Heat Pump Running Cost Calculator

How the calculation works

Heat Pump Running Costs: Are They Worth It?

Heat pumps are often presented as the obvious replacement for electric heaters and older boilers, but the answer depends on the building, the equipment and the electricity tariff. This calculator estimates the yearly electricity use of a heat pump from its heating capacity, its SCOP and the pattern of use. It then multiplies that figure by the price of a kilowatt-hour to give the annual running cost. It also shows the electricity that electric resistance heating would need to deliver the same heat, so the difference can be seen directly.

How the Calculation Works

The heat delivered over a heating season is the product of three quantities: the heating capacity in kW (C), the hours of operation per day (h) and the number of heating days per year (d). Dividing that heat output by the SCOP gives the electricity the heat pump needs, because the SCOP expresses how many units of heat are delivered for each unit of electricity over a full season. Multiplying the electricity figure by the price per kWh (p) gives the running cost.

A worked example makes the logic clear. Take a 5 kW unit running 8 hours a day for 140 heating days, with a SCOP of 3.5. The heat output is 5 × 8 × 140, which equals 5,600 kWh. Dividing by 3.5 gives 1,600 kWh of electricity per year. An electric resistance heater of the same capacity has an efficiency of 1, so it would use the full 5,600 kWh. The difference, 4,000 kWh a year, is the electricity saved. The annual running cost then follows from the unit rate on your electricity bill.

The calculation treats capacity as if the unit ran at its full nominal output for every hour entered. This is a simplification. A modern inverter heat pump varies its output to match demand, so the real electricity use may be higher or lower than the estimate, depending on how the home is heated and how the controls are set. The estimate is most useful for comparing options on the same basis, rather than for predicting the bill to the penny.

Why SCOP Matters More Than Headline Capacity

Capacity tells you how much heat the unit can deliver at its maximum output. It does not show how efficiently the unit runs across a heating season. The coefficient of performance (COP) measures the ratio of heat delivered to electricity consumed at one set of conditions. Because the outdoor temperature changes the COP, a single test point can flatter or understate a unit. For this reason, manufacturers and the European standard BS EN 14825 use the seasonal figure, the SCOP, which averages performance across the season.

The COP tends to fall as the outdoor temperature drops, so a unit tested in mild conditions shows a higher figure than its typical winter performance. Flow temperature matters too. A system that needs water at high temperatures for radiators reduces the efficiency of an air-to-water unit, compared with underfloor heating, which runs at lower temperatures. Check the efficiency at the flow temperature your system actually needs, not only at the headline figure.

A higher SCOP means less electricity for the same heat. In the example above, a SCOP of 4.0 instead of 3.0 reduces the electricity needed by a quarter, because the same heat then requires three quarters of the input. The model choice therefore affects the running cost more than many buyers expect.

Types of Heat Pump Used in the UK

Air-to-air split systems move heat between an outdoor unit and one or more indoor units, heating rooms directly without a water circuit. They are often the least complicated to install, which makes them common in flats and smaller houses. They heat individual rooms rather than radiators, and they do not provide hot water for taps or a cylinder. Multi-split systems, in which one outdoor unit serves several indoor units, suit homes with several rooms that need heating.

Air-to-water units heat water for radiators or underfloor heating and can also supply domestic hot water through a cylinder. They suit homes that already have a wet central heating system, including homes replacing a gas boiler. Installation involves more pipework and work on the existing hydraulic system, so the cost depends on the condition of the radiators, the pipework and the cylinder. Seasonal efficiency can be a little lower than an air-to-air system in similar conditions, but heat is distributed more evenly through the building.

Ground-source systems draw heat from the ground through buried loops or boreholes. Ground temperatures change less across the year than air temperatures, which is why these systems can achieve high seasonal efficiency. Installation costs are the highest of the three types, because the ground loop requires excavation or drilling and suitable outdoor space. Ground-source systems are usually justified for larger homes with high heating demand, or where grant support helps with the cost.

Common Mistakes When Sizing and Buying

The most common mistake is buying a unit that is too small to save money. An undersized heat pump runs near full output for long periods during cold spells and may not reach the set temperature. Its real seasonal efficiency then falls below the declared SCOP, and electricity use rises. The choice of capacity should follow the heat loss of the building, which depends on insulation, glazing, air leakage and ventilation as well as the floor area. Ceiling height matters too: a room with a 2.4 m (about 8 ft) ceiling has less air to heat than a room of the same floor area with a much taller ceiling.

The second mistake is judging a unit by its energy label band alone. Energy labels group units into bands, and the difference between adjacent bands can be meaningful over a unit's working life. Read the SCOP on the technical data sheet and compare like with like.

The third mistake is installing a heat pump in a building with high heat loss and expecting low running costs. Draught-proofing, loft and wall insulation and better glazing usually reduce the capacity needed, which lowers both the purchase price and the running cost. An installer who carries out a heat-loss calculation will normally identify these improvements.

The fourth mistake is ignoring defrosting. In cold, damp weather, an air source unit periodically defrosts its outdoor coil. The defrost cycle uses energy and interrupts heating, so real seasonal performance is lower in humid cold than in dry cold at the same temperature.

When to Use This Calculator, and When Not To

This calculator is suited to comparing options, budgeting for running costs and seeing how the SCOP, the hours of use and the electricity price change the result. It is also useful for a rough comparison with electric resistance heating.

It is not suited to sizing equipment. It does not model the heat loss of a building, which is the basis for choosing capacity. It does not model domestic hot water, which can add a significant share of the energy use in a home. It cannot compare a heat pump with a gas boiler, because that comparison needs the gas unit rate and the boiler's efficiency, and neither is included. It also does not cover commercial premises, buildings with unusual construction, or systems that rely on a backup heater for much of the season. Use it as a starting point, then check the result with a qualified installer.

Regulations and When to Call a Qualified Installer

In England and Wales, installing a heat pump is subject to the Building Regulations, and the work must comply with the requirements in Approved Document Part L, which covers the conservation of fuel and power. In Scotland, the equivalent framework is the Building Standards. Handling the refrigerant gases used in heat pumps is regulated under the UK F-gas rules, so the refrigerant work should be carried out by a certified engineer. Look for installers certified under the Microgeneration Certification Scheme (MCS), which is generally required for grant-funded installations.

The design capacity should come from a heat-loss calculation to BS EN 12831. This calculation estimates the heat the building loses on a design winter day and sets the capacity needed to maintain the target temperature. Many installers include it in their quotation, so ask for it in writing before you commit.

An energy performance certificate (EPC), prepared by an accredited energy assessor, is a useful starting point for understanding a building's energy use. Consider a formal assessment before choosing equipment if the required capacity is large, or if the building is old and has not had insulation work. The assessment identifies the main heat losses and helps to size the system on the building's actual needs, rather than on rough estimates of floor area.

Limits of the Estimate

The calculator is deliberately simple. It assumes a constant SCOP across the season, a fixed number of hours and days, and a single price per kWh. Real tariffs can include standing charges, time-of-use rates and price caps, and these change over time. Ofgem regulates the energy price cap, but the unit rate on your own bill is the most reliable figure for the price input. Standing charges are not part of the calculation, so the total bill will be higher than the calculated running cost for the heat pump alone. The result should be read as an order of magnitude for planning, not as a forecast.