Glossary
Jahresarbeitszahl (JAZ)
Jahresarbeitszahl (JAZ): The Jahresarbeitszahl (JAZ, German for seasonal performance factor or SPF) states how many kilowatt hours of heat a heat pump produces from one kilowatt hour of electricity over a whole year. A JAZ of 3.5 means 1 kWh of electricity gives 3.5 kWh of heat.
What is the Jahresarbeitszahl?
The Jahresarbeitszahl (JAZ) is the key figure for a heat pump. It divides the heat produced in a year by the electricity used to produce it. A heat pump with a JAZ of 3.0 needs 5,000 kWh of electricity for 15,000 kWh of heat.
Electricity needed = heat demand ÷ Jahresarbeitszahl
Typical values
| Radiators and flow temperature | Air-source | Ground-source |
|---|---|---|
| Underfloor heating (up to 35 °C) | about 4.0 | about 4.6 |
| Low-temperature radiators (up to 45 °C) | about 3.5 | about 4.1 |
| Classic radiators (up to 55 °C) | about 3.0 | about 3.6 |
These are guide values. The real JAZ also depends on design, settings and local climate.
What does this mean for you?
A higher JAZ cuts electricity costs directly. Replacing a few radiators can lower the flow temperature needed and raise the JAZ. The Heating Cost Comparison Calculator shows what that means over 20 years compared with gas, pellets or district heating.
This page is for information only and does not replace an energy consultation.
Frequently asked questions
Jahresarbeitszahl translates as 'annual performance factor'. In English it is usually called the seasonal performance factor (SPF). It is the ratio of heat delivered to electricity used by a heat pump over a full year.
For an air-source heat pump in an older German house, 3.0 is decent and 3.5 or more is good. Ground-source heat pumps often reach 4 or more. The flow temperature matters most: the lower it is, the higher the JAZ.
Since 29 July 2026, § 559f BGB generally lets landlords pass the full cost of a new heat pump on to tenants as a modernisation only with proof of a JAZ of at least 2.5. Otherwise only half of the cost counts. Exceptions include buildings built after 1996.