Heat Pump Sizing: From Heat Load to the Right Capacity
Why the Right Capacity Matters
A 6 kW heat pump costs less than a 12 kW unit – but is it suitable for the building? The answer lies in the heat load: it determines how much heating capacity a house needs on the coldest days of the year.
An undersized heat pump cannot keep rooms warm during frost. The electric backup heater kicks in – driving up electricity costs. An oversized heat pump, on the other hand, constantly cycles: it switches on, quickly reaches the target temperature, switches off, cools down, switches on again. This on-off pattern stresses the compressor and reduces efficiency by 10–15%.
This article shows three ways to determine the right capacity: professional heat load calculation, the rule-of-thumb method, and estimation from previous energy consumption.
Step 1: Determine the Heat Load
The heat load is the heating capacity in kilowatts that a building needs at the lowest outdoor temperature of the year (design outdoor temperature) to maintain the desired room temperature. It forms the basis for any heat pump sizing.
The Gold Standard: Calculation According to EN 12831
The most accurate method is a room-by-room heat load calculation according to EN 12831. This captures the transmission heat losses (through walls, windows, roof, floor) and ventilation heat losses for each room. The regional design outdoor temperature, building location, and desired indoor temperatures are also considered.
Tip: With our free Heat Load Calculator, you can calculate your building's heat load yourself – room by room and according to standards.
The Rule-of-Thumb Method
When detailed calculation is not possible, a rough estimate based on living space and building type helps:
Formula: Heat load (kW) = Living space (m²) × specific heat load (W/m²) ÷ 1,000
The specific heat load depends heavily on the insulation standard:
| Building type | Specific heat load | Example 150 m² |
|---|---|---|
| Passive house | 10–15 W/m² | 1.5–2.3 kW |
| Low-energy new build | 25–35 W/m² | 3.8–5.3 kW |
| Standard new build | 35–45 W/m² | 5.3–6.8 kW |
| Well-renovated old building | 50–70 W/m² | 7.5–10.5 kW |
| Partially renovated old building | 70–100 W/m² | 10.5–15 kW |
| Uninsulated old building | 100–150 W/m² | 15–22.5 kW |
Calculation example: A renovated old building with 150 m² and 60 W/m² results in: 150 × 60 ÷ 1,000 = 9 kW heat load.
Estimation from Energy Consumption
Those who know their previous heating costs can also derive the heat load from annual consumption. The conversion uses so-called full load hours – approximately 2,000 hours per year during which a heating system theoretically runs at full capacity.
For natural gas: Heat load (kW) = Annual consumption (kWh) ÷ 2,000
For heating oil: Heat load (kW) = Annual consumption (litres) × 10 ÷ 2,000
Natural gas example: With 20,000 kWh gas consumption: 20,000 ÷ 2,000 = 10 kW heat load.
Heating oil example: With 2,000 litres of heating oil: 2,000 × 10 ÷ 2,000 = 10 kW heat load.
Note: This method provides only guide values. Actual consumption depends on user behaviour and weather conditions. A standard-compliant heat load calculation is more reliable for heat pump sizing.
Step 2: Consider Allowances
The pure heat load is not sufficient for heat pump sizing. Two additional factors must be included: hot water preparation and any utility lock-out periods.
Hot Water Preparation
If the heat pump is also to heat domestic hot water, additional capacity must be planned. VDI 4645 recommends approximately 0.25 kW per person in the household as a rule of thumb.
| Household size | Hot water allowance |
|---|---|
| 2 persons | 0.5 kW |
| 3 persons | 0.75 kW |
| 4 persons | 1.0 kW |
| 5 persons | 1.25 kW |
For households with high hot water demand (daily baths, multiple showers simultaneously), the allowance may be higher.
Utility Lock-out Periods
Many energy suppliers offer favourable heat pump electricity tariffs. In return, they may switch off the heat pump at certain times – typically three times daily for two hours each, totalling six hours per day.
During the lock-out period, the building must draw on stored heat. For the heat pump to produce enough heat in the remaining 18 hours, it needs more capacity.
Formula: Additional capacity = Heat load × (Lock-out time in hours ÷ 24)
Calculation example: With 9 kW heat load and 6 hours lock-out: 9 × (6 ÷ 24) = 9 × 0.25 = 2.25 kW additional capacity.
