What is a heat pump COP? Why it matters for your tariff choice and running costs

What COP means

COP stands for Coefficient of Performance. It's the ratio of heat output to electrical input:

COP = heat energy delivered ÷ electrical energy consumed

A heat pump with a COP of 3.0 produces 3 kWh of useful heat for every 1 kWh of electricity it draws. A COP of 1.0 would mean pure electrical resistance heating — every unit in becomes one unit of heat. A heat pump is typically two to four times more efficient than that.

This is possible because a heat pump doesn't generate heat from scratch — it moves heat from the outside air into your home, using a refrigerant cycle. Moving heat takes much less energy than creating it.

COP vs SCOP: which figure matters?

COP is a snapshot — the efficiency at one specific operating condition (usually measured in a lab at a fixed outdoor temperature, often 7°C).

SCOP (Seasonal COP) is the average across a full heating season, taking into account real outdoor temperature variation. It's the number that actually determines your annual bill.

In the UK climate, typical real-world SCOP values are:

When evaluating your heat pump's running costs, use SCOP — not the best COP figure from a product brochure.

Why electricity pricing makes COP critical

At the April–June 2026 Ofgem price cap:

• Electricity: 24.7p/kWh
• Gas: 5.7p/kWh

Electricity costs roughly 4.3 times as much as gas per unit. For a heat pump to match a gas boiler on running costs (assuming 90% boiler efficiency), it needs a COP of at least 3.9 — delivering enough extra heat per unit to compensate for electricity's higher price.

At the standard electricity rate, many real-world heat pumps fall marginally short of that breakeven. On a standard variable tariff, the economics are tight.

This is where tariff choice changes everything.

On a heat pump tariff with an effective blended rate of 16p/kWh (achievable with active load-shifting on Cosy Octopus or similar), the breakeven COP drops to around 2.5 — well within the range of any reasonably installed system.

The lower your effective electricity rate, the more forgiving the maths — and the wider the range of systems that come out ahead of gas.

What affects your COP day-to-day?

Outdoor temperature

Heat pumps extract heat from outdoor air. The colder it is outside, the harder the system has to work, and the lower the COP. On a cold January night at −5°C, a typical UK air source heat pump might achieve a COP of 1.8–2.2. On a mild October day at 12°C, the same system might hit 3.5–4.0.

This is why load-shifting during mild daytime hours (when cheap windows happen to coincide with higher outdoor temperatures) has a double benefit: cheaper electricity and more efficient operation.

Flow temperature

The temperature the heat pump needs to heat water to has a major effect on efficiency. A system running at 35°C flow temperature (typical for underfloor heating or oversized radiators) will achieve a significantly higher COP than one running at 55°C (needed for small radiators or domestic hot water).

Every 10°C reduction in flow temperature adds roughly 0.5–1.0 to your COP. If your system is running at high flow temperatures, this is often the single biggest improvement available — sometimes without any hardware changes, just recalibration.

Legionella cycles and immersion use

Your immersion heater has a COP of 1.0. Every kilowatt-hour of electricity your immersion draws is one kilowatt-hour of heat — no multiplication effect. Running the immersion unnecessarily drags your whole-system average SCOP down and inflates your electricity consumption.

COP and tariff choice: putting it together

The table below shows the effective cost of heat (per kWh delivered) at different electricity rates and COP values. The gas breakeven line is approximately 6.3p/kWh of heat (5.7p gas ÷ 90% boiler efficiency).

❌ = more expensive than gas. ✓ = cheaper than gas. ≈ = near breakeven.

A system with a seasonal COP of 3.0 on the standard tariff is only marginally more expensive than gas. The same system on Cosy off-peak rates beats gas by a significant margin. The right tariff can make the difference between a heat pump that's cheaper than gas and one that isn't.

How to find your system's actual SCOP

The most reliable way is to read your heat pump's energy monitoring data (most modern units log both electrical input and heat output) and calculate the ratio over a 12-month period. If your heat pump doesn't log this, a clamp meter on the electrical supply and a heat meter on the flow/return will give you the figures.

Alternatively, look at your annual electricity consumption attributable to the heat pump (visible in your smart meter data via Bright or similar), and compare it to your home's estimated annual heat demand. A rough SCOP estimate follows.

If your SCOP is below 2.5, it's worth investigating why before optimising your tariff — the issue is more likely to be flow temperature, sizing, or commissioning than tariff choice.

The short version

• COP is heat output divided by electrical input. Higher is better.
• SCOP (seasonal average) is what determines your annual bill — not the lab-tested peak figure.
• Real-world UK SCOPs range from 2.5 to 3.5 for well-installed systems.
• On the standard electricity cap, a COP of ~3.9 is needed to beat gas. Most systems fall short.
• On a heat pump tariff with active load-shifting, the breakeven drops to ~2.5 — achievable by most systems.
• Flow temperature has a larger effect on COP than most people realise. Lower is better.
• The immersion has a COP of 1.0. Use it sparingly.

Want to see what a better tariff is actually worth for your system? Run a comparison on Heat Pump Tariffs — using your real half-hourly usage data.