Guide to Using the Heat Pump and SPF Calculator

Welcome to our heat pump calculator! With this tool, you can calculate the Seasonal Performance Factor (SPF) of your heat pump according to VDI 4650 and make an informed decision for your heating system. In this guide, I will explain step by step how to use the calculator and interpret the results.

Introduction

What is the Seasonal Performance Factor (SPF)?

The Seasonal Performance Factor (SPF), also known as JAZ in German, is the most important efficiency measure for heat pumps. It indicates how much heat a heat pump generates on average per unit of electrical energy over a year.

Simply explained: An SPF of 4.0 means that the heat pump produces 4 kWh of heat from 1 kWh of electricity – 3 kWh come from the environment (air, ground or water), 1 kWh from the electricity.

SPF = Heat produced [kWh/year] / Electricity consumed [kWh/year]

What does this calculator compute?

The heat pump calculator determines based on your inputs:

• Seasonal Performance Factor (SPF) according to VDI 4650
• SCOP (Seasonal Coefficient of Performance) – European standard
• Annual electricity consumption for heating and hot water
• Operating costs based on your electricity price
• CO₂ emissions for environmental assessment
• Monthly breakdown with heat demand and efficiency

Why is the SPF so important?

Calculation Principles and Formulas

2.1 Basic Principle of SPF Calculation according to VDI 4650

VDI 4650 defines a standardized procedure for calculating the Seasonal Performance Factor. The basic principle is based on the weighted averaging of performance coefficients across different operating states:

SPF = Σ (Qi × COPi) / Σ Qi

Where:

• Q_i = Heat demand at operating point i [kWh]
• COP_i = Coefficient of performance at operating point i [-]

2.2 Coefficient of Performance (COP) at Different Temperatures

The COP of a heat pump strongly depends on temperature conditions. The calculator uses three characteristic operating points:

2.3 Temperature Dependence of COP

The COP decreases with increasing temperature difference between heat source and heating circuit. A rule of thumb:

COP ≈ η × Th / (Th - Tc)

Where:

• η = Efficiency factor of the heat pump (typically 0.4-0.5)
• T_h = Flow temperature [Kelvin]
• T_c = Source temperature [Kelvin]

Practical consequence: Reducing the flow temperature by 5 K increases the COP by approximately 10-15%!

2.4 Calculating Annual Heating Demand

The calculator uses the degree-day method to calculate the annual heating demand from the design heat load:

Q_Heat = P_Design × FLH

Where:

• Q_Heat = Annual heating demand [kWh/a]
• P_Design = Design heat load at outdoor design temperature [kW]
• FLH = Full load hours [h/a]

Full load hours by building standard:

The calculator uses 1,900 full load hours by default, which corresponds to an average building.

2.5 Domestic Hot Water Heat Demand

Hot water demand is calculated according to DIN 4708 or VDI 2067:

Q_DHW = V_Day × ρ × c × ΔT × 365 × f_L

Where:

• V_Day = Daily water consumption [liters]
• ρ = Water density (1 kg/L)
• c = Specific heat capacity (1.163 Wh/kg·K)
• ΔT = Temperature difference (typically 50 K: 10°C → 60°C)
• f_L = Loss factor for storage and distribution (1.15)

Simplified:

Q_DHW [kWh/a] = Liters/Day × 365 × 1.163 × 50 × 1.15 / 1000

2.6 Monthly Calculation

The calculator performs a monthly calculation to reflect seasonal variations:

1. Outside temperature per month: From climate data (location)
2. Heating degree days per month: Only days below heating limit temperature (15°C)
3. Proportional heat demand: Proportional to heating degree days
4. COP per month: Interpolated from manufacturer data
5. Electricity consumption per month: Heat demand / COP

2.7 Effect of Legionella Prevention

With activated legionella prevention (weekly heating to 65°C), the electricity consumption for hot water increases:

Q_Legionella = 52 × V_Storage × ρ × c × ΔT_Leg

With:

• 52 = Weeks per year
• ΔT_Leg = Additional heating (65°C - 55°C = 10 K)

Step-by-Step Guide

3.1 Project Management

Starting a New Project

On the start page, you have two options:

1. "Start calculation" – Starts the input wizard
2. Load project – Enter an existing 5-character project key

Editing an Existing Project

1. Load the project using the project key
2. Click on "Edit" in the results view
3. The wizard opens with all pre-filled data
4. Make your changes and recalculate

3.2 Wizard Step 1: Capturing Heat Demand

The first step determines how much heat your heat pump must provide.

