Heat pump size calculator: an indicative kW figure for your house, with the assumptions visible
Updated: 2026-10-23 · ~6 min read
- Enter the heated area, the building standard and your design outdoor temperature: you get the peak heat demand in kW, the capacity to look for in a catalogue, the yearly kWh and an indicative electricity bill.
- The result is a range, not a number: between „new build” and „old house, partly renovated” the demand differs by a factor of two to three at the same floor area.
- The calculator also prices oversizing: each unnecessary kW costs roughly €150–400 at purchase and shortens compressor life through shorter on/off cycles.
- With 55 °C radiators, read the catalogue figure at A−7/W55; with underfloor heating at A7/W35 — the same heat pump has two different numbers there.
Calculator
— kW — peak heat demand (heating) at the selected design temperature
Range —. Heat the pump itself must deliver: — kW; the rest is covered by the second source (— kW).
Per year: — kWh of heat → — kWh of electricity → — per year at the COP and price entered.
What the calculator actually computes
The method is the one installers use for a first estimate: area × unit demand, corrected for the design temperature. Values used:
| Building standard | Peak power (W/m²) | Annual energy (kWh/m²·year) |
|---|---|---|
| new / well insulated | 40–60 (calculator: 50) | ≤60 (calculator: 55) |
| partly renovated | 70–100 (calculator: 90) | 80–110 (calculator: 95) |
| old, poorly insulated | 120–180 (calculator: 150) | 150–200 (calculator: 180) |
The design temperature enters as a temperature-difference ratio: heat loss is roughly proportional to the gap between 20 °C inside and the frost outside. The calculator uses −18 °C as its base, so choosing −24 °C raises the result by about 16%, and −16 °C lowers it by about 5%.
Annual kWh of electricity = heat ÷ COP. A COP of 3.0 means 1 kWh of electricity becomes 3 kWh of heat. This is the number that decides the bill — which is why with 60 °C radiators (COP about 2.2) the same heat pump costs roughly 35% more to run than with underfloor heating (COP about 3.0).
How to read the result
- The „peak” figure is what the house needs on the coldest day — not the nameplate rating of a device. The unit is selected slightly above it, with a margin for defrosting the outdoor unit.
- Catalogue condition: for 55 °C radiators look for the capacity at A−7/W55; for underfloor at A7/W35. The gap between those two conditions can be 30–40% for the same machine.
- Bivalent point: if you choose a 70% share, the missing kW is covered by the electric element (typically 3–9 kW inside the heat pump) or by a second source. The calculator prints how much must be covered — if it exceeds 9 kW, the built-in element alone will not be enough.
- Oversizing: at roughly €150–400 per unnecessary kW, sizing the demand in advance produces a real saving. Add the cost of short start/stop cycles — compressor wear and more frequent defrost.
This is a tool for the conversation with an installer, not a design. It ignores thermal bridges, mechanical ventilation with heat recovery (which lowers demand by 10–25%), room height, glazing area and how airtight the building really is. When you order a quote, ask for a heat-loss calculation to EN 12831 — an offer without one is made „from the catalogue”.
What to do with the result
- Write down three numbers: the peak in kW, the capacity to look for in a catalogue, and the annual heat in kWh.
- Cross-check against the history of your current source: annual fuel use × calorific value × boiler efficiency gives an independent control (described in the sizing method).
- Check whether lowering the flow temperature pays off — see underfloor vs radiators and how to read COP, SCOP and SEER.
- In an old, poorly insulated house, plan the order of work: insulation and windows first, heat pump capacity second — see what makes up the installation cost.
Frequently asked questions
Is the calculator result enough to place an order?
No. It is for screening quotes and for preparing the conversation with an installer. For ordering, and for most grant applications, you need a heat-loss calculation for the specific building (EN 12831) and a defined bivalent point.
Why did I get a different figure than in the 120 m² worked example?
Because that example used a design temperature of −18 °C and the „partly renovated” standard. At −16 °C with the same standard you get a few percent less; if you select „old, poorly insulated”, the result doubles at the same floor area.
How much margin is safe?
For a modulating heat pump (typically 30–100% of capacity) a safe margin is about 10–20% above the calculated peak. A larger margin brings no benefit in hard frost, and you pay for it at purchase and in every start/stop cycle.
Does the calculator include hot water?
In the annual part — ticking the option adds about 2 MWh for 3–4 people. In the peak-power part domestic hot water has almost no effect on the selection, because water is heated when the building does not need full heating capacity.
What if the house is 200 m² and the result is 22 kW?
That does not disqualify a heat pump, but it changes the plan: first check whether insulation alone removes several kW of demand, then compare a machine of that capacity with a hybrid keeping the existing boiler. At this demand level, get two independent quotes.
Basis: the W/m² method and annual demand ranges follow published installer practice for buildings in Central Europe (state: October 2026); calculation standard: EN 12831 (design heat load); catalogue rating conditions for compression heat pumps — EN 14825. COP values and the electricity price are entered by the user of the calculator.