What does your heat-pump electricity result mean?
The calculator separates space heating, domestic hot water and extra auxiliary electricity so you can see which part drives annual electricity use and running cost.
Change an input above and this interpretation updates automatically.
Continue your calculation
Heat-pump electricity is only one part of the heating decision. Use the same useful-heat assumptions when you compare system cost, hot water and low-temperature emitters.
How is annual heat-pump electricity use calculated?
The basic annual model is simple: divide the useful heat that must be delivered by a seasonal performance factor. Space heating and domestic hot water are kept separate because they can operate at different temperature levels and therefore at different performance.
Annual cost is then total electricity multiplied by your electricity price, plus any fixed annual charge you deliberately include. This is an operating-energy estimate. It does not calculate heat-pump size, design heat loss, installation cost or payback.
SCOP for space heating
SCOP is a seasonal ratio: useful space-heating output divided by electricity used over a defined heating season. A SCOP of 4 means the simplified annual model delivers 4 kWh of useful space heat for each 1 kWh of electricity assigned to that space-heating component.
A product-sheet SCOP is useful for a scenario, but real system performance can differ with climate, flow temperature, emitter sizing, controls, defrosting, cycling and installation quality.
Why hot water gets a separate factor
Domestic hot water often requires a higher water temperature than space heating. That can change heat-pump performance, so using one annual space-heating SCOP for both tasks may hide an important difference.
Use a hot-water performance value that matches your own evidence or scenario. If you only have a single-point COP, treat the annual result cautiously because one operating point is not automatically a seasonal average.
Where can you get the input data?
| Input | Useful source | Check before entering it |
|---|---|---|
| Useful space-heating demand | Energy assessment, design calculation or a consistent annual heat-demand model. | It must be energy in kWh/year, not design heat load in kW and not automatically the purchased fuel from a bill. |
| SCOP | Manufacturer product data, an approved design calculation or monitored seasonal performance. | Check climate and operating-temperature assumptions. A catalogue SCOP is not a guarantee of the installed system result. |
| Useful hot-water heat | Hot-water calculation based on water volume, temperature rise and use, or another justified annual estimate. | Enter useful heat delivered to hot water, not electricity already consumed by the heat pump. |
| Hot-water performance factor | Manufacturer information, monitoring or a design assumption suitable for DHW operation. | Do not automatically reuse the space-heating SCOP. |
| Auxiliary / backup electricity | Metering, controller data or a separate estimate for loads not already represented. | Avoid double counting pumps, controls or backup heaters if your seasonal performance figure already includes them. |
| Electricity price | Your tariff, recent bill or a scenario price. | Use a price per kWh. For time-of-use tariffs, use a justified average if you want a single annual estimate. |
Important distinction: kW, kWh and SCOP are not interchangeable
kW is power at a moment in time, such as the design heat load or heat-pump output. kWh is energy over time, such as annual heating demand or electricity consumption. SCOP is a dimensionless seasonal ratio between useful heat and electricity. This calculator needs annual energy in kWh, not heat-pump capacity in kW.
Worked example: space heating, hot water and auxiliaries
Assume a home needs 10,000 kWh/year of useful space heat and 2,500 kWh/year of useful hot-water heat. Space-heating SCOP is 3.5, the hot-water performance factor is 2.7, auxiliary electricity is 200 kWh/year and electricity costs €0.30/kWh.
- Space-heating electricity: 10,000 ÷ 3.5 = 2,857 kWh/year.
- Hot-water electricity: 2,500 ÷ 2.7 = 926 kWh/year.
- Add 200 kWh of extra auxiliary electricity: 2,857 + 926 + 200 = 3,983 kWh/year.
- Variable annual electricity cost: 3,983 × €0.30 = about €1,194.92/year.
The 12,500 kWh of useful heat divided by 3,983 kWh of total electricity gives an overall heat/electricity ratio of about 3.14. This derived ratio includes the entered auxiliaries and should not be confused with the standardized space-heating SCOP.
What affects heat-pump electricity cost the most?
| Factor | Effect on the result |
|---|---|
| Useful heat demand | More required heat increases electricity use approximately in direct proportion when performance factors stay unchanged. |
| SCOP | A higher seasonal space-heating factor reduces the electricity needed for the same useful space heat. |
| Hot-water demand and factor | High DHW use or a lower DHW performance factor can make hot water a significant annual electricity component. |
| Auxiliary / backup electricity | Every extra kWh entered is added directly to total electricity. Backup resistance heating can therefore matter even if the compressor performs well. |
| Electricity tariff | The same kWh consumption can produce very different running costs at different tariffs. |
| Operating temperatures and controls | They are not direct inputs here, but they can change the real SCOP or seasonal factor that belongs in the calculation. |
Compare two SCOP scenarios fairly
To see what SCOP changes by itself, keep the building heat demand, hot-water assumptions, auxiliary electricity and tariff fixed. With the default example, changing only space-heating SCOP from 3.0 to 4.0 reduces space-heating electricity from about 3,333 to 2,500 kWh/year.
