ORIGINAL ARTICLE

Disaggregating the causes of falling consumption of domesticheating energy in Germany

Ray Galvin & Minna Sunikka-Blank

Received: 8 March 2013 /Accepted: 12 March 2014# Springer Science+Business Media Dordrecht 2014

Abstract Consumption of domestic heating energy(space and water heating combined) in Germany hasbeen falling in recent years. Official figures indicate itfell by 17 % in 2000–2011, from 669 to 564 TWh(temperature adjusted), while the population reducedby 2 % and the number of occupied dwellings increasedby 3.4 %. German policy has strongly promoted deepthermal retrofits through regulation, information cam-paigns and subsidised loans. An important question iswhat portion of the reductions are due to progressiveenergy efficiency upgrade policy and what are due toother, non-technical factors such as demographic andbehaviour change. We use national statistics andexisting empirical studies to disaggregate the contribu-tion of energy-efficiency improvements and nontechni-cal factors to the reduction in consumption. Our analysissuggests that around 20 % of the reductions are likely tobe due to thermal retrofits of existing dwellings (insula-tion and new windows), 31 % due to boiler or heatingsystem replacements, 1 % due to replacement of olddwellings with new, energy-efficient buildings, whilesome 45–50 % of the savings cannot be explained bythese technical improvements. Most of these reductionsappear to have occurred in non-upgraded, non-newdwellings. Although we do not know what caused thesereductions, the finding is robust to very wide

inaccuracies in figures for savings through technicalimprovements in buildings’ energy efficiency. Moreresearch is needed to explore the extent to which thisimplies increasing fuel poverty, increasing skills andmotivation among non-poor households to heat moreeconomically, or the effects of demographic and life-style changes.Highlights• German home heating energy consumption fell by105 TWh in 2000–2011

• Approximately 31 % of this was due to boiler/heatingsystem upgrades

• Approximately 20 % was due to window replacementand thermal envelope upgrade

• Less than 1 % was due to replacement of old with newstock

• Over 45 % cannot be explained by technical upgradefactors

• This could imply rising fuel poverty or more skilfulheating behaviour

Keywords Home heating behaviour . Domestic energysaving . German energy policy . Thermal retrofits .

Heating fuel consumption

Introduction

There has been a steady fall in heating energy consump-tion (including both space and water heating) in theGerman domestic sector since this peaked in the year2000, reducing from 669 to 564 TWh or 17 % by 2011

Energy EfficiencyDOI 10.1007/s12053-014-9259-5

R. Galvin (*) :M. Sunikka-BlankDepartment of Architecture, University of Cambridge,1-5 Scroope Terrace, Cambridge CB2 1PX, UKe-mail: rg445@cam.ac.uk

M. Sunikka-Blanke-mail: mms45@cam.ac.uk

in temperature-adjusted figures (Destatis 2010, 2013a,2013c). This reduction, of 105 TWh, indicates thatGerman householders spent €8 billion less on heatingfuel in 2011 than they would have if their consumptionhad stayed steady at the 2000 level, and caused around20 million tonnes less CO2 emissions, even though thenumber of occupied dwellings increased by 3.4 % in2000–2011. The 2.8 million new dwellings constructedduring this period were built to high energy-efficiencystandards, while about half that number of older, lessenergy-efficient buildings became unoccupied.Meanwhile, approximately 4 million existing homesbenefited from thermal retrofits, including insulationand/or window replacement; windows were replacedin approximately 2.5 million further dwellings; and theheating systems in approximately 13 million dwell-ings were upgraded to newer, more energy-efficient models. These improvements resultedpartly from the legal requirement in the Energy-Saving Regulations (Energieeinsparverodnung—EnEV), introduced in 2002, to undertake thermal im-provements whenever a portion of a building envelopeis being repaired or replaced (EnEV 2009); partly fromnew regulations concerning boilers; partly from a seriesof well-funded, persistent government campaigns toencourage and persuade homeowners to thermally up-grade their buildings; and partly from the effect ofsubsidies from the German Development Bank(Kreditanstalt für Wiederaufbau—KfW) (DENA 2009;DENA 2012a; UBA 2010).

