Summary
This is one of the most common questions tradespeople get asked on-site, and it's also one of the easiest to answer badly. The internet is full of confident claims in both directions — "heat pumps are always cheaper to run" and "heat pumps cost a fortune, don't bother" — and both are wrong as blanket statements. The real answer depends on four numbers: the price of a unit of gas, the price of a unit of electricity, the boiler's efficiency, and the heat pump's coefficient of performance (CoP) in that specific installation. Get those four numbers for the customer's actual tariff and the actual system design, and the maths does itself.
The reason this matters for quoting: customers increasingly ask installers to justify a heat pump quote against "just replacing the boiler," and if you can't walk them through the running-cost comparison with real numbers, you lose the sale to whoever can — or you oversell savings that don't materialise and get the blame a year later when their bills don't drop as promised.
The core physics: a modern condensing gas boiler converts roughly 90-94% of the gas energy into heat (efficiency), so 1 kWh of gas input gets you about 0.9 kWh of heat. A heat pump doesn't convert electricity to heat directly — it moves heat from outside air into the building using a refrigeration cycle, and for every 1 kWh of electricity it consumes, it typically delivers 2.5-4.0 kWh of heat (this ratio is the CoP or seasonal average, SCOP). Because electricity costs roughly 3.5-4x more per kWh than gas on typical UK tariffs, the heat pump's efficiency advantage needs to be roughly in that same range just to break even on running cost, and needs to exceed it to show a genuine saving.
Key Facts
- Gas boiler efficiency — modern condensing boilers: 88-94% SEDBUK efficiency rating; older non-condensing boilers: 70-78%
- Heat pump CoP (Coefficient of Performance) — instantaneous ratio of heat output to electrical input; typically 2.5-5.0 depending on outdoor temperature, flow temperature, and unit
- SCOP (Seasonal Coefficient of Performance) — the annualised average CoP across a full heating season, accounting for varying outdoor temperatures — this is the number that matters for running cost calculations, not the peak/marketing CoP figure
- Typical ASHP SCOP range — 2.8-4.0 for a well-designed, correctly-sized UK domestic installation at 35-45°C flow temperature; poorly designed or oversized systems commonly underperform this
- Flow temperature impact — CoP drops significantly as required flow temperature rises; a heat pump running at 55°C flow (to suit unmodified small radiators) delivers a materially lower CoP than the same system running at 35°C flow with upsized radiators or UFH — this is the single biggest factor in real-world heat pump running cost disappointment
- Gas price (Ofgem price cap, indicative) — historically around 6-7p/kWh under the price cap, though this fluctuates with wholesale energy markets —
- Electricity price (Ofgem price cap, indicative) — historically around 24-28p/kWh under the price cap on standard tariffs —
- Heat pump-specific tariffs — several suppliers offer discounted off-peak or flat-rate tariffs specifically for heat pump customers (e.g. time-of-use tariffs), which can improve the running-cost comparison materially versus a standard variable tariff — always factor this into the customer conversation
- Boiler Upgrade Scheme (BUS) grant — £7,500 towards ASHP or GSHP installation in England and Wales, paid to the MCS-certified installer, not the homeowner, reducing net capital cost — scheme funded to 2028 at time of writing
- Break-even CoP formula — Heat pump running cost beats gas boiler running cost when: SCOP > (electricity price per kWh ÷ gas price per kWh) × boiler efficiency
- Insulation standard impact — poorly insulated properties need higher flow temperatures or run heating for longer, both of which depress the heat pump's effective CoP; a heat pump is not a substitute for basic fabric improvements (loft, cavity/wall insulation) and should ideally follow, not precede, a fabric-first upgrade
- Radiator sizing — heat pumps run efficiently at lower flow temperatures (typically 35-45°C design) than a gas boiler system designed around 70-80°C flow — existing radiators are almost always undersized for a heat pump at these lower temperatures and need upsizing (typically 2-3x surface area) or supplementing, which is a real cost most "like-for-like" running cost comparisons omit
- MCS 3005 sizing methodology — the standard heat pump sizing calculation method requiring room-by-room heat loss assessment (BS EN 12831), not a rule-of-thumb kW-per-m² estimate — under-sizing or over-sizing both damage real-world running costs and comfort
- Standing charge differences — electricity standing charges are typically higher than gas standing charges on UK tariffs, which affects total annual bill comparisons even before usage is considered —
- Hot water performance — heat pumps typically produce hot water less efficiently (lower CoP, around 2.0-2.5) than space heating, because higher temperatures are needed for legionella-safe cylinder storage (60°C+) — factor this into the annual running cost estimate separately from space heating
Quick Reference Table
Quoting a heating job? squote turns a 2-minute voice recording into a professional quote.
