Heat Pump Calculator

TL;DR: To calculate the required heat pump size, sum your home's heating load (area in m² × W/m²) and domestic hot water demand (people × 0.2 kW), then divide the total thermal kW by your estimated Coefficient of Performance (COP) — typically 3.5 for air-source heat pumps — to find the electrical power draw, and finally select a commercial model with a thermal output at least equal to that total.

What Is the Heat Pump Calculator?

This calculator determines the correct size for an air-source heat pump (aerothermal) system based on two critical inputs: your building's heating load and your desired flow temperature. It converts physical space requirements into a concrete electrical power figure, allowing you to select a commercial unit with confidence.

Heat pumps are not like conventional boilers — you cannot simply match the boiler's output. Because a heat pump's efficiency (COP) varies with outdoor temperature and flow temperature, you must calculate the thermal demand first, then account for the COP to find the electrical input. This tool bridges that gap, helping homeowners, HVAC installers, and energy auditors avoid the two most common errors: buying an oversized unit (wasting money) or undersizing (freezing in winter).

This calculator is specifically relevant for anyone planning a retrofit or new build in climates where winter temperatures drop below 5°C. It uses the simplified method: heating area multiplied by a standard thermal load per square metre, plus a fixed allowance for domestic hot water (ACS) based on occupant count.

How to Use the Calculator

Using this tool is straightforward, but each input directly feeds the formula. Follow these steps in order:

1. Enter the heated floor area (m²): Input the total square metres of living space you need to heat. Do not include unheated garages, basements, or attics. For example, a 120 m² house means approximately 120 m² of conditioned living space.
2. Enter the specific heating load (W/m²): Use the calculator's default of 50 W/m² for a reasonably well-insulated modern home. For older, poorly insulated buildings, you might enter 70–90 W/m². For a passive house, use 25–30 W/m².
3. Enter the number of occupants: Count every person living in the home permanently. This determines the domestic hot water (ACS) demand — each person is allocated 0.2 kW of thermal power for water heating.
4. Enter the design flow temperature (°C): This is the water temperature your heat pump must supply to radiators or underfloor heating. 35°C is typical for underfloor heating; 45–55°C is for radiators. Higher flow temperatures reduce the COP.
5. Review the COP estimate: The calculator applies an estimated COP of 3.5 for air-source (aerothermal) systems. This is an average seasonal figure for a 35°C flow temperature in moderate climates.
6. Read the outputs: The calculator displays your total thermal demand in kW and the estimated electrical power draw in kW. Use the thermal figure to select the commercial unit size.

Formula and Calculation Method

The calculation follows a logical three-step method: determining heating demand, adding hot water demand, then converting thermal to electrical power using COP.

Step 1: Heating Demand
Heating demand (W) = Heated area (m²) × Specific heating load (W/m²)

Step 2: Hot Water Demand (ACS)
ACS demand (kW) = Number of occupants × 0.2 kW per person

Step 3: Total Thermal Demand
Total thermal (kW) = [Heating demand (W) ÷ 1000] + ACS demand (kW)

Step 4: Electrical Power
Electrical power (kW) = Total thermal (kW) ÷ Estimated COP (3.5)

Worked Example: Consider a 120 m² home with 4 occupants and a 35°C flow temperature.
Step 1: 120 m² × 50 W/m² = 6000 W
Step 2: 4 people × 0.2 kW = 0.8 kW
Step 3: (6000 W ÷ 1000) + 0.8 kW = 6.0 kW + 0.8 kW = 6.8 kW thermal
Step 4: 6.8 kW ÷ 3.5 = 1.94 kW electrical

Your commercial selection: you need a heat pump with a thermal output of at least 6.8 kW at your design flow temperature. A 7–8 kW commercial unit would be appropriate. The electrical draw of 1.94 kW tells you the impact on your electrical panel and running costs.

Practical Examples

Below are three realistic scenarios showing how inputs change the result. Each uses the same formula logic.