Option A: Import from Heat Load Calculation (recommended)

If you have already performed a heat load calculation:

1. Select "Import from heat load calculation"
2. Enter the project key of your heat load calculation
3. The calculator automatically imports:
   • Design heat load [kW]
   • Location data (postal code, city)
   • Outdoor design temperature
   • System temperatures (if defined)

Option B: Manual Entry

If you don't have a heat load calculation:

1. Select "Manual entry"
2. Enter the design heat load [kW]
   • If unknown: Approximately 40-60 W/m² for unrenovated old buildings, 20-40 W/m² for renovated buildings
3. Enter the postal code
   • The calculator automatically determines the location and outdoor design temperature

Specifying Hot Water Demand

Regardless of the data source:

1. Enter the annual hot water demand [kWh/a]
   • Typical: 1,500 - 3,000 kWh/a for 2-4 people
2. Or click "Calculate" for the hot water assistant (more details in Chapter 5)

3.3 Wizard Step 2: Selecting a Heat Pump

In this step, you select your heat pump. You have two options:

Option A: Choose from Catalog

1. First select the heat pump type:

   • Air-water – uses outside air (most common)
   • Brine-water – uses ground heat (probes or collectors)
   • Water-water – uses groundwater

2. The catalog shows suitable models with:

   • Manufacturer and model designation
   • Nominal output at A2/W35
   • COP at A2/W35
   • SPF (manufacturer specification, if available)

3. Select a model by clicking on the row

Option B: Manual Entry

If your heat pump is not in the catalog or you have specific values:

1. Select "Enter manually"
2. Enter (optionally) manufacturer and model
3. Enter the performance data:
   • Nominal output [kW] at A2/W35
4. Enter the COP values:
   • COP A-7/W35 (at -7°C outside temperature) – for cold days
   • COP A2/W35 (at +2°C) – required field, standard operating point
   • COP A7/W35 (at +7°C) – for mild days

3.4 Wizard Step 3: System Parameters

In the last step, you configure the system temperatures and operating settings.

Heating Circuit Temperatures

Hot Water Settings

Electricity Price

Enter your current electricity price (in cents/kWh):

• Standard household electricity: approx. 28-35 ct/kWh
• Heat pump tariff: approx. 22-28 ct/kWh (with blocking times)

3.5 Starting the Calculation

After entering all data, click "Calculate SPF". The calculator now performs the following calculations:

1. Determination of annual heating demand
2. Monthly distribution by degree days
3. COP interpolation for each month
4. Calculation of electricity consumption
5. Economic analysis

The results are displayed immediately and the project is saved automatically.

Understanding Results

The results are organized into 6 clear tabs that provide a comprehensive overview of all aspects of your heat pump calculation. On mobile devices, navigate conveniently between tabs using arrow buttons or the dropdown menu.

Tab Overview

4.1 "Overview" Tab

The Overview tab displays the most important results at a glance.

Hero Metrics

Four large metric cards present the core values:

SPF Rating with Traffic Light System

The SPF is rated with a color-coded traffic light system:

Sankey Diagram (Energy Flow)

The Sankey diagram visualizes the energy flow through your heat pump:

Electricity (Input) ────────┐
                            ├──► Space heating (Output)
Environmental heat (Input) ─┤
                            └──► Hot water (Output)

• Left: Inputs – Electricity and environmental heat (air/ground/water)
• Right: Outputs – Space heating and domestic hot water
• Bar width: Proportional to energy quantity

Cost Overview

Additional cost metrics:

Heat Pump and Location Info

Two info cards show:

• Heat pump: Manufacturer, model, flow temperature
• Location: Postal code, city, state, outdoor design temperature

4.2 "Annual Profile" Tab

The Annual Profile tab shows the monthly distribution of heat demand and electricity consumption.

Monthly Energy Distribution

You can switch between Chart and Table view:

Chart View:

• Bar chart with 12 months
• Dark blue bars: Heat demand [kWh]
• Dark green bars: Electricity consumption [kWh]

Table View:

The table shows annual totals and the average SPF in the footer.