Try squote free →| Scenario | Gas Boiler (94% eff.) | ASHP (SCOP 3.0) | ASHP (SCOP 3.8, low-temp design) |
|---|---|---|---|
| Heat delivered per 1 kWh fuel/electricity | 0.94 kWh | 3.0 kWh | 3.8 kWh |
| Cost per kWh heat @ gas 6.5p / elec 26p | ~6.9p | ~8.7p | ~6.8p |
| Cost per kWh heat @ gas 7p / elec 22p (HP tariff) | ~7.4p | ~7.3p | ~5.8p |
| Requires radiator upsizing? | No | Usually yes | Usually yes |
| Capital cost (indicative, gross) | £2,500-£4,500 | £10,000-£14,500 | £10,000-£14,500 |
| Net cost after BUS grant | N/A | £2,500-£7,000 | £2,500-£7,000 |
| Typical annual heating bill impact vs like-for-like boiler | Baseline | Roughly comparable to slightly higher | Typically lower |
Figures are indicative and vary by property, tariff, and design —.
Detailed Guidance
Why "heat pumps are always cheaper" is misleading
The claim usually comes from comparisons done at a fixed CoP (often the manufacturer's peak-condition CoP, tested at a mild outdoor temperature and low flow temperature) against a national average gas price, ignoring that most existing radiator systems weren't designed for heat pump flow temperatures. A heat pump retrofitted onto an unmodified radiator system running at 55-60°C flow to compensate for undersized radiators can see its effective CoP drop to 2.0-2.5 — at that point, on a standard variable tariff, running costs are frequently higher than the gas boiler it replaced. The saving claim is only true when the whole system — insulation, flow temperature, radiator sizing, and tariff — is designed properly around the heat pump, per the MCS 3005/Heat Emitter Guide methodology. See heat pump sizing and heat pump radiator correction for the sizing detail.
Why "heat pumps always cost more to run" is also misleading
This claim usually comes from real-world cases where a heat pump was installed without proper heat loss calculation or radiator upsizing — a genuine and common failure mode, not evidence the technology is inherently worse. On a correctly designed system, particularly one on a heat-pump-specific tariff, running costs are frequently at or below gas boiler running costs. The distinction that matters for the customer conversation is "was this heat pump correctly designed" not "are heat pumps good or bad."
The conversation structure that works on-site
- Establish current gas and electricity unit rates from the customer's actual bill — never assume the national price cap average applies to their tariff.
- Get or estimate the property's heat loss (a proper MCS 3005 heat loss survey is the accurate way; a rough W/m² estimate is a starting point only — see heat loss).
- Establish whether existing radiators/UFH can run at 45°C or below without upsizing, or what upsizing is needed — see boiler to heat pump migration for a full swap-over checklist.
- Apply the break-even formula (SCOP vs price ratio × boiler efficiency) using the customer's real tariff numbers.
- Present capital cost separately, including BUS grant eligibility — see bus grant guide — and be clear that even where running costs are roughly comparable, the capital cost difference (even after grant) is a real factor in the decision, alongside carbon reduction goals that matter to some customers regardless of running cost parity.
