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BBSR calculation tool for measures to refurbish the envelop of existing buildings

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In order to estimate the economic feasibility of an energy-related refurbishment various methods can be used. Based on the methods “equivalent energy price”, “Annuity” and “Amortisation period”, a transparent MS Excel-based calculation tool was developed for this purpose and is available for free download.

Cost functions and energy price matrices for specific measures are given with the respective examples of modernisation; their application is explained in that context.

Input and output of the calculation tool

With just a few adjustments, the tool

  • generates a matrix showing the equivalent energy price, the annuity and the amortisation period for the possible correlations of the total investment costs and the basic maintenance costs (occurring independently of any improvement of energy performance), and
  • shows estimates of these cost portions using the cost functions for the specific measure.
  • Instead of using the costs derived from cost functions included in the results of the tool, individual costs from other sources – e. g. from tenders – can be used alternatively.

Equivalent energy price

The “equivalent energy price” for the individual case can be read from the matrix. It marks the minimal energy price above which the measure is cost efficient; if the really paid or expected energy price is higher than the equivalent energy price the investment is recommendable from an economic point of view.


Furthermore, the annuity for the assessed case can be obtained from a second matrix. If the annuity is positv, the investment is recommendable from an economic point of view.

Amortisation period

A third matrix gives the amortisation period for the individual case. If the amortisation perod is shorter than the lifetime of the refurbished parts of the building envelop, the investment is recommendable from an economic point of view.

Further results

In addition, the result-table provides information about the amount of energy – expressed on the level of delivered energy – transmitted per square meter of surface of the element in question before and after the measure and – subsequently – the amount of delivered energy saved due to the measure per square meter refurbished surface. These values are calculated using certain simplifications (degree days, efficiency of heating system). The degree day factors can be manually adjusted.
The calculation tool is applicable for measures concerning outer walls, pitched roofs, upper ceilings, cellar ceilings and windows of existing residential buildings.

Necessary input data

Necessary input data are the status before the measure and the intended status after the measure (u-values). The economic boundary conditions are pre-set, but mostly can be changed by the user (e. g. interest rate).

Calculation examples

Screenshots with results of the calculation tool are displayed and discussed under the menu item “Examples of modernisation”.

Conditions of use, download and operation of the calculation tool

The calculation tool can be downloaded for free. The conditions of use for BBSR software and the disclaimer on liability have to be respected. Information how to operate the tool, how to adjust the conditions of the calculation and how to read the results is provided within the Excel file itself (German language only).

System requirements: Microsoft Excel 2007 (or newer)

The calculation tool was developed by

  • Prof. Dr.-Ing. Thomas Lützkendorf, KIT Karlsruhe, and
  • Dr. rer. pol. Andreas Enseling, Institut Wohnen und Umwelt, Darmstadt
    and configured and supplemented by BBSR.

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