Air source heat pump running costs are one of the most searched questions in UK heating — and one of the most confusingly answered. This guide cuts through the vague estimates with a step-by-step calculator approach: real maths, real tariff figures, and real-world data from the BEIS Heat Pump Ready programme so you can work out what a heat pump will actually cost to run in your home. We’ll use a worked example of a 3-bedroom semi-detached home fitted with an 8kW Vaillant Arotherm+ at a measured SCOP of 3.5, running on the Octopus Cosy tariff.
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The basic calculation is straightforward once you know the three inputs:
The formula is: Annual cost = (Heat demand ÷ SCOP) × tariff rate
So if your 3-bed semi needs 14,000 kWh of heat per year, your heat pump runs at SCOP 3.5, and your electricity costs 18p/kWh (blended Cosy Octopus rate — more on this below):
(14,000 ÷ 3.5) × £0.18 = 4,000 kWh × £0.18 = £720 per year for space heating.
Add roughly 1,500 kWh/year for domestic hot water at the same efficiency: another £77. Total: approximately £797 per year for heating and hot water in a well-insulated 3-bed semi.
SCOP — Seasonal Coefficient of Performance — is the number that determines your annual running cost more than any other factor. It measures how many kWh of heat you get for every kWh of electricity your heat pump consumes, averaged across a full heating season including cold snaps when efficiency drops.
The Vaillant Arotherm+ 8kW achieves an independently measured SCOP of 4.2 at A7/W35 (7°C ambient air, 35°C flow temperature) under EU testing conditions. In UK real-world use, the BEIS Heat Pump Ready trial found typical installed SCOP values of 2.9–3.8 for well-installed systems — with the spread explained mainly by flow temperature and home insulation quality.
For our worked example we’ll use SCOP 3.5, which is realistic for a 3-bed semi that’s had some loft and cavity wall insulation, with radiators slightly oversized for low-temperature running. Our full guide to SCOP and COP explains how to interpret the manufacturer’s published figures and what to expect in practice.
The Energy Saving Trust estimates typical heat demand for UK home types as follows:
These figures include space heating only. Domestic hot water typically adds 2,000–3,500 kWh depending on household size. For our 3-bed semi with 3 occupants, we’ll add 2,500 kWh for hot water — giving a total heat demand of 16,500 kWh.
At SCOP 3.5 for space heating and COP 2.5 for hot water (heat pumps are less efficient at producing 55°C hot water than 45°C radiator flow temperatures):
Octopus Cosy is a time-of-use tariff designed specifically for heat pump households. It offers very cheap electricity during off-peak periods — particularly 4–7am and 1–4pm — when your heat pump should do the bulk of its work. At the time of writing, the tariff structure is approximately:
A well-programmed heat pump on Cosy Octopus will run most of its compressor hours in the cheap windows, achieving a blended rate of approximately 16–18p/kWh on your heating electricity. We’ll use 17p/kWh for our example.
5,000 kWh × £0.17 = £850 per year for all heating and hot water.
The table below shows estimated annual running costs for different home sizes and electricity tariffs. All figures assume a well-installed air source heat pump with SCOP 3.2 (average), including space heating and hot water. Savings vary depending on property size, insulation, and energy tariff.
| Home type | Heat demand (kWh) | Electricity used (÷3.2) | Standard tariff (25p) | Octopus Cosy (17p) | E.ON Next Drive (21p) |
|---|---|---|---|---|---|
| 1-bed flat | 7,000 | 2,188 | £547 | £372 | £460 |
| 2-bed house | 10,500 | 3,281 | £820 | £558 | £689 |
| 3-bed semi | 16,500 | 5,156 | £1,289 | £876 | £1,083 |
| 4-bed detached | 22,000 | 6,875 | £1,719 | £1,169 | £1,444 |
The Cosy tariff makes a significant difference — roughly 30% off compared to a standard price-cap rate for a heat pump household that uses it well. That’s why choosing the right tariff is arguably as important as which heat pump you install.
The BEIS Heat Pump Ready programme monitored real installations across the UK over a full heating season. Key findings relevant to running costs:
Translating that to costs: the difference between a bottom-quartile installation (SCOP 2.2) and a top-quartile one (SCOP 3.8) on a 3-bed semi is roughly £600–£700 per year at current tariff rates. That gap is almost entirely down to installation quality, commissioning, and the home’s insulation — not the heat pump brand.
This comparison uses the same 3-bed semi with 16,500 kWh annual heat demand. Gas at Ofgem’s current price cap rate of approximately 6p/kWh, with a modern gas boiler at 90% efficiency:
| Heating system | Fuel cost per kWh of heat | Annual fuel cost (16,500 kWh heat) |
|---|---|---|
| Gas boiler (90% efficient, 6p/kWh gas) | 6.7p | £1,097 |
| Heat pump SCOP 2.2 (poor), standard tariff 25p | 11.4p | £1,875 |
| Heat pump SCOP 2.8 (average), standard tariff 25p | 8.9p | £1,473 |
| Heat pump SCOP 3.2 (good), standard tariff 25p | 7.8p | £1,289 |
| Heat pump SCOP 3.2 (good), Octopus Cosy 17p | 5.3p | £876 |
| Heat pump SCOP 3.5 (excellent), Octopus Cosy 17p | 4.9p | £800 |
The pattern is clear: on a standard electricity tariff, a heat pump only beats gas on running costs if it achieves a SCOP above roughly 3.5. On Octopus Cosy or a similar heat pump tariff, even an average-performing system at SCOP 2.8 starts to become competitive — and a good installation at SCOP 3.2+ is noticeably cheaper than gas.
This is why we always recommend that anyone getting a heat pump also switches to a time-of-use tariff. See our full heat pump running costs guide for a deeper comparison including oil, LPG, and electric boilers.
Octopus Cosy is specifically designed for heat pump households and it shows. The cheap periods — 4–7am and 1–4pm — are the windows when a smart heat pump controller (Vaillant’s sensoAPP, Mitsubishi’s MELCloud, or a Homely controller) can schedule the bulk of heating demand. The peak period (4–7pm) is when you should avoid running the compressor wherever possible — your stored heat in a well-insulated home will carry you through.
The practical implication: to get full value from Cosy, your heat pump needs either a smart thermostat with scheduling capability or a controller that responds to time-of-use tariff signals. Basic on/off thermostats won’t work well — the system needs to “pre-heat” during cheap periods.
If you’re interested in smart controls that work with heat pumps, SmartHomeWise has a detailed guide to compatible smart thermostats including which ones integrate with Octopus tariff APIs.
The best savings come from a combination of these actions. Not all apply to every home, but work through the list and estimate which are realistic for you:
The single best thing you can do for your running costs happens before the heat pump is switched on: have an honest conversation with your installer about the target flow temperature and SCOP. Ask them to:
These aren’t unusual requests — any competent MCS-certified installer will be familiar with all of them. If an installer dismisses questions about flow temperature or doesn’t do a heat loss calculation, that’s a red flag worth taking seriously.
To estimate your own heat pump running costs, you need three numbers:
Then: (Gas kWh × boiler efficiency 0.9) ÷ SCOP × electricity rate = your estimated heat pump annual cost.
Remember that most heat pump quotes will include a Boiler Upgrade Scheme grant of £7,500 for air source heat pumps, which significantly changes the installation cost comparison with a new gas boiler. The running cost advantage, once you’re on the right tariff and your system is performing well, then adds up year on year.
For a broader comparison of heat pump running costs against other fuel types, see our main running costs guide. And if you’re wondering what SCOP figures to expect from specific models, our SCOP and COP explainer has model-by-model data.
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