Ground source heat pumps
Ground source heat pumps (GSHP) typically achieve higher CoP (3.5-5.0) than air source because ground temperature is more stable than air temperature through winter, but the higher installation cost (borehole or horizontal loop) usually makes ASHP the default recommendation for standard domestic retrofits unless plot size and budget support a GSHP. See ground source heat pumps for loop sizing and cost detail.
Don't skip the fabric-first conversation
A heat pump installed into a poorly insulated Victorian solid-wall property with no loft insulation will need a higher flow temperature and run for longer hours than the same heat pump in a well-insulated modern home, materially worsening its effective running cost. Where budget allows, recommending loft insulation, cavity/wall insulation, and draught-proofing ahead of or alongside a heat pump installation is not upselling — it's what makes the heat pump numbers actually work. See loft insulation and cavity wall.
Frequently Asked Questions
What SCOP figure should I use when quoting a customer?
Use the SCOP figure from the specific unit's MCS product listing or manufacturer test data for the design flow temperature you're actually specifying — never use a generic "heat pumps achieve CoP 4" marketing figure. A system designed for 55°C flow will have a materially lower real-world SCOP than the same unit designed for 35°C flow, even though it's the identical piece of kit.
Should I recommend a heat pump-specific electricity tariff?
Where the customer is getting a heat pump installed, checking eligibility for a heat pump tariff (several suppliers offer these, often requiring a smart meter and sometimes a specific brand/monitoring integration) is worth doing before finalising the running cost comparison — the difference in unit rate can materially change the outcome. This is a genuine part of the advice, not just a nice-to-have.
Does the Boiler Upgrade Scheme grant affect running costs?
No — the £7,500 BUS grant reduces capital/installation cost only, applied by the MCS-certified installer at point of sale; it has no effect on the ongoing electricity cost of running the heat pump. Keep the capital-cost and running-cost conversations distinct so the customer understands both halves of the value proposition.
Is it ever right to recommend a straight gas boiler replacement over a heat pump in 2026?
Yes, in plenty of cases — properties with poor fabric that can't be upgraded within budget, very high existing flow temperature radiator systems where upsizing isn't practical (e.g. no wall space, listed building restrictions), or households on a standard tariff where the running-cost sums genuinely favour a boiler. Recommending a heat pump regardless of suitability, purely because it's the "future," is bad advice and damages trust when the customer's bills don't improve.
What about hybrid systems (heat pump + boiler)?
Hybrid systems can make sense where fabric or radiator constraints make a full heat pump conversion impractical or expensive — the heat pump handles baseline heating at high efficiency and the boiler covers peak demand or hot water. Running cost calculations for hybrids require modelling the split between the two systems across the heating season, which is more involved than the direct swap comparison above — treat this as a specialist design exercise, not a rule of thumb.
Regulations & Standards
MCS 3005 — Heat pump sizing methodology and heat loss calculation standard required for MCS-certified installations and BUS grant eligibility
BS EN 12831 — Method for calculating the design heat loss of a building, the basis for correct heat pump and boiler sizing
MCS Heat Emitter Guide — Industry-standard methodology for assessing and upsizing radiators for lower flow temperature heat pump operation
Boiler Upgrade Scheme (BUS) — Government grant scheme administered by Ofgem, requiring MCS installer certification and a valid EPC
Building Regulations Approved Document L — Sets efficiency and control requirements for both boiler and heat pump installations
F-Gas Regulations (UK) — Govern refrigerant handling and certification requirements for heat pump installation and servicing engineers
Boiler Upgrade Scheme guidance — UK Government
MCS Standards and installer search — Microgeneration Certification Scheme
Ofgem energy price cap — Ofgem (check current rates before quoting)
Energy Saving Trust heat pump guidance — Energy Saving Trust
heat pump sizing — MCS 3005 sizing methodology, design flow temperature selection, typical UK domestic load ranges
bus grant guide — Boiler Upgrade Scheme eligibility, grant amount, and application process
boiler to heat pump migration — full conversion checklist: heat loss calc, radiator de-rating, pipework, cylinder requirements
heat loss — heat loss calculator with worked examples for boiler and heat pump sizing