There has been no attempt to quantify the proportionsof domestic heating fuel savings being made from non-technical as compared to technical measures. By ‘tech-nical’, we mean anything to do with the substance ofbuildings including window or boiler replacement, in-sulation of the building envelope, new builds, or dwell-ings becoming unoccupied. By ‘non-technical’, we in-clude such things as demographic changes, which mightresult in empty rooms in large homes; lifestyle changes,such as people spending less time in the home; behav-ioural changes, such as households’ attempts to keepheating consumption low by heating or ventilating moreefficiently; and changes due to any other unknownphenomena. Rehdanz (2007) noted that it would beuseful to disaggregate these proportions, and Gram-Hanssen (2010, 2011) argues from empirical evidencethat both behavioural and technical measures are signif-icant in attempts to lower household energy consump-tion. The Federal Environment Office (UBA 2006)

surveyed households in 1995–2005 and concluded thatchanges in user behaviour were making a significantlygreater contribution to energy savings than technical,energy-efficiency improvements. Koch et al. (2008)investigated the types of user behaviour that wouldenhance the German government’s domestic heatingenergy saving goals, while others have surveyedGerman households to find out what skill and knowl-edge shortages need to be addressed to enable willinghouseholds to reduce heating energy (Brohmann et al.2000; Hacke 2007). However, as yet there has not beenan attempt to disentangle the various causes of con-sumption reduction to see what proportion, if any, canbe attributed to non-technical measures.

This paper considers the possible contribution to do-mestic heating reductions of technical measures, i.e. ther-mal retrofits of existing homes (insulation and windowupgrades), boiler or heating system replacement, newlybuilt dwellings, and dwellings becoming unoccupied.Weuse the term ‘abandoned dwellings’ for the latter,as statistics show that there is no direct correspon-dence between the numbers of dwellings beingdemolished annually and the number of inhabiteddwellings becoming unoccupied (Destatis 2004;2010; 2012). We address the period 2000–2011,as temperature-adjusted consumption peaked in2000, while 2011 is the most recent year forwhich confirmed data is available.

The German Federal Statistics Office (StatistischesBundesamt—Destatis), the Housing Ministry(Bundesmin i s t e r i um für Verkehr, Bau undStadtentwicklung—BMVBS), the Ministry for theEconomy (Bundesministerium für Wirtschaft undTechnologie—BMWi), the German Energy Agency(Deutsche Energie-Agentur—DENA) and the FederalEnvironment Office (Umweltbundesamt—UBA) pub-lish national statistics of heating fuel consumption, thenumber of new builds and the number of occupied andunoccupied dwellings. In Germany, there is now also anumber of credible, existing empirical studies, eitherpeer-reviewed or from major research institutes, thatoffer empirical findings on other factors that need tobe known for a study such as this. These are the follow-ing: the measured heating energy consumption of newlybuilt dwellings, the quantity of living area that hasbenefited from thermal upgrades, the number ofother dwellings that have benefited from windowreplacements, the measured energy consumptionsavings that have resulted from thermal upgrades,

Energy Efficiency

and the number of boiler/heating system replace-ments and their increased efficiency.

By examining the results of these studies, togetherwith national statistics, we will test the hypothesis thatnot all the reductions in domestic heating energy con-sumption in 2000–2011 can be explained by these tech-nical factors. The hypothesis will fail to be confirmed ifeither (a) the technical factors are seen to have producedsufficient fuel savings to account for the fall in con-sumption, or (b) uncertainties in the data are so large asto make a clear conclusion impossible. If the hypothesisholds true, we will then suggest what other avenues ofresearch would be necessary to identify the nontechnicalfactors that have been contributing to this steady fall inconsumption.

Our methodology is set out schematically in Fig. 1.The quantities denoted by variables A, B, C, etc., whichare displayed on this schematic diagram, are given inTable 1. Other quantities we will be referring to in ouranalysis are given in Table 2.

Starting at the top right of the schematic diagram inFig. 1, in ‘New builds in 2000–2011’ we estimate the

number of dwellings newly constructed in 2000–2011and their total heating consumption in 2011, to give thequantity G. In ‘Dwellings abandoned in 2000–2011’,we estimate the total number of dwellings that were‘abandoned’ in 2000–2011 and their total consumptionin 2000, to give the quantity B. Here, the word ‘aban-doned’ is a mathematical variable equal to the differencebetween the number of new dwellings built in 2000–2011 and the rise in the number of occupied dwellings in2000–2011. This is not the same as the numberdemolished in 2000–2011. The demolition rate bearsno direct relation to the rate at which dwellings fall outof use (Destatis 2004; 2010). It also fails to take accountof shifting numbers of unoccupied dwellings thatare, for example, between tenants or in underutilisedapartment blocks, for which there are no plans fordemolition. A certain number of the new builds canbe seen as replacing, one-for-one, the abandoneddwellings