COP and SCOP
A heat pump moves heat instead of making it, so it can deliver more heat than the electricity it uses. COP describes that ratio at one moment. SCOP describes it over a whole heating season. Both answer the question your bill asks: for each kilowatt-hour of electricity you pay for, how many kilowatt-hours of heat end up in the house?
COP: efficiency at one operating point
The coefficient of performance (COP) is the heat delivered divided by the electricity used, in the same units. A COP of 4 means 1 kWh of electricity produces 4 kWh of heat. An electric radiator gives 1 for 1, a COP of 1.
Datasheets quote COP at test points set by the standard EN 14511-2:2022 . A7/W35 means outdoor air (A) at 7 °C and water (W) leaving the heat pump at 35 °C. That water temperature is the flow temperature: what the heat pump sends into your floor or radiators. A-7/W55 is outdoor air at -7 °C and water at 55 °C. Ground-source units, which take heat from a buried loop, use the temperature of the antifreeze liquid in that loop (the brine) instead, for example B0/W35.
A COP figure is only true at its own test point. Change either temperature and it changes.
Why COP depends on both temperatures
Think of the heat pump as lifting heat uphill, from the cold outdoors to the warm water in your heating circuit. The longer the climb, the more electricity the compressor needs for each unit of heat. A colder day lengthens the climb, and so does hotter water. The best any machine could do in principle is the Carnot COP: the hot temperature divided by the gap between hot and cold, both in kelvin (Celsius plus 273.15).
| Outdoor air | Flow 35 °C | Flow 55 °C |
|---|---|---|
| 7 °C | 11.0 | 6.8 |
| -7 °C | 7.3 | 5.3 |
These are theoretical limits, and real units reach a fraction of them. The pattern still holds: a colder day and a hotter flow both cost efficiency. That is why low-temperature emitters (underfloor heating, or radiators big enough to warm a room with cooler water) matter so much, and so does weather compensation, where the controller lets the water run cooler on mild days.
SCOP: efficiency over a season
SCOP, the seasonal COP, is the useful heat delivered over a heating season divided by the electricity used over the same season. EN 14825:2022 defines how manufacturers calculate it. It takes performance measured at several part-load conditions (a heat pump often runs below its full output) and weights it by how often each outdoor temperature occurs in a reference climate, a standard pattern of temperatures offered as average, colder or warmer. The standard has two versions of the figure. SCOPon covers only the hours the unit runs. SCOP adds the electric backup heater and the electricity the unit draws while idle: with the thermostat off, in standby, and for a crankcase heater, a small heater that keeps the compressor’s oil warm.
The label figure compares products under identical rules. It is not a forecast for your house, because your climate, heating demand and flow temperature differ from the reference ones.
How the model gets COP
A 16 kW heat pump and a 10 kW one are easy to picture. The less obvious difference is how each machine’s heat output and efficiency change as the outdoor temperature falls. Plotted against outdoor temperature, that change is its performance curve.
For air-source heat pumps, the model holds data for six size classes, called 4, 6, 8, 10, 13 and 16 kW, and for two identical units working together (2 x 10, 2 x 13 and 2 x 16) when a house needs more than one unit gives. Each class is a real model from one manufacturer’s mid-range line, not its premium one, and every value is a published datasheet figure. The equipment selector uses generic size classes. A class has a capacity (how much heat it can deliver) and a COP at several outdoor temperatures, from -15 to 7 °C, for four water temperatures: 35, 45, 55 and 65 °C. The model joins those published points with straight lines instead of fitting a smooth curve, so a larger unit has its own pattern instead of a scaled copy of one reference unit.
The water temperature comes from the table, not from a correction formula. For a size 10 unit at 7 °C outdoors, the published COP is 5.2 with water at 35 °C, 3.7 at 45 °C and 2.7 at 55 °C. The model reads the table at the flow temperature the heating circuit delivers in each hour, so a 50 °C radiator circuit sits between the 45 and 55 °C columns. Below 35 °C the model keeps the 35 °C figures, which is cautious, because cooler water would give a better COP. Above 65 °C the model refuses the setup instead of guessing, and none of the emitters it offers runs that hot.
