The right heat pump covers the actual heat demand of your house on the coldest design day – no more. The most reliable basis for this is your real consumption in recent years (oil deliveries, gas bill), not a flat rate per square metre. And contrary to intuition, too large is a worse mistake than too small: An oversized heat pump cycles itself to death. (As of: July 2026)
Key points in brief
- Dimensioning is based on the heating load: the power that the house needs on the coldest standard day – a key figure of the building, not of the device.
- The best data source is in your documents: The real energy consumption of the last few years beats any square metre flat rate.
- Oversizing leads to cycling: many short starts instead of quiet runs – this costs efficiency and lifespan.
- Modern devices modulate their output – this defuses the issue, but does not replace proper dimensioning.
- Hot water, blocking times of the utility and planned renovations belong in the calculation; a «fear surcharge» does not belong in it.
What is actually being dimensioned for?
The size of a heat pump describes its heating capacity – and this must match a number that is not in the catalogue, but is attached to your house: the heating load. That is the power that the building needs on the coldest design day of the region to keep the living rooms warm. It depends on size, insulation, windows and location – and it is significantly smaller than many suspect, because the coldest day is the exception, not the winter.
Important is the distinction between two values that often blur in the consultation: The heating load (kilowatts) determines the device size; the annual energy demand (kilowatt hours), together with the seasonal performance factor, determines the electricity bill. A larger heat pump does not heat «more» – the house needs what it needs. It can only do it faster, and that is exactly where the problem begins.
Why oversizing is harmful
The intuition «better one size larger, then there is a reserve» comes from the world of oil and gas boilers – there it was harmless. With a heat pump it is not: A device that is too large reaches the target temperature too quickly, switches off, and kicks in again shortly afterwards – it cycles. Many short starts instead of long, quiet runs mean poorer efficiency, more wear on the compressor and a more restless operating noise. In the list of errors for heating replacement, oversizing is not at the top by chance – it is the silent error that is never noticed because the house is warm.
The opposite direction is milder than its reputation: A tightly dimensioned heat pump simply runs continuously on the few extreme days – and practically all devices have an additional electrical heating element for exceptional cases, which intercepts the peak. If this only kicks in on a handful of days a year, it is planned and unproblematic; if it runs regularly, the dimensioning was indeed too tight. Between these two errors lies a broad, good-natured corridor – you just have to want to hit it, instead of reflexively rounding up.
The best data source: your real consumption
For an existing house with a heating history, there is a data source that beats any estimation formula: the actual energy consumption of recent years. The oil delivery notes or the gas bill show in black and white how much heat the house really needs – averaged over real winters, with real user behaviour. From these values, the planner reliably derives the heating load and energy demand; the conversion is his craft.
That is why these documents belong in the inventory, even before the first consultation. And that is why a flat rate «per square metre of living space» as the sole basis is a warning signal: It ignores everything that distinguishes your house from the statistics – insulation condition, window replacement, usage. A provider who asks for your consumption data is calculating for your house; one who only wants the living space is calculating for the catalogue.
Two corrections are part of this, however: Anyone who renovates in the future (insulation, windows) plans for the falling demand – otherwise the heat pump will be oversized from the renovation onwards. And anyone who previously used a fireplace for additional heating or left rooms cold that should be warm in the future, corrects upwards.
Hot water, blocking times, modulation: the subtleties
Hot water is added to the heating demand – noticeable as an energy item, usually uncritical as a capacity item, because the storage cylinder can be charged flexibly in terms of time. In the combination with photovoltaics, this flexibility even becomes an advantage: The boiler charges at noon.
Blocking times of the grid operator, on the other hand, belong in the capacity calculation: If the utility is allowed to interrupt the heat pump at certain times, it must be able to make up for the heat in the remaining time – the dimensioning takes this into account with a defined surcharge. This is the one legitimate «markup» in the whole calculation, and it is calculated, not felt.
Modulation finally defuses the size issue: Modern heat pumps regulate their output continuously within a range, instead of just knowing on/off. A modulating device forgives small dimensioning deviations – but even the best control range will not save a heat pump that is twice as big as the house. Modulation is the safety net of proper dimensioning, not its replacement.
How you recognise a serious dimensioning
| Good sign | Warning signal |
|---|---|
| asks for oil/gas consumption of recent years | only calculates with living space |
| considers planned renovations and future usage | «let's take the bigger one just to be safe» |
| shows heating load and assumptions | only names a device without derivation |
| explains blocking time surcharge, if relevant | flat-rate reserve surcharge without justification |
| talks about modulation range and cycling | sells performance as a quality feature |
Guidance, As of: July 2026 – the same logic as when checking solar quotes: Something concrete is a good sign, something flat-rate is worth a query.
The sentence that we invalidate most frequently in dimensioning discussions is: «But what if it gets really cold?» The answer is in your own documents: The house has already experienced the cold winters of the last decades, and its consumption during them is documented – an extreme case cannot be proven better. It is striking how often the clean calculation falls victim to gut feeling: Houses whose owners expect «certainly a big one» regularly turn out to be candidates for the compact class after window replacement and quiet insulation upgrades. The heating element, which everyone is afraid of, kicks in so rarely in well-dimensioned systems that some owners ask after years whether it is even connected. That is the state that a good dimensioning strives for – reserve that you never notice, instead of size that you pay for daily.
Frequently asked questions
What happens if the heat pump is too small?
On the few extreme days, it runs continuously, and the additional electrical heating element intercepts the peak – that is what it is there for. It only becomes problematic when the heating element runs regularly: Then the dimensioning was too tight, and the electricity bill shows it. The corridor between «just right» and «too small» is broader than fear might suggest.
Why is too big worse than too small?
Because the error remains invisible and costs permanently: The oversized device cycles – many short starts instead of quiet runs –, wears out faster and works inefficiently, while the house is warm and nobody notices anything. The system that is too small at least stands out and can be evaluated.
Do I need a heating load calculation or are the consumption data enough?
For existing buildings with a documented heating history, the real consumption data are the strongest basis; a calculated heating load determination supplements them in the case of conversions, extensions or if the usage changes. For new builds without a history, the calculation is the only way.
Does the heat pump have to be larger if the utility has blocking times?
Yes, that is part of the dimensioning: The system must be able to make up for the heat outside the blocking windows, for which a defined surcharge is calculated. This is a calculated item – no reason to take a blanket approach and go one device class higher.
Does a planned solar system change the dimensioning of the heat pump?
No – the heat pump is dimensioned according to the heat demand of the house, not the roof. The solar system changes where the electricity comes from, not how much heat the house needs. The reverse is true, however: The PV system is planned larger if a heat pump is installed.
Free initial consultation
Your heat pump in the right size – derived, not guessed.
Bring your oil or gas bills with you: We dimension the system to your real needs, with clearly stated assumptions and without a fear surcharge.
Prefer to talk? +41 78 830 83 35
Swissolar-certified specialist company · ESTI installation authorisation (Art. 14 NIV) · in Zurich since 2017 · over 150 systems completed · a personal answer from the specialist company, no call centre
Sources: Fachvereinigung Wärmepumpen Schweiz FWS (dimensioning basics); empirical values from the planning and project practice of ecoEn GmbH, Zurich region.
Last updated: 9 July 2026 · Author: ecoEn editorial team
