If you are considering installing a heat pump and wondering what aerothermal power do I need, you are in the right place. Choosing the correct output is essential to ensure comfort, efficiency and a fast return on investment.
In this practical guide, you will find the key factors that influence sizing, a simple formula to calculate aerothermal heat pump power, a full worked example and a reference table based on climate zones and insulation levels. This approach applies to residential properties as well as small commercial buildings, where aerothermal power must match the real thermal demand of the space.
Factors That Determine the Aerothermal Power You Need
The required output depends on the building’s heat load, meaning the amount of heat needed on the coldest days to maintain indoor comfort temperature.
These are the key factors:
Property Size and Layout
- Usable floor area (m2): the larger the surface area, the greater the heating demand.
- Ceiling height (m): higher ceilings increase the air volume that must be heated. If ceilings exceed 2.5 m, an adjustment factor should be applied.
- Building compactness and layout: elongated layouts or homes with many external openings lose more heat than compact structures.
- Thermal bridges and windows: large glazed areas without thermal break or low performance glazing can significantly increase demand.
Insulation and Energy Efficiency
- Quality of insulation: façades, roofs and floors play a major role. New builds or refurbished homes perform much better.
- Airtightness: air infiltration through gaps may require slight oversizing.
- Climate zone: coastal mild climates differ significantly from cold inland areas. Design outdoor temperature strongly affects heat load.
- Heat emitters: underfloor heating typically operates at lower flow temperatures than traditional radiators, improving efficiency and increasing the effective useful output of the heat pump.
How to Calculate Aerothermal Power
The professional method involves a full heat loss calculation including thermal transmittance, infiltration and temperature differentials. However, for a reliable preliminary estimate, you can use the specific heat load per square metre method.
Formula
Basic Steps
- Select the specific heat load (W/m2) from the table below based on climate zone and insulation level.
- Multiply by the usable floor area: Power (W) = Area x Specific heat load
- Apply a height correction if ceilings exceed 2.5 m: Height factor = ceiling height / 2.5
- Add a safety margin of 10 to 15 percent to cover peak demand and potential infiltration.
Power (kW) = Area (m2) x Specific heat load (W/m2) x Height factor / 1000 x 1.10 to 1.15
Important Notes:
- If using high temperature radiators with flow temperatures of 45 to 55 degrees Celsius, check the unit’s useful output at those conditions. Output may decrease compared to 35 degrees Celsius operation.
- Domestic hot water (DHW) is not added directly to heating power. It is normally covered via a storage cylinder with DHW priority control. As a practical guideline, a 150 to 200 litre cylinder is typical for a 2 to 3 bedroom home.
Practical Example: What Aerothermal Power Do I Need for a 120 m2 House?
Data:
- Area: 120 m2
- Average ceiling height: 2.5 m
- Climate: temperate inland
- Insulation: medium level, refurbished property
- Heat emitters: underfloor heating
From the reference table, for temperate inland climate with medium insulation, we select 60 W/m2 as a reasonable estimate.
Calculation:
- Base power = 120 x 60 = 7,200 W = 7.2 kW
- Height factor = 1.00
- Apply 15 percent margin: 7.2 x 1.15 = 8.28 kW
Result:
An 8 to 9 kW heat pump rated at A7/W35 conditions would comfortably cover heating demand. If radiators operating at 45 to 55 degrees Celsius are used instead, check the useful output at W45 to W55 conditions, as a larger unit may be required.
For domestic hot water, a 180 to 200 litre cylinder with priority control is typically sufficient for a household of three to four people without increasing nominal power.
Reference Table by Climate Zone and Insulation Level
The following table provides indicative specific heat load values (W/m2) for quick sizing. These values are suitable for preliminary estimation. A full heat load calculation is recommended for complex refurbishments, passive houses or buildings with variable occupancy.
Indicative Winter Heating Load (W/m2)
| Climate Zone (Indicative) | Basic / Old Insulation | Medium / Refurbished | Good / New Build |
|---|---|---|---|
| Very cold / Mountain | 90 to 120 | 65 to 90 | 45 to 65 |
| Cold inland | 80 to 110 | 60 to 85 | 40 to 60 |
| Temperate inland | 70 to 100 | 55 to 70 | 35 to 55 |
| Atlantic humid | 65 to 95 | 50 to 70 | 35 to 50 |
| Mediterranean inland / South | 60 to 90 | 45 to 65 | 30 to 50 |
| Mild coastal | 50 to 75 | 35 to 55 | 25 to 40 |
Note: Cooling load calculations require different criteria, including solar gains, shading and ventilation.
Final Conclusions
Answering the question what aerothermal power do I need requires looking beyond square metres. Insulation level, ceiling height, climate zone and type of heat emitter are decisive factors.
Using the formula and reference table above, you can pre size your system and determine the appropriate output range in kilowatts. Always verify the useful output under your specific design conditions, such as A7/W35 or A minus 7/W45.
If you would like a more precise calculation based on full heat loss modelling, equipment selection at different operating temperatures and domestic hot water sizing, professional advice is strongly recommended to ensure optimal performance and long term efficiency.
