of occupants n and the reference value per person.
Q2 = n × 0.10
The reference value per person is 0.10 kW and corresponds to light activity indoors.
Heat gain from household appliances
The appliance load Q3 (kW) converts the total electrical power of the appliances Pel (W) into an additional heat gain.
Q3 = Pel × 0.0003
The coefficient 0.0003 assumes that approximately 30% of the stated total appliance power is converted into heat at the design moment. If the actual simultaneous power consumption of the equipment is known, the simplified calculation may underestimate the heat gain.
Ventilation adjustment based on air change rate
The ventilation addition Qvent (kW) accounts for the load required to cool the supply air. It is included only when ventilation is taken into account and depends on the air change rate N (1/h) and the room volume V (m³).
Qvent = N × V × 0.000335 × ΔT
The coefficient 0.000335 accounts for the volumetric heat capacity of air. The ventilation load increases as the air change rate and temperature difference increase.
- For living rooms, N =
0.5-1.0is often used. - For kitchens, bathrooms, workshops and rooms with elevated humidity, N =
1.5-3.0or higher is often used. - If the air change rate is specified by the project, the project value of N is normally used.
Top-floor adjustment
The top-floor option adds a proportion of the base load Q1 to the calculation.
Qtop = 0.15 × Q1
The coefficient 0.15 accounts for additional room heating from the roof and upper building envelope.
How the recommended capacity range is determined
The recommended range is derived from the final Q to help select the nearest available capacity with a small allowance.
Qmin = 0.95 × Q
Qmax = 1.15 × Q
Selection principle: the range serves as a reference for comparison with available capacity sizes. It is preferable to select a model whose rated cooling capacity is not lower than the calculated value Q and does not include an excessive allowance.
Converting the result to BTU/h
BTU/h units are used for comparison with the common rating system for residential air conditioners. The conversion is performed using a constant factor.
BTU/h ≈ kW × 3412.142
The conversion factor is 3412.142 BTU/h per 1 kW of cooling capacity.
Standards reference
EN 14511 specifies test conditions and methods, as well as rules for determining the performance characteristics of air conditioners and heat pumps. The calculated result should be compared with the rated cooling capacity of the equipment declared in accordance with EN 14511.
EN 16798-1 specifies indoor environmental input parameters for building design and energy performance assessment. The standard is not the calculation method used by this calculator. If the outdoor airflow rate is specified by the project, the ventilation load should be determined using the project air change rate.
FAQs
Why is the calculation based on volume rather than floor area alone?
The simplified method uses a specific reference value in W/m³, so the room floor area is multiplied by the ceiling height. For the same floor area, a room with a higher ceiling receives a higher calculated base load.
How does cooling capacity differ from electrical power consumption?
Cooling capacity Q indicates how much heat must be removed from the room. Electrical power consumption depends on the efficiency of the specific model and is usually lower than Q.
When does ventilation have the greatest effect on the result?
When the air change rate N is high or the room volume V is large, the Qvent addition increases linearly and may become comparable to the base load Q1. If there is a continuous supply of outdoor air, ignoring ventilation often leads to insufficient air-conditioner capacity.
How does temperature affect the result?
The greater the difference between the design outdoor temperature and the desired indoor temperature, the higher the load through the building envelope and ventilation. The coefficient 0.15 additionally accounts for increased heat gains on the top floor.
How should a model be selected using the Qmin-Qmax range?
Compare the model's rated cooling capacity with the calculated value Q and the displayed range. It is preferable to select the nearest available capacity that is not lower than Q. A substantial allowance is also undesirable because it may cause frequent on-off cycling and less stable temperature control.