Room-by-Room Distribution (with Heating Load Import)

If you imported data from the heating load calculator, you'll also see the room-by-room energy distribution:

• Room Overview: Color-coded boxes (up to 12 rooms) with:

  • Heating load [kW]
  • Share of total demand [%]
  • Annual energy demand [kWh]
  • Floor area [m²]

• Room Table: Detailed monthly breakdown per room

SPF Annual Profile

An additional line chart shows how the SPF changes throughout the year – higher in summer (warmer outside air), lower in winter.

4.3 "Efficiency" Tab

The Efficiency tab provides detailed insights into your heat pump's performance.

SPF Breakdown

If calculated separately, you'll see:

Heat Pump COP Values

Three cards show COP values at different outdoor temperatures:

COP Temperature Curve

A line chart shows the COP progression from -15°C to +20°C outdoor temperature:

• X-axis: Outdoor temperature [°C]
• Y-axis: COP [-]
• Reference points: -15°C, +7°C (highlighted), +20°C

Monthly SPF Table

A detailed table with:

Efficiency Tips

A blue info box with three practical tips:

1. Lower flow temperature – Every Kelvin less brings 2-3% more efficiency
2. Use underfloor heating – Enables lowest flow temperatures
3. Regular maintenance – Maintains optimal performance

4.4 "Economics" Tab

The Economics tab analyzes the financial aspects of your heat pump over 20 years.

Main Metrics

Four large metric cards:

Extended Metrics

Three additional financial metrics:

Annual Operating Costs

Breakdown of operating costs:

Monthly Cost Calendar

A calendar view shows monthly electricity costs – ideal for identifying winter months with higher costs.

Cash Flow Projection

A bar chart visualizes the cumulative cash flow over 16 years:

• Red bars: Negative years (before payback)
• Green bars: Positive years (after payback)
• Payback point: Highlighted in yellow

Cash Flow Projection Table

Detailed annual breakdown:

The payback year is highlighted in yellow, all positive years have a green background.

Heating System Comparison

A comparison chart shows annual operating costs of different heating systems:

• Heat pump
• Gas heating
• Oil heating
• Pellet heating

Electricity Price Scenarios

How does profitability change with different electricity prices?

Sensitivity Analysis

What happens if...?

• Electricity price rises/falls?
• Investment costs higher/lower?
• SPF better/worse?

4.5 "Environment" Tab

The Environment tab shows the ecological balance of your heat pump.

CO₂ Main Metrics

Four large metric cards:

Illustrative Equivalents

Three color-coded boxes make CO₂ savings tangible:

CO₂ Emissions Comparison

A stacked bar chart compares annual CO₂ emissions:

Oil heating   ████████████████████  3,500 kg
Gas heating   ██████████████        2,800 kg
Heat pump     ████                    850 kg

The difference between gas and heat pump is your annual savings.

Electricity Mix Scenarios

A table shows how the electricity mix affects the CO₂ balance:

The PV scenario is highlighted in green as the best option.

Primary Energy Balance

Lifecycle CO₂

Also considers:

• CO₂ emissions from heat pump manufacturing
• Refrigerant type and its climate impact (GWP)

Environmental Tip

A green info box recommends:

PV combination: With a photovoltaic system, you can operate your heat pump almost CO₂-neutrally! A battery storage increases self-consumption even further.

4.6 "Professional" Tab

The Professional tab is aimed at specialists, installers, and technically interested users.

Info Banner

A blue notice explains: "The following information is intended for planners and installers."

Technical Metrics

Electric Backup Heater Analysis (Bivalent Operation)

Details about the electric backup heater:

Status boxes:

• Building heating load [kW]
• Heat pump capacity at design temperature [kW]

Assessment box:

• Green: Low backup heater share (<5%) – optimal
• Yellow: Acceptable share (5-15%) – okay
• Red: High share (>15%) – larger HP or building renovation recommended

Subsidy Information

Information about federal subsidies for efficient buildings:

• Eligibility based on SPF
• Bonuses for natural refrigerants
• Reference to current subsidy rates

SPF vs. SCOP Explanation

Two columns explain the difference:

VDI 4650 Calculation Details

Technical details about the calculation methodology:

• Climate data used
• Degree-day calculation
• Interpolation method for COP values

Grid Operator Lock-out Time Simulation

Simulation of the effects of grid operator lock-out times:

