How to size a heat pump for your house (kW) — the method without magic numbers (2026)
Updated: 2026-10-02 · ~2 min read
Why capacity matters
- Too small — the backup (electric/gas) takes over for most of winter; bills grow and the real system COP drops.
- Too big — you pay more per kW (the biggest cost factor in the investment), short compressor cycling, faster wear.
Estimating demand (three steps)
- Annual specific demand in kWh/m²·year: well-insulated new house ~40–80; renovated ~80–130; old uninsulated ~150–250. This is the first filter — without insulation, no heat pump will be optimal.
- Peak demand (kW): the installer calculates heat loss (standard energy-audit method). Rule of thumb: the heat pump should cover most of the load at the design outdoor temperature (typically −7…−10 °C in this climate), and the backup covers the coldest days.
- Add DHW: domestic hot water typically adds 1.5–3 kW on top of space-heating peak.
Benchmark for typical houses (table)
| House type (120 m²) | Annual demand | Total capacity incl. DHW (indicative) |
|---|---|---|
| new, well insulated (≤80 kWh/m²·yr) | ~5–10 MWh/yr | 8–12 kW |
| renovated / average insulation | ~10–16 MWh/yr | 12–16 kW |
| old, poorly insulated (>150 kWh/m²·yr) | ~18–30 MWh/yr | 16–24 kW — check retrofit economics first |
Signs the sizing failed
- Backup runs regularly below −7 °C (visible in logs/app) → capacity shortage.
- Winter electricity bill clearly above the indicative range in the seasonal-cost article → undersizing or bad settings.
- The pump starts/stops every few minutes (short cycles) → possible oversizing relative to the load.
What to demand in the installer’s quote
- Capacity chosen from a heat-loss calculation, not “from the catalogue”.
- SCOP stated for your configuration (floor heating/radiators, buffer, climate zone).
- A clear description of the backup (power, source, control logic).
Affiliate slot: sizing calculators / quote comparison (CPA Money2Money) — before signing the quote.
Sources
- Energy audit and heat-loss calculation standards (EN 12831 / installer practice).
- Manufacturer sizing tools — 2026 practice.