What happens if a heat pump is oversized?

Oversizing can increase upfront cost and reduce efficiency if the system short cycles. Proper design and controls matter.

Calculator

Heat Pump Sizing Calculator

Use floor area, insulation level, age, bedrooms, climate, and heating emitters to get a broad planning estimate.

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What this sizing estimate can and cannot do

This tool gives you a broad sizing range by combining home size, insulation quality, climate, age, and heating emitters. It helps you understand whether a home may be closer to a smaller or larger system.

It cannot replace a professional room-by-room heat loss survey. Final system selection should always be based on detailed building fabric and emitter calculations.

How the formula works

The calculator applies a rough watts-per-square-metre heat loss factor based on insulation level, then adjusts for property age, climate zone, emitters, and bedroom count.

The result is converted from watts to kW and shown as a range. It is a sense-check, not a professional design load.

Example calculation

A 120 m2 home at roughly 60 W per m2 has an estimated peak heat loss near 7.2 kW before survey-level adjustments.

If insulation improves and the estimate falls to 45 W per m2, the rough load becomes 5.4 kW, which can change equipment and radiator discussions.

Country or regionHome sizeInsulation levelProperty ageNumber of bedroomsClimate zoneRadiator or underfloor heating type

United Kingdom assumptions

Currency: GBP. Electricity: Check current official sources. Gas: Check current official sources. Heating oil: Check current official sources.

Last reviewed: 2026-04-27. Confidence: medium. Prices are not pre-filled because supplier tariffs and price caps change. Enter your current unit rates from a bill.

Estimated heat loss range

9.7 to 11.9 kW

Suggested heat pump size range

9.2 to 13.1 kW

Important warning

This is only a broad planning estimate and is not a replacement for a professional room-by-room heat loss survey.

Suggested next steps

Book a professional heat loss survey before choosing equipment. Ask installers to confirm emitter suitability at lower flow temperatures. Review insulation and draught proofing before final sizing.

Calculators on this site are estimates only. Always get a professional heat loss survey and verify current grant rules, tariffs, and product suitability before making purchase decisions.

Sizing FAQs

Only roughly. Floor area helps provide a starting estimate, but accurate sizing depends on fabric losses, airtightness, ventilation, glazing, and local climate.

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