SCOP, COP and ηs: what a heat pump’s efficiency figures mean

A heat pump turns one kWh of electricity into several kWh of heat. How many depends on conditions, and datasheets describe it with three related numbers: COP, SCOP and seasonal space heating efficiency (ηs). They sound alike but measure different things, and mixing them up leads to unfair comparisons and wrong running-cost estimates.

The figures in this guide come from the 1,350 heat pumps in our database that are sold in the Netherlands, Germany or the UK: 996 air-to-water, 250 ground source and 100 water-to-water. Almost all values come from the German BAFA list of eligible heat pumps and the Heat Pump KEYMARK, which both publish standardised test data ^1,2. Unless stated otherwise, figures refer to air-to-water models. MCS certification is a separate UK requirement; check each product’s MCS status before relying on it for a grant.

COP: efficiency at one test point

COP (coefficient of performance) is heat output divided by electricity input at a fixed test point. “A7/W35” means 7 °C outdoor air and 35 °C water leaving the heat pump. A COP of 5 means 5 kWh of heat for every kWh of electricity at that moment.

The test point matters a lot. Medians for air-to-water in our database (the A2 and A-7 figures are available for just over half of the models):

At −7 °C a typical air-to-water heat pump delivers only about 60% of its efficiency at 7 °C. A single COP at a mild test point says little about a whole year.

SCOP: the seasonal average

SCOP (seasonal COP) weights efficiency across a standardised heating season with many mild days and a few cold ones. It is calculated for an “average climate” with a design temperature of −10 °C ³, and declared for two flow temperatures ³:

• 35 °C (low-temperature application): underfloor heating or generously sized radiators.
• 55 °C (medium-temperature application): typical radiators in a less well-insulated home.

Medians per type, with the middle half (25–75%) in brackets:

For every type, the median SCOP at 55 °C is about 25% lower than at 35 °C. That gap is bigger than the difference between a good and a mediocre product at the same temperature. In other words, the flow temperature your home needs affects your bills more than the brand you choose.

ηs and the energy label

Seasonal space heating efficiency ηs is SCOP converted to primary energy. The formula is ηs = SCOP ÷ 2.5 − 3 percentage points ³; the 2.5 represents the efficiency of electricity generation. A SCOP of 4.73 therefore gives 4.73 ÷ 2.5 × 100 − 3 ≈ 186%. Our air-to-water data follows this almost exactly; for ground source and water-to-water models the declared ηs is typically about 5 percentage points lower than this formula gives.

The energy label comes from ηs, with separate scales for 35 °C and 55 °C ³:

Of the air-to-water heat pumps in our database with a label, 90% reach A+++ at 35 °C and 23% at 55 °C; almost all the rest are A++. For ground source models it is 91% and 53%. The label barely separates products.

SCOP in the MCS design and the grant

For the Boiler Upgrade Scheme, an air-to-water or ground source system must reach a SCOP of at least 2.8 in the MCS calculation for your home ⁴. That is a system figure: MCS installers must state the design flow temperature and hand over the installation’s SCOP based on that temperature ⁵. A product with a high lab SCOP can still produce a poor system SCOP if it is designed to run hot. Grant amounts are covered in our heat pump grants guide.

What it means for your bills

The rule of thumb: electricity use = heat demand ÷ SCOP. For a home needing 10,000 kWh of space heating a year:

These are the air-to-water medians. That is roughly 720 kWh a year, purely from flow temperature. Multiply by the unit rate on your tariff to put a price on it. To estimate your heat demand from gas use, take the kWh on your gas bill and multiply by your boiler’s efficiency, since not all of the gas ends up as heat; hot water is heated to a higher temperature and runs at a lower efficiency than space heating.

How to get a better SCOP in practice

• Lower the flow temperature. Between 35 and 55 °C, heat pumps in our data lose a little over 1% efficiency per degree on average. Ask for a weather-compensation curve that starts as low as possible.
• Compare the right column. With radiators, compare SCOP at 55 °C, or at your design temperature. Our list of most efficient heat pumps for radiators sorts on that.
• Don’t oversize. A unit that cycles on and off runs less efficiently. Of the air-to-water models for which our data records it, 98% can modulate their output, but every unit has a minimum.
• Compare within a size band, for example the most efficient 8–10 kW heat pumps.

SCOP figures are laboratory values under standard conditions. They are good for comparing products; how efficient your installation really is depends on the design, the emitters, hot-water use and settings.