The JAZ (Jahresarbeitszahl), internationally the seasonal performance factor (SPF), is the ratio of heat a heat pump delivers to the electricity it consumes over a full year of real operation. A JAZ of 3.0 means 1 kWh of electricity produced 3 kWh of heat. Air-to-water systems typically achieve 3.0–4.0; Germany's BEG subsidy requires a calculated JAZ of at least 3.0.
What does the JAZ actually measure?
The JAZ (Jahresarbeitszahl, annual performance factor) divides the heat a heat pump delivers over twelve months by the electricity it consumes in the same period. A JAZ of 3.0 means one kilowatt-hour of electricity became three kilowatt-hours of heat. Internationally the same metric is called the seasonal performance factor (SPF).
Unlike laboratory ratings, the JAZ reflects the real installation: local climate, actual flow temperatures, defrost losses, standby power and — depending on the system boundary — circulation pumps and the electric backup heater. The EU SEPEMO project standardised those boundaries as SPF H1 to H4; H4 includes backup heating plus all pumps and is the figure that matches the electricity bill.
How is the JAZ different from COP and SCOP?
COP and SCOP are laboratory values; the JAZ is what the building actually achieves. All three divide heat output by electrical input, but over very different scopes:
| Metric | Scope | Basis |
|---|---|---|
| COP | One operating point (e.g. A7/W35) | Lab test, EN 14511 |
| SCOP | Modelled heating season, reference climate | Lab test, EN 14825 |
| JAZ / SPF | Real year, real building | Field metering or VDI 4650 calculation |
No heat pump rated at a best-point COP of around 4.9 at A7/W35 — an excellent rating by current standards — will run a JAZ of 4.9 in the field: winters are colder than 7 °C, radiators often need more than 35 °C, and defrost takes its share. Comparing a best-point COP against a required annual JAZ is the most common error in heat pump sizing conversations. The laboratory value closest to the JAZ is the SCOP, and the EU label metric ηs is derived directly from it.
What JAZ values are realistic?
Air-to-water heat pumps in European homes typically reach a JAZ between 3.0 and 4.0; ground-source systems reach 4.0 to 5.0. Fraunhofer ISE field monitoring measured average seasonal performance factors of about 3.1 for air-source and 4.1 for ground-source units in existing buildings — evidence that well-planned retrofits clear the efficiency bar even without underfloor heating. New builds with low-temperature circuits routinely land between 3.5 and 4.5.
Why does Germany's BEG subsidy require a JAZ of 3.0?
The KfW heating grant (programme 458) funds only heat pumps whose planned annual efficiency, calculated with the VDI 4650 method, reaches at least 3.0. The threshold marks the point where a heat pump clearly beats fossil and direct-electric heating on both primary energy and running cost: at JAZ 3.0, a kilowatt-hour of heat costs one third of the electricity price, undercutting gas heating whenever electricity costs less than roughly three times as much as gas per kWh.
How do you raise the JAZ of an installed heat pump?
Lower the temperature the compressor must produce and the JAZ follows — each 1 °C reduction in flow temperature improves efficiency by roughly 2–3%. The levers, in order of impact:
- Correct the heating curve with weather compensation instead of running a fixed, defensively high flow temperature.
- Balance the hydraulics so mixing never forces the compressor to overshoot the emitter temperature.
- Keep hot water realistic: store at 50–55 °C with a scheduled anti-legionella cycle rather than 60 °C around the clock.
- Size honestly from a room-by-room heat load calculation — oversized compressors cycle, and cycling erodes the annual figure.
The spread between a well-configured and a badly configured identical unit is regularly a full JAZ point. With European heat pump sales reaching 2.88 million units in 2025 (+13%, EHPA), system configuration has become a continent-scale efficiency question.
Frequently asked questions
What is a good JAZ for an air-to-water heat pump?
Anything from 3.5 upward is a well-designed system; 3.0–3.5 is solid in retrofits with radiators. Below 3.0, investigate flow temperatures, heating-curve settings and hot water configuration — Fraunhofer ISE field averages show 3.1 is achievable even in unmodernised existing buildings.
Is the JAZ the same as the SCOP?
No. The SCOP is a laboratory seasonal rating to EN 14825 under a reference climate; the JAZ is measured (or calculated per VDI 4650) for one specific building and year. The JAZ usually lands somewhat below the SCOP because real systems include defrost, standby and imperfect settings.
How do I measure the JAZ of my own heat pump?
Divide the reading of a heat meter on the heat pump output by the reading of a dedicated electricity meter over the same twelve months. Many controllers estimate both values internally, but physical meters are more reliable — and include the backup heater if you want the honest whole-system figure.
Does a higher JAZ always mean lower heating bills?
Almost always, since electricity consumed per unit of heat falls in direct proportion. The exceptions are tariff-driven: a slightly lower JAZ achieved by shifting operation into cheap or self-generated PV electricity can still win on cost.