Scenario 1 (new build): The 4.6 kW thermal total means you need a 5–6 kW unit. The low electrical draw confirms energy efficiency.
Scenario 2 (retrofit): The 11.3 kW thermal requirement is high because of poor insulation and high flow temperature (55°C for radiators). This will require a 12 kW unit, and the COP may actually drop below 3.0 at this flow temperature, meaning real electrical draw could be higher than 3.23 kW.
Scenario 3 (cottage): The 7.6 kW result shows that even a small area needs a big unit if insulation is terrible.

Tips for Accurate Results

Getting the right size depends on honest inputs and understanding the system's limitations. Follow these specific guidelines:

• Adjust the W/m² for insulation quality: Do not blindly use 50 W/m². If your home has cavity wall insulation, double glazing, and a modern roof, 35–45 W/m² is accurate. If you have solid walls and single glazing, use 70–90 W/m². When in doubt, choose the higher value — it is safer.
• Never ignore the design outdoor temperature: The COP of 3.5 is an estimate for moderate winter days (0°C to 7°C). When outdoor temperatures drop below -5°C, the COP can fall to 2.0–2.5. If you live in a cold climate (e.g., Scandinavia, northern Canada), multiply your electrical result by 1.3–1.5 to ensure the unit can still heat at low temperatures.
• Account for ACS correctly in winter: In winter, incoming water temperature drops from 15°C to 5°C. This means the heat pump works harder to heat water to 50°C for domestic use. The 0.2 kW per person is a year-round average; for winter-only sizing, increase to 0.3 kW per person.
• Check the backup electrical resistance: Most heat pumps include an integrated backup heating element (resistencia eléctrica de apoyo). Do not rely on this for your primary sizing — it is a safety net for extreme cold. If you have a large home in a cold region, you may need a more powerful backup element, which affects your electrical panel sizing.
• Flow temperature is not negotiable: Underfloor heating works at 30–35°C, which is ideal for heat pumps. If you are using radiators, note that older radiators sized for 70°C boilers will be undersized for 45°C heat pump water — you will need larger radiators or lower your expectations.
• Unit conversions: Double-check that you are inputting area in square metres and load in watts per square metre. Some datasheets use BTU/h (British Thermal Units). 1 kW ≈ 3,412 BTU/h. If you have a BTU/h figure for a heat pump, divide by 3,412 to compare with your kW result.

Frequently Asked Questions

Q: What size heat pump do I need for a 100 m² house?
Using the standard formula: 100 m² × 50 W/m² = 5,000 W = 5.0 kW heating demand. With 3 occupants, ACS adds 0.6 kW, making 5.6 kW thermal total. At a COP of 3.5, electrical draw is 1.6 kW. You would select a commercial heat pump with a thermal output of 6–7 kW. However, if your home is a retrofit with poor insulation, recalculate with 70 W/m², giving 7.0 kW heating + 0.6 kW ACS = 7.6 kW total, meaning you need an 8 kW unit. Always use your actual insulation quality, not the default.

Q: How does flow temperature affect heat pump efficiency and size?
Flow temperature is the temperature of the water sent to radiators or underfloor heating. At 35°C (underfloor), the heat pump operates at a high COP of 3.5–4.5. At 55°C (standard radiators), the COP drops to 2.5–3.0 because the compressor must work harder to raise the water temperature. This means for the same thermal demand, a radiator system requires more electrical power. For example, a 6.8 kW thermal demand at COP 3.5 draws 1.94 kW, but at COP 2.7 it draws 2.52 kW. This also means your commercial unit may run longer and wear faster at higher flow temperatures — keep the flow temperature as low as your heating system allows.

Q: Should I size the heat pump for the coldest day of the year?
Yes, for the heating load calculation you should use the design outdoor temperature for your region (e.g., -10°C in central Europe, -5°C in southern UK). However, you should not buy a heat pump that is oversized for 99% of the year. The correct approach is to size for the coldest design day (using the W/m² adjusted for your climate) but check that the chosen unit can modulate down its output in mild weather. Modern inverter heat pumps can operate at 20–100% of capacity. If your calculated thermal demand is 6.8 kW, buy a 7–8 kW unit, not a 12 kW unit. The CO2 emissions and electrical costs of an oversized unit are significantly higher because it will short-cycle in shoulder seasons, reducing efficiency.