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The Degree-Day Method: What the Calculation Can Do, and Where It Stops

4 min read

At some point in a client conversation, the question comes up: “So what will I actually save?” An honest answer needs a method — and knowing where that method stops. Both are covered here.

How a degree-day figure is derived

At its core, the method is a sum. For every heating day, the difference between indoor temperature and mean daily outdoor temperature is calculated and added up over all heating days:

GTZ = Σ (Ti − Ta) over all heating days

A heating day is a day on which the mean outdoor temperature falls below the heating limit temperature. The standard approach in Germany is G20/15: 20 °C indoor temperature, 15 °C heating limit. For older buildings, 15 °C is sensible; well-insulated buildings manage with 12 °C or less, because they only start heating later.

From this follows the approximate heating energy demand of a building element:

Q = U · A · Gt · 24 / 1000  [kWh/a]

U is the thermal transmittance, A the area, Gt the degree-day figure in Kelvin-days. The factor 24 converts days into hours, the 1000 converts watts into kilowatts.

Where the method stops

This is the more important part. The degree-day method captures only the transmission heat loss. Not included are:

  • Ventilation heat losses — a substantial share, depending on use
  • Solar gains through windows
  • Internal gains from occupants, lighting, appliances
  • Thermal bridges — the decisive factor in a mould context
  • Hot water — the comparison must explicitly be run “excluding energy for hot water”

On top of that comes a methodological imprecision: the standards set the heating limit temperature as a fixed value, but in reality it shifts by the day and hour with solar radiation, sky thermal radiation and wind.

And: degree-day figures are strongly location-dependent and only ever apply to one climate zone — even within a zone, the specific site can deviate. By their nature, they also describe the past; for a future year, they are an assumption.

What this means for a savings calculation

An order of magnitude for the conversation — nothing more. Specifically, this means:

  1. No percentage figure without a starting U-value. The same absolute improvement produces a completely different percentage at U = 1.4 than at U = 0.24. A percentage without a reference basis is arbitrary.
  2. No savings figure without boundary conditions. Area, U-value before, assumed improvement, energy price, system efficiency, degree-day figure — all six must be stated, otherwise the figure cannot be checked.
  3. The assumed improvement is an assumption. It is not a product guarantee unless a verified proof exists. Calculating conservatively in a client conversation is not modesty — it is self-protection.

Factoring in CO₂ — but with the correct factor

The CO₂ saving follows directly from the amount of energy saved, multiplied by the emission factor of the energy carrier. Natural gas is around 0.2 kg CO₂ per kWh, heating oil higher. The factor belongs alongside the result, otherwise the figure cannot be put into context.

A word on the legal position

An approximate calculation is not formal proof. For requirements under the GEG and for funding programmes, design values of thermal conductivity from DIN 4108-4, a general building authority approval, a European Technical Assessment or a harmonised product standard are required.

Terms such as “effective U-value”, “dynamic U-value” or “U-value under real-world conditions” are explicitly not recognised for this purpose. Basing a funding application on them risks its rejection.

To calculate it yourself

The savings calculator on the Wirkprinzip page works on exactly this method. It runs entirely in the browser — no transmission, no storage — and exposes every assumption as an input field, so it can be changed and discussed during the conversation.

It provides an order of magnitude. It does not provide a final account, and that is not a shortcoming — it is the limit of the method.

Sources

  • VDI 3807 Sheet 1 — Consumption benchmark values for buildings, fundamentals
  • Bund der Energieverbraucher e. V., “Die Anwendung der Gradtagzahlen GTZ”
  • Umweltbundesamt, emission factors for fuels
  • Gebäudeenergiegesetz (GEG) and the BEG-EM FAQ on the recognition of U-value proofs
  • DIN 4108-4 — Thermal insulation and moisture protection design values

All formulae in this article are calculation methods, not measured values. Results depend entirely on the assumptions used.

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