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Heat pump sizing calculator

Enter your building's design heat loss and your heat pump's rated capacity and COP at a few outdoor temperatures from its datasheet, and get the balance point, the backup heat needed below it, and capacity/COP curves across the outdoor temperature range.

Balance point + bivalent sizing Datasheet-driven capacity curve Free, no sign-up
Heat pump with a capacity and COP curve against outdoor temperature

Building & heat pump data

Inputs only — every result is on the right.

How do you want to work?

Your building

Building, at design conditions

Calculate this from your building →
Estimate annual running cost (optional)

A simplified degree-day estimate: annual heat demand from your building's design heat loss and local heating degree days, divided by the heat pump's seasonal COP (SCOP — on its EU energy label or datasheet) for annual electricity use and cost. Add your current system's efficiency and fuel price below to compare running costs.

Heat pump performance from your unit's datasheet

Three capacity/COP points at different outdoor temperatures, low to high. The example values are illustrative only — replace them with your own unit's datasheet figures.

Point 1 — cold

Point 2 — mid

Point 3 — mild

Backup / bivalent heater (optional)

Leave blank to size the backup heater automatically from the gap between building load and heat pump capacity at your design temperature.

Recommended HP size
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Estimated heat loss
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Building type
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Insulation rate used
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Enter your building details to estimate a heat pump size.

This is a floor-area rule-of-thumb, not a room-by-room heat loss calculation — good for a first estimate, not for final equipment selection. Once you have a candidate unit's datasheet, switch mode above to check its actual balance point for this building.

Estimated design heat loss
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Heat loss rate used
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Recommended heat pump size
Balance point
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Backup heat needed
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HP capacity at design T
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Building load at design T
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COP at balance point
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COP at design T
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Margin at 0°C
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Backup size used
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Enter your building and heat pump data to calculate.

Balance point temperature
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Building load at balance point
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HP capacity at design temperature
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Building load at design temperature
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Backup heat needed at design temperature
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Backup heater capacity used
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COP at balance point
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COP at design temperature
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Datasheet points entered

Link copied — it reopens with these exact inputs.

How to use this calculator

  1. Enter the building's design heat loss and the design outdoor/indoor temperatures — from a heat loss calculation (e.g. this site's heating load calculator) or a manual survey.
  2. Enter three capacity/COP points from your heat pump's datasheet, from cold to mild outdoor temperature — the wider the spread, the more accurate the curve between them.
  3. Optional: enter a fixed backup heater size, or leave it blank to size one automatically.
  4. Read the balance point, the backup heat needed, and the capacity/COP curves.

What this calculates

An air-source heat pump's heating capacity falls as outdoor temperature drops, while a building's heat loss rises as outdoor temperature drops — the two curves cross at the balance point: the outdoor temperature above which the heat pump alone can cover the building's heat loss, and below which it can't, needing backup heat to make up the gap. This is the central sizing concept behind every "how big a heat pump do I need" question, and this calculator builds exactly that chart from your inputs, plus the backup heater size implied by it.

Building load(T) = UA · (T₂ − T) UA = designLoss / (T₂ − Tₛ) from your one design point HP capacity(T) = piecewise-linear through your 3 datasheet points, extrapolated beyond them Balance point = the outdoor temperature T where Building load(T) = HP capacity(T) Backup needed = max(0, Building load(Tₛ) − HP capacity(Tₛ)) at design outdoor temp Tₛ T₂ = indoor design temperature Tₛ = design outdoor temperature

The building side uses the standard simplified "steady-state UA" model for this exact chart type: one design heat-loss figure at one design outdoor temperature implies a constant UA (kW/K), which then scales linearly to any other outdoor temperature. It ignores solar and internal gains (a real building's actual load is somewhat lower than this line at mild outdoor temperatures, since gains cover part of the loss) — the same simplification textbooks make for this chart, useful for sizing, not a substitute for a full dynamic building simulation.

Why three datasheet points, not a built-in curve

A generic heat pump capacity-vs-temperature curve doesn't exist — real performance depends heavily on refrigerant, compressor type (fixed-speed vs inverter), defrost strategy and unit size, and varies a lot between manufacturers and models. Rather than guess at a curve and present it as if it were physics (which this site avoids doing elsewhere too, e.g. refrigerant density on the refrigerant pipe pressure drop calculator), this asks for three real points from your unit's own datasheet and interpolates linearly between them, extrapolating the end segments' slope beyond the given range. Three points capture the curve's general shape (capacity dropping faster than COP as it gets colder) without needing a fitted polynomial — more points would help extrapolation accuracy at the extremes, but three is the minimum that lets the balance-point solver actually work with a resolved position between the coldest and mildest data.

Backup / bivalent heater sizing

"Bivalent" describes any system with two heat sources — the heat pump plus a backup (electric resistance, boiler, or a second heat pump stage) that covers the shortfall below the balance point. This calculator sizes that shortfall at your design outdoor temperature specifically — the coldest condition the system has to meet — using the same capacity curve. Some real installations set the "bivalent point" a little warmer than the pure thermodynamic balance point for practical reasons (defrost derating margin, faster morning warm-up, a smaller/cheaper heat pump); this calculator reports the thermodynamic balance point and lets you size backup capacity from your own design temperature directly, which is the more conservative and directly checkable number.

COP and the balance point

COP typically improves as outdoor temperature rises (a smaller temperature lift between outdoor and indoor means less compressor work per unit of heat delivered), interpolated from your three datasheet points the same way as capacity. The COP at the balance point is a useful reference: it's roughly the efficiency the heat pump is running at during its hardest continuous duty before backup heat has to help.

The building side uses a simplified constant-UA (steady-state) heat-loss model from one design point — it does not model solar/internal gains, thermal mass, or ventilation schedule changes. The heat pump side interpolates linearly between three user-supplied datasheet points and extrapolates the end segments' slope beyond them — accuracy depends entirely on how representative those points are of your actual unit; defrost cycling, part-load behaviour and manufacturer-specific capacity control are not modelled. This does not yet estimate annual energy consumption or seasonal COP (SCOP) — that needs an hour-by-hour or bin-temperature method against real climate data, planned as a future addition. Provided for engineering guidance — verify the balance point and backup sizing against your heat pump manufacturer's own selection software for critical designs.