• Typical lock-out times (3× 2h daily)
• Impact on operating hours
• Recommendation for buffer tank size

Monthly Detailed Data

Comprehensive table:

Building Characteristic Curve

A line chart shows the heating load depending on outdoor temperature:

• X-axis: Outdoor temperature [-15°C to +20°C]
• Y-axis: Heating load [kW]
• Heating limit visible at ~15°C

COP Contour Chart (Heatmap)

A heatmap visualizes the COP depending on:

• Outdoor temperature (X-axis)
• Flow temperature (Y-axis)
• Color scale: Blue (low COP) to Green (high COP)

Additional Professional Features (Planned)

A gray info box announces:

• Hourly load profile
• Modulation behavior graphics

Calculating Hot Water Demand

The hot water assistant helps you realistically determine the domestic hot water demand – because in well-insulated houses, this often accounts for 30-50% of the total heat demand!

5.1 Opening the Assistant

In Wizard Step 1, click on the "Calculate" icon next to the hot water demand. The assistant is divided into two clear tabs:

• Tab "Consumption" – Determine hot water demand
• Tab "Heating schedule" – When should the water be heated?

5.2 Tab "Consumption" – Number of People

The most important input! Select by clicking on the person pictograms:

5.3 Shower Behavior

How extensively do the residents shower?

5.4 Bathtub Use

5.5 Dishwasher

Does the household have a dishwasher?

• Yes: Reduces hot water demand (no hand washing)
• No: +5 L/person/day for hand washing

5.6 Tab "Heating Schedule" – Preparation Strategies

In the second tab, you can choose when the hot water should be heated. This has a direct impact on the heat pump's efficiency!

Available Strategies

Strategies in Detail

Heat once daily Ideal for most households. Choose 11am-2pm as heating time when the outdoor temperature is highest. The tank keeps the heat until the next day.

Heat twice daily Good for households with high morning and evening consumption. Example: 6:00 AM (before showering) and 6:00 PM (before dinner).

Solar-optimized Perfect for households with photovoltaics! Heating automatically occurs between 10am-3pm when:

• The PV system produces electricity
• The outdoor temperature is highest

Night operation Only useful with a special night electricity tariff (HT/NT). Efficiency is lower (colder outdoor temperature), but can be compensated by cheaper electricity price.

Individual Time Selection

For the strategies "Once daily" and "Twice daily", you can choose the exact time:

• First heating time: Dropdown selection from 00:00 to 23:00
• Second heating time: (only for "Twice daily")

Recommended times:

• Optimal: 11:00 AM - 2:00 PM (warmest time of day)
• With PV: 10:00 AM - 3:00 PM (highest solar irradiation)
• Avoid: 10:00 PM - 6:00 AM (coldest time)

5.7 Result and Adjustment

The assistant always shows at the top:

• Calculated demand [kWh/a] – based on your inputs
• Daily consumption [liters] – for plausibility check
• Per person/day [liters] – should be around 30-50 L

5.8 Efficiency Display in Tab

In the "Heating schedule" tab, you'll see an efficiency badge as soon as you choose a strategy other than "Continuously". This shows the expected efficiency improvement (or reduction for night operation).

5.9 DHW Profile is Saved

The complete hot water profile is saved with the project:

• Number of people and shower behavior
• Bathtub usage and dishwasher
• Preparation strategy and heating times

When reloading, you can adjust all settings directly without having to re-enter everything.

Heat Pump Entry

6.1 Catalog Selection

The heat pump catalog contains current models from renowned manufacturers with verified performance data.

Displayed information:

• Manufacturer – e.g., Buderus, Viessmann, Stiebel Eltron
• Model – Complete type designation
• Output – Nominal heat output at A2/W35 [kW]
• COP – Coefficient of performance at A2/W35
• SPF – Manufacturer specification (if available)

Filtering by type:

• Air-water (AirWater) – Outside air as heat source
• Brine-water (BrineWater) – Ground via probes/collectors
• Water-water (WaterWater) – Groundwater

6.2 Manual Entry

For heat pumps not in the catalog, use manual entry:

Required Fields

Optional Fields (recommended)

6.3 Interpreting COP Values Correctly

The COP values in the data sheet refer to standardized test conditions according to EN 14511:

Notation: A/W or B/W
A = Air, B = Brine, W = Water (flow)
Number = Temperature in °C

