CalcHeat Pump Calculator
Last updated: 2026-08-10
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| Home area (m²) | People (DHW) (personas) | |
|---|---|---|
| Small room | 25 m² | 2 personas |
| Medium room | 40 m² | 3 personas |
| Large room | 50 m² | 4 personas |
| Office | 75 m² | 6 personas |
| Warehouse | 100 m² | 8 personas |
The Heat Pump Calculator is a practical tool designed to help you determine the correct size of a heat pump for your home or building. Whether you are planning a new installation or upgrading an existing system, this calculator provides a quick estimate of the thermal power (in kilowatts) required to meet your heating needs, based on your home's area, insulation quality, and the number of people using hot water. It uses a simple but effective formula to convert heating load and domestic hot water demand into a total output, making it easier to choose the right heat pump for comfort and efficiency.
What the Calculator Does and When to Use It
The Heat Pump Calculator estimates the required heating capacity of a heat pump by combining two main factors: the heating load of the building and the hot water demand from occupants. The result is given as a thermal output (kW) and an electrical input (kW), based on an assumed coefficient of performance (COP) of 3.5. This helps you understand not only the heat pump's size but also its energy consumption under typical conditions.
You should use this calculator during the early planning stages of a heat pump installation, such as when comparing quotes, selecting a unit, or checking if your current system is oversized or undersized. It is especially useful for residential homes in climates where heating is required, but it can also be applied to small commercial spaces. Common scenarios include replacing an old boiler, installing a heat pump in a new build, or retrofitting an existing property. However, always remember that this is a preliminary tool; a professional heat load calculation (like a Manual J in North America or an EN 12831 in Europe) is recommended for final sizing.
The Formula Explained Variable by Variable
The calculator's internal formula is straightforward. Here is a breakdown of each variable:
| Variable | User Input | How It Works |
|---|---|---|
| area_m2 | Total floor area in square meters | This is the heated floor area of your building. A larger area requires more heating power. The calculator multiplies this by a coefficient based on insulation quality. |
| aislamiento | Insulation level (deficiente, medio, bueno, passivhaus) | This selects the heat loss coefficient, which represents how many watts per square meter your home loses in cold conditions. Values are: deficiente (poor insulation) = 70 W/m², medio (average) = 50 W/m², bueno (good) = 35 W/m², passivhaus (Passive House standard) = 15 W/m². These numbers are typical for design winter temperatures. |
| acs_personas | Number of people for domestic hot water | This estimates hot water demand. Each person is assumed to need 0.2 kW of continuous heating for water. For example, a family of 4 adds 0.8 kW to the total load. |
| kw_cal | Calculated (not user input) | Heating load = (area_m2 × insulation coefficient) / 1000. This converts the load from watts to kilowatts. |
| kw_acs | Calculated | Hot water load = number of people × 0.2 kW per person. |
| kw_total | Calculated | Sum of kw_cal and kw_acs, rounded to one decimal place. This is the recommended thermal output of the heat pump. |
| cop_estimado | Assumed constant | Set to 3.5, this means the heat pump produces 3.5 kW of heat for every 1 kW of electricity used at average conditions. It is a reasonable default for modern air-to-water heat pumps. |
| kw_electrico | Calculated | Electrical input = kw_total / COP. This tells you how much electricity the heat pump will draw at full load. |
Two Worked Examples with Concrete Numbers
Example 1: An Average Home with Good Insulation
Consider a 120 m² house with good insulation (bueno, coefficient 35) and 4 occupants.
- Heating load calculation: 120 m² × 35 W/m² = 4200 W. In kilowatts, that is 4.2 kW.
- Hot water load: 4 people × 0.2 kW = 0.8 kW.
- Total thermal output required: 4.2 + 0.8 = 5.0 kW.
- Electrical input: 5.0 kW / 3.5 COP = 1.43 kW.
This means you would look for a heat pump with a rated heating capacity around 5 kW. A typical 5–6 kW air-to-water heat pump would be suitable.
Example 2: A Large Home with Poor Insulation
Now imagine a 200 m² home with poor insulation (deficiente, coefficient 70) and 6 occupants.
- Heating load calculation: 200 m² × 70 W/m² = 14,000 W = 14.0 kW.
- Hot water load: 6 people × 0.2 kW = 1.2 kW.
- Total thermal output required: 14.0 + 1.2 = 15.2 kW.
- Electrical input: 15.2 kW / 3.5 COP = 4.34 kW.
Here, a larger heat pump (15–16 kW) is needed. Note that poor insulation dramatically increases the required size, which highlights the importance of upgrading insulation before or alongside installing a heat pump.
For quick conversion to imperial: a 120 m² home is roughly 1,292 ft², and 5 kW is about 17,000 BTUs per hour (1 kW ≈ 3,412 BTU/h). So you might see residential heat pumps rated at 3–5 tons (1 ton = 12,000 BTU/h) in the US market.
Common Mistakes When Sizing a Heat Pump
Avoid these frequent errors to ensure your heat pump installation performs well:
- Using gross floor area instead of heated floor area. Basements, garages, and unheated attics should not be included. Only count spaces where you maintain the desired temperature.
- Overlooking hot water demand. Many people size only for space heating, forgetting that heat pumps in many systems also supply domestic hot water. Our calculator adds 0.2 kW per person, which is a reasonable baseline, but larger families or high-use households may need more.
- Choosing an insulation coefficient too optimistically. Selecting passivhaus for a standard 1950s home will significantly undersize the unit. Be honest about your building's envelope—if in doubt, use the next lower insulation quality to be safe, then plan for improvements.
- Ignoring backup heating. Heat pumps lose efficiency at very low outdoor temperatures. In cold climates, you may need a supplementary heating element. The calculator assumes an average COP of 3.5, but real performance can drop to 2.0 or lower in freezing conditions, which affects electrical consumption.
- Relying solely on this calculator. This tool is a preliminary check. Professional sizing accounts for duct losses, window orientation, air infiltration, and local climate data. Always cross-check with a certified installer for final specifications.
Frequently Asked Questions
Can I use this calculator for a heat pump that also cools my home?
This calculator is specifically designed for heating load. Cooling loads are different because they depend on solar gain, internal heat from appliances and people, and outdoor temperature peaks. While the thermal output may give a rough idea for a reversible heat pump, cooling sizing should be done separately using a cooling load calculation (e.g., Manual J or equivalent). In many cases, the cooling load is lower than the heating load, but not always—especially in warm climates or homes with large windows.
What if my home's flow temperature is very high (e.g., 60°C for old radiators)?
High flow temperatures reduce the COP of most heat pumps because the compressor has to work harder to deliver heat at a higher temperature. The calculator assumes a typical COP of 3.5, which is realistic for flow temperatures around 35–45°C (common for underfloor heating or low-temperature radiators). For older radiator systems requiring 60°C or more, the actual COP might drop to 2.5–3.0. In that case, the electrical input (kw_electrico) will be higher than shown. Consult your heat pump's performance data sheet for exact values at your design flow temperature.
Is the 0.2 kW per person for hot water accurate for all climates?
This is a general estimate based on average daily hot water usage in temperate climates. In colder regions where incoming water temperature is lower, or for households with multiple showers, baths, or high consumption, you may need up to 0.3–0.4 kW per person. Conversely, in warm climates or for low-use households, the figure might be less. The calculator provides a conservative baseline; adjust upward if your situation involves high hot water demand.
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