EnergyPlus, the building simulation program the model runs on, reads the table at every step of simulated time (15 minutes, or 5 for a ground-source house), using the actual outdoor and flow temperatures. On a house with radiators, the flow temperature follows a heating curve, a fixed rule that sets the water temperature from the outdoor temperature. The water runs cooler on mild days and the modelled COP rises. A floor heating loop follows a lower curve, but its design supply is already the 35 °C floor of the table, so its cooler water on mild days earns no further credit. Ground-source units are tabulated the same way, against the brine temperature from -10 to 10 °C.
The seasonal figure in your results
The results show a heat pump COP for the simulated year: the heat the heat pump delivered to the heating circuit, divided by the electricity it used for heating while running, summed over every hour. Like SCOPon, it leaves standby out, so a fixed idle draw does not weigh on a house that needs little heat. The SCOP behind the label includes standby for a reference house, so the two are close but not the same figure. It does not group hours into temperature bins as EN 14825 does, because the model has the real hourly sequence of outdoor temperatures and heat demand.
Standby is its own line. The EU product sheets of the units in the model declare 16 W in standby, 14 W with the thermostat off and 0 W for a crankcase heater. The model adds 16 W per unit in every hour the unit does not run, about 120 kWh a year for one unit on a new 182 m² house with underfloor heating. That electricity is in the heat pump electricity and in the cost. Heat pump size and cycling shows what standby and compressor starts add up to for six sizes on one house.
Next to it sits a system figure: all the heat delivered to the rooms, divided by the electricity of the heat pump, its standby and the backup heater (an electric element that tops up the heat pump when it cannot keep up). A big gap between the two numbers means the backup heater is doing a lot of the work. In a house that needs little heat, standby alone widens the gap, because 16 W in every idle hour is a large share of a small total. The system figure is not shown for setups that burn fuel.
The seasonal figure covers heating the rooms only. Hot water is a separate unit, and circulation pump and ventilation fan electricity are left out. Because the weather year, house, emitters and settings are yours, it will not match a datasheet SCOP. Use it to compare your own options with each other.
Limits of the model
- Performance data exists for six air-source and five ground-source size classes, plus cascades of two units, with no blending between neighbouring sizes.
- Each class is a real model from one manufacturer’s mid-range line, shown without the brand. Its COP is the declared datasheet value, which is slightly optimistic when the unit runs at full output. Your unit will differ. Compare the modelled COP with the manufacturer’s datasheet at your actual flow temperature before relying on its absolute value.
- The air-source tables end at 7 °C outdoors, so the modelled COP does not rise further in milder weather.
- Cycling below the unit’s minimum output carries no efficiency loss. EN 14825 describes that loss with a degradation coefficient, Cdh, and the product sheets of these units declare a Cdh of 1, meaning none. The model estimates how often the compressor starts and saves it with each run, but never adds it to the energy.
- There is no separate defrost cycle. Defrost is the periodic step in which an air-source heat pump melts the frost that builds up on its outdoor unit, using some of its energy. Datasheet COPs at low outdoor temperatures normally include its effect.
Reference performance tables
The equipment choices use generic size classes. The underlying heating tables come from these manufacturer-authored planning guides:
- Viessmann Vitocal 150-A/250-A planning guide, 6246340 DE, March 2026 , sections 8.5 to 8.7, water-temperature tables on PDF pages 133 to 142. This copy is hosted by Heizungsdiscount24. The size-10 example above uses the A7 rows on PDF pages 133 and 134, with the displayed COP values rounded.
- Viessmann ground-source planning guide, 5811541 DE, April 2023 , BWC 201.B operating-point tables, including PDF pages 19 and 20 for the size-10 reference. This copy is hosted on the manufacturer’s community site.
These are reference profiles for the simulation. They do not establish the performance of an installed unit or certify a simulated result.