Examples:

• A2/W35 = Outside air 2°C, flow 35°C
• A-7/W55 = Outside air -7°C, flow 55°C
• B0/W35 = Brine 0°C, flow 35°C

Tips and Best Practices

7.1 Choosing Optimal Flow Temperature

The flow temperature is the most important lever for a high SPF:

Optimization tips:

1. Replace radiators with larger ones → lower flow possible
2. Perform hydraulic balancing
3. Optimize individual room control
4. Adjust heating curve (reduction at mild outside temperatures)

7.2 Optimal Heat Source Selection

7.3 Consider Sizing

7.4 Efficient Hot Water Production

7.5 Plan Monitoring

After installation, you should regularly check:

• Electricity meter for heat pump (separate meter recommended)
• Heat meter (often mandatory for subsidies)
• Operating hours and compressor starts

Check real SPF:

SPF_real = Heat meter [kWh] / Electricity meter [kWh]

Frequently Asked Questions (FAQ)

What is the difference between COP and SPF?

The SPF is always lower than the best COP, as it also considers cold days, hot water preparation and defrost cycles.

Why is my calculated SPF lower than the manufacturer states?

Possible reasons:

1. Higher flow temperature – The manufacturer often specifies SPF at 35°C
2. Colder location – Your outdoor design temperature is lower
3. Hot water share – High DHW demand lowers overall SPF
4. Legionella prevention – Weekly 65°C heating costs efficiency

What flow temperature do I need for radiators?

This depends on the radiator size:

• Adequately sized (1:1 replacement): 50-55°C
• Oversized (e.g., after window replacement): 40-45°C possible
• Undersized: >60°C required → replacement recommended

Use our heat load calculator to calculate the optimal flow temperature!

Is a heat pump worthwhile with radiators?

Yes, if:

• Flow temperature ≤55°C achievable
• SPF ≥ 3.0 realistic
• Electricity tariff ≤ 30 ct/kWh
• Alternatively: Gas boiler old and inefficient

No, if:

• Flow temperature >60°C required
• Radiators significantly undersized
• No possibility to increase heating surface area

How accurate is the SPF forecast?

With correct input data (especially COP values and flow temperature), the deviation is typically ±10-15% compared to actual operation.

Factors not considered:

• User behavior (ventilation, setback periods)
• Defrost cycles (for air HP in winter)
• Control losses
• Buffer storage losses

What SPF do I need for BAFA funding?

As of 2024 (BEG):

• Air-water HP: SPF ≥ 3.0
• Brine-water HP: SPF ≥ 3.5
• Water-water HP: SPF ≥ 3.5
• Natural refrigerants: Bonus (R290 propane, R744 CO₂)

Technical Background Information

9.1 Heat Pump Principle

A heat pump works like a "reverse refrigerator":

1. Evaporator: Refrigerant absorbs environmental heat (air/brine/water)
2. Compressor: Refrigerant is compressed → temperature rises
3. Condenser: Heat is released to heating water
4. Expansion valve: Pressure is released → cycle begins again

Environmental heat (3 parts) + Electricity (1 part) = Heating (4 parts)
→ COP = 4

9.2 Typical Values by Heat Source

Air-Water Heat Pump

Brine-Water Heat Pump

Water-Water Heat Pump

9.3 Effect of Flow Temperature on COP

The following table shows typical COP values for an air-water HP:

Insight: The COP decreases with:

• Colder outside temperature (→ less environmental heat available)
• Higher flow temperature (→ greater lift required)

9.4 Monthly Outside Temperatures (Germany)

The calculator uses location-specific climate data. Typical average values for reference:

Standards and Further Information

10.1 Relevant Standards and Guidelines

10.2 Further Links

• BWP Climate Map – Outdoor design temperatures for Germany
• BAFA Heat Pump List – Eligible devices
• Our Heat Load Calculator – For design heat load
• Our Solar Calculator – PV self-consumption for heat pump

10.3 Funding

Currently (as of 2024), heat pumps are funded through the BEG (Federal Funding for Efficient Buildings):

To the Calculator

Ready for your heat pump calculation?

Start Heat Pump Calculator

If you have questions about the heat load, we recommend first performing our heat load calculation – the results can be imported directly into the heat pump calculator.

Last updated: December 2025