Modern heat pumps with large buffer tanks can often bridge lock-out periods without additional capacity. With underfloor heating, the building mass itself acts as storage.
Step 3: Calculate Total Capacity
Combining all factors gives the required heat pump capacity:
Total capacity = Heat load + Hot water allowance + Lock-out allowance
Worked Practical Example
A detached house is to be equipped with a heat pump:
| Building data | Value |
|---|---|
| Living space | 160 m² |
| Year built, renovated | 1985, insulation 2015 |
| Specific heat load | 55 W/m² |
| Persons in household | 4 |
| Utility lock-out | 6 hours/day |
Calculation:
| Item | Calculation | Result |
|---|---|---|
| Heat load | 160 m² × 55 W/m² ÷ 1,000 | 8.8 kW |
| Hot water | 4 persons × 0.25 kW | 1.0 kW |
| Lock-out | 8.8 kW × (6 ÷ 24) | 2.2 kW |
| Total | 12.0 kW |
A heat pump with 10–12 kW nominal capacity would be suitable here. Most manufacturers offer units in capacity steps such as 8, 10, 12, or 14 kW.
Guide Values by Building Type
| Building type | Area | Heat load | With allowances | Recommended HP |
|---|---|---|---|---|
| Passive house | 140 m² | 2.0 kW | 3.5 kW | 4–5 kW |
| New build standard | 150 m² | 6.0 kW | 8.5 kW | 8–10 kW |
| Renovated old building | 160 m² | 8.8 kW | 12.0 kW | 10–12 kW |
| Partially renovated | 180 m² | 14.4 kW | 18.5 kW | 16–18 kW |
| Uninsulated old building | 150 m² | 18.0 kW | 22.5 kW | 20–24 kW |
Common Sizing Mistakes
Problem: Oversizing
The common thought "Better more capacity, then I'm on the safe side" leads to problems with heat pumps. An oversized heat pump reaches the target temperature too quickly and switches off. Shortly after, the temperature drops, the heat pump starts again. This so-called cycling has several disadvantages: the compressor wears faster due to frequent start-ups, efficiency drops by 10–15%, and acquisition costs were unnecessarily high.
Modern inverter heat pumps can reduce their output, but they also have a minimum capacity. If actual demand is permanently below this threshold, inverter units also cycle.
Problem: Undersizing
A heat pump that is too small cannot keep the house warm on cold days. The electric backup heater kicks in – with a COP of 1.0 instead of 3–4. With frequent backup heater use, electricity costs rise noticeably, and comfort suffers.
The Golden Rule
Experienced planners size heat pumps rather tightly than generously. On a few very cold days per year, the backup heater can provide short-term support – this is more economical than a permanently oversized system. For inverter heat pumps with a modulation range of 30–100%, sizing to 90–100% of the calculated heat load makes sense.
Conclusion
The key point: The right heat pump capacity results from the building's heat load plus allowances for hot water and any utility lock-out periods. The heat load can be determined through a standard-compliant calculation according to EN 12831, through rules of thumb based on living space, or from previous energy consumption. The crucial factor is sizing neither too large nor too small – a heat pump matched to the heat load works more efficiently and lasts longer than an oversized unit that constantly cycles.
More on Heat Pumps
| No. | Article | Focus |
|---|---|---|
| 0 | Heat Pump: The Complete Guide | Overview and introduction |
| 1 | How Does a Heat Pump Work? | Physical principle |
| 2 | The Components | Evaporator, compressor, condenser |
| 3 | Key Figures and Sizing | COP, SPF, SCOP |
| 4 | SCOP Explained | Evaluating efficiency |
| 5 | Sizing Calculation | You are here |
Those who want to check their heat pump's efficiency after installation will find practical tips on heating curves and hydraulic balancing in the article Optimisation & Settings. The different operating modes – monovalent, bivalent, hybrid – are explained in a separate article.
Sources
- EN 12831-1: Heat load calculation
- VDI 4645: Planning and sizing of heating systems with heat pumps
- VDI 4650: Calculation of seasonal performance factor
- German Heat Pump Association: Heat Load Calculator
- German Heat Pump Association: SPF Calculator
Calculate Your Heat Load Now
With our free Heat Load Calculator, determine your building's design heat load according to EN 12831 – the most important basis for heat pump sizing.
With the Heat Pump Calculator, you can then calculate the seasonal performance factor and operating costs of your heat pump according to VDI 4650.