Heat pump bills overrun for four measurable — and usually fixable — reasons: the quote used a laboratory SCOP instead of a field SPF, the flow temperature is running above design, the compressor cycles instead of running steadily, or the tariff is unfavourable. Well-set-up radiator systems reach an SPF of 3.2–3.6 and underfloor systems often exceed 3.8, so most overruns are recovered through settings rather than new equipment.
Why is the real efficiency lower than the SCOP I was quoted?
Because SCOP is a laboratory number and your bill is not. SCOP is measured to EN 14825 at one flow temperature under a standard climate profile; it excludes domestic hot water, defrost losses, circulation pumps, controls and any backup immersion heater. The field metric is SPF, and it is always the lower of the two.
The UK's Electrification of Heat Demonstration Project installed 742 heat pumps across a broad spread of house types and ages. Across 428 monitored air source systems the median SPF came out at 2.78 — about 0.3 better than the 2.44 recorded in the earlier RHPP field trial, but far below the headline figures on brochures. A quote built on SCOP 4.2 and a bill built on SPF 2.8 differ by 50% before anything has actually gone wrong. See SCOP and seasonal performance factor for exactly what each figure counts.
What is actually pushing my bill up?
Five things explain almost every overrun. Measure them before replacing anything.
| Symptom | What to measure | What good looks like |
|---|---|---|
| Bill far above the quote | Annual heat out ÷ electricity in (SPF) | 3.0 – 4.0 |
| Runs constantly, rooms still cool | Flow temperature on the coldest days | 35 °C underfloor, 45–50 °C radiators |
| Starts and stops all day | Compressor cycles per hour | 2 – 3 maximum in mild weather |
| Hot water costs more than heating | Cylinder charge temperature | 48–52 °C, not 60 °C daily |
| Unexplained kWh on the meter | Share of energy used by the backup immersion | under 5% of annual kWh |
Flow temperature is the biggest single lever. Each additional degree of flow temperature costs roughly 2–2.5% of COP, so a system running at 55 °C when it was designed for 45 °C burns about a fifth more electricity for the same heat. The fix is usually a curve setting, not a new machine — see what flow temperature should I run my heat pump at?
Is it the heat pump, or is it the tariff?
Do the arithmetic before blaming the machine. A heat pump only beats a gas boiler on running cost when its SPF exceeds the electricity-to-gas price ratio multiplied by the boiler's seasonal efficiency. At the EU-average ratio of about 2.4 (Eurostat, H2 2025) and a boiler achieving 85%, break-even sits at SPF 2.04 — so even the UK field median of 2.78 heats about a quarter more cheaply than gas. Only in high-ratio markets, such as the UK at about 3.5 on a standard tariff, does break-even rise to SPF 2.98; there a well-set-up SPF of 3.5 still wins comfortably, and heat pump tariffs widen the margin. The EU's 2026 Electrification Action Plan targets a household electricity-to-gas ratio of 2.5 or below by 2030 precisely because this single ratio decides the outcome. From 2028, ETS2 adds a carbon price to heating fuels — around 1 cent per kWh of gas for every €50 per tonne of CO₂. The full comparison sits in heat pump vs gas boiler running costs.
What can I fix this winter without spending money?
- Drop the weather compensation curve 3 °C at a time and live with each setting for a week. Rooms that still reach target are telling you the previous setting was waste. See weather compensation.
- Open the radiator valves in the reference room fully. A throttled reference room forces the whole system to a higher flow temperature.
- Stop the night setback. Reheating a cooled house demands a higher flow temperature at exactly the coldest hour of the day.
- Move the anti-legionella cycle to weekly and overnight rather than daily.
- Check the immersion heater is not silently enabled as a backup — it runs at COP 1.0 and hides in the bill.
- Log heat output and electricity separately. Without a heat meter you are guessing, and guessing is how a solvable 15% overrun turns into a replaced heat pump.
A well-designed retrofit on radiators should land between SPF 3.2 and 3.6; underfloor systems regularly exceed 3.8. Anything below 2.5 is a design or commissioning fault, not a property of heat pumps.
Frequently asked questions
Is an SPF of 2.5 bad?
For a modern inverter heat pump, yes. An SPF of 2.5 usually means the design flow temperature is too high, the unit is cycling, or a backup heater is contributing. Well-executed radiator retrofits reach 3.2–3.6 and underfloor systems commonly exceed 3.8.
Why is my hot water so expensive to heat?
Domestic hot water needs a much higher water temperature than space heating, so its COP is typically 2.0–2.5 against 3.5 or better for heating. Charging the cylinder to 48–52 °C through a large coil, instead of 60 °C every day, is the single biggest hot water saving available.
Does a heat pump cost much more during a cold snap?
Yes, and this is normal. Output and COP both fall as outdoor air gets colder while the building simultaneously needs more heat, so weekly consumption can double or triple. It is the seasonal average that matters, not the worst week.
Should I switch the heat pump off when I go out?
No. Heat pumps are most efficient running steadily at a low flow temperature. Switching off and reheating a cold house forces a higher flow temperature and more compressor starts, which costs more than the standing heat you saved.