A project by qlines.net
energyhub
Deutsch

Concepts · Chapter 08

Efficiency decoded: COP, SCOP, and JAZ

Three numbers that all mean "efficiency" but answer different questions — and why SCOP55 is the one that matters in an old radiator house.

Updated: 2 min read Deutsch →

Chapter 7 showed why a heat pump delivers more heat than the electricity it consumes. That ratio gets measured with three different figures — and they don't answer the same question.

COP — the snapshot

COP (Coefficient of Performance) is efficiency at exactly one operating point. A COP of 3.15 at "A−7/W35" means: at −7 °C outdoor temperature and 35 °C flow, the heat pump delivers 3.15 kW of heat per 1 kW of electricity. The problem: a single COP figure says nothing about how the system performs across a whole heating season of changing weather.

SCOP — the seasonal figure

SCOP (Seasonal COP) averages efficiency over a standardised heating season — a far more useful number for comparison. Importantly, there are SCOP figures for different flow temperatures:

  • SCOP35 — at 35 °C flow, typical for underfloor heating.
  • SCOP55 — at 55 °C flow, the relevant figure for old buildings with radiators.

The mistake that shows up most often in offers: only SCOP35 gets quoted — the flattering number — even though the house has radiators and runs at a much higher flow temperature. Ask specifically for SCOP55 if you don't (yet) have underfloor heating.

JAZ — what actually happens in your house

JAZ (Jahresarbeitszahl, annual performance factor) is the real, measured efficiency in your specific house over a year. It depends heavily on installation quality — above all on proper hydraulic balancing, the flow temperature actually run, and the radiators themselves. Theory (SCOP, from the datasheet) and practice (JAZ, measured at your house) can diverge significantly if the installation isn't done carefully.

An example from a real project

These figures come from one specific model (Stiebel WPL-A 10.2 Plus, per its datasheet as of mid-2026) — as an example, not a universal value. Check the current datasheet for whichever model you're actually offered:

  • SCOP35: 4.96 · SCOP55: 3.99
  • Heating output at A−7/W35: 10.07 kW
  • COP at A−7: 3.15 · COP at A7: 5.40

A roughly 0.3–0.4 difference in SCOP55 between two models corresponded, in this project, to roughly €250–300 a year in heating cost — as a rough order of magnitude, not a guarantee: the actual effect depends on electricity prices, heat demand, and real-world JAZ.

What this means for you

Ask every offer specifically for:

  • SCOP55 (not just SCOP35), if your house has radiators.
  • The A−7 COP, not just the flattering mild-weather COP (A7).
  • After installation, ask your installer how the JAZ can be monitored or read out — that's the figure that determines your actual running cost.

The biggest lever for keeping SCOP theory and JAZ practice close together is flow temperature — the next chapter.

In this section

Energy basics: kW, kWh, and what "kWp" actually means The single most common mix-up in offers and conversations, cleared up once — because everything else builds on it. How a PV system works From sunlight on the roof to power at the socket — the chain in four steps, and what actually drives yield. Understanding modules Mono or bifacial, cell quality, and the difference between product and performance warranty — without the marketing. Battery basics Nominal vs usable capacity, LFP vs NMC, depth of discharge, and sizing the battery to your array. How a heat pump works A fridge, thought about backwards — why a heat pump turns 1 kWh of electricity into several kWh of heat, without breaking physics.