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Estimation of energy savings

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Estimation of energy savings

Rules of thumb and freely available calculation tools enable initial, quick estimations of energy savings caused by modernisation projects.

The BBSR calculation tool – among other issues – also estimates the energy demand of building shell elements in original as well as refurbished status, specified as values per square meter of the building element in question. The values are given on the level of “delivered energy” assuming a typical heating system and thus directly allow the estimation of the energetic impact of a measure. The possibility of different degreeday factors allows taking into account the shortening of the heating period due to the refurbishment. This – however – is only relevant in case of major refurbishment.
(The calculation approach of the BBSR tool is different from the rule of thumb described below.)

Rule of thumb

If the values in the following table (rows 1 to 4) are multiplied, the result is an initial estimation of the annual demand of delivered energy for heating required to cover the losses of heat by one square meter of building surface. The calculation should be performed separately for initial and target scenarios of modernisation. The "rule of thumb" is also suitable to account for higher room temperatures after modernisation ("rebound effect", e.g., by switching from partial heating to full comfort):

BezeichnungEinheitTypische Werte
1U-value W/(m²·K)effective U-value of component,
see also table "Typical U-values"
2Expenditure factor of heating systemmean expenditure factor of heating system,
see also table "Typical expenditure factors"
3Difference between indoor and outdoor temperaturesK
  • mean indoor temperature ( heating period):
    15 to 22 °C,
  • mean indoor temperature (heating period):
    for cellar and ground 8 to 12°C,
    for all other surroundings 3 to 6 °C,
4duration of yearly heating periodh/atypical range between 4500 and 6000 h/a

Example of a calculation: Modernisation of an exterior wall

  • U-value before modernisation: 1.0 W/m²K
  • U-value after modernisation: 0.24 W/m²K
  • Expenditure factor of heating system: 1.15 (gas condensing boiler, insulated heat distribution system)
  • Difference of temperatures: 20 °C – 5 °C = 15 K
  • Duration of yearly heating period: For a mild winter, the orientation value is 4,500 hours, for a harsh winter, around 6,000 hours. On average, one should calculate 5,000 hours.

Rule of thumb for state before modernisation:

Faustformel vor der Modernisierung

Rule of thumb for state after modernisation:

Faustformel nach der Modernisierung


Faustformel nach der Modernisierung

Per square meter of renovated exterior wall, approx. 65 kWh are saved per year (without rebound effect).

If the total exterior wall of a house, e.g., has a surface area of 100 m² and if the living space is approx. 120 m², one would save per year about 6,500 kWh or 55 kWh per m² of living space.

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