With 2,500 kWh/year of useful DHW at a factor of 2.7, 200 kWh/year of auxiliaries and €0.30/kWh electricity, total annual electricity falls from about 4,459 to 3,626 kWh. The variable annual cost falls from about €1,337.78 to €1,087.78: a difference of €250/year in this scenario.
The total does not improve in exactly the same proportion as the space-heating SCOP because hot-water electricity and auxiliaries were deliberately left unchanged.
Does your result look realistic?
There is no single “correct” annual kWh figure for every home. A useful sanity check is to look for input mismatches before judging the total. If the result seems implausibly high or low, check these points first:
- Did you enter annual heat energy in kWh/year, rather than heat-pump capacity or design heat load in kW?
- Does your SCOP apply to a similar climate and water-temperature regime?
- Did you accidentally use purchased electricity as useful heat and then divide by SCOP again?
- Are hot-water demand and hot-water performance separated rather than copied from space heating without justification?
- Is auxiliary or backup electricity already included in the seasonal performance value or measured annual electricity?
- Is the tariff really a price per kWh, and does it represent the periods when the heat pump is likely to run?
When monitored electricity is available for a full representative year, compare the model with that measurement and investigate large differences instead of forcing the assumptions to match a desired answer.
Low flow temperatures can matter because they affect real performance
This calculator does not ask for flow temperature directly. Instead, its effect is represented indirectly through the SCOP or seasonal factor you enter. A heat pump generally has less temperature lift to provide when the heating system can run at lower water temperatures, so emitter design and controls are important to the real seasonal result.
Use the Underfloor Heating vs Radiator Calculator to compare required output at different emitter conditions, and the underfloor heating or radiators guide for the wider design trade-offs. For radiators at lower temperatures, the radiator output correction-factor table helps translate catalogue output to a different ΔT.
What should you do with the result?
- Compare tariffs: keep the kWh result fixed and change only the electricity price to see the cost sensitivity.
- Compare performance: keep useful heat fixed and test a justified lower or higher SCOP rather than changing several assumptions at once.
- Separate hot water: if DHW is a large share, review the hot-water demand and its performance factor independently.
- Check auxiliaries: if extra pumps or backup heat dominate the result, verify whether they are actually additional or already included elsewhere.
- Use the result in a wider heating comparison: compare the same useful heat demand with the Heating Cost Calculator for a conventional-efficiency scenario.
For equipment sizing or detailed system design, use a design heat-loss calculation and manufacturer performance data at the required operating conditions. Annual kWh alone does not size a heat pump.
For building-services and HVAC students: from useful heat to electricity
At school or during technical training, treat heat demand and heat-pump electricity as different energy flows. The building receives useful heat. The heat pump consumes electricity to move that heat. SCOP or another seasonal performance factor connects the two over a longer period.
Exercise: A building needs 12,000 kWh/year of useful space heat and 2,400 kWh/year of useful domestic-hot-water heat. The space-heating SCOP is 4.0, the hot-water performance factor is 2.5, extra auxiliary electricity is 150 kWh/year and electricity costs €0.25/kWh. Calculate space-heating electricity, DHW electricity, total electricity and annual variable cost.
Common heat-pump electricity calculation mistakes
- Entering design heat load in kW where the calculator expects annual useful heat in kWh/year.
- Using one laboratory COP as though it were guaranteed seasonal performance.
- Using the same factor for domestic hot water and space heating without checking the operating conditions.
- Counting pumps, controls, immersion backup or other auxiliary loads twice.
- Changing the building heat demand while trying to compare two heat pumps for the same building.
- Treating the derived overall heat/electricity ratio as the standardized SCOP from a product label.
- Assuming annual cost divided by 12 predicts every monthly bill; heating demand and tariff prices can both vary through the year.
- Assuming this operating-cost estimate includes installation, maintenance, financing or replacement costs.
Practical tip
When comparing scenarios, change one major assumption at a time. Keeping useful heat demand constant while changing SCOP, DHW performance or tariff makes it much easier to understand why the annual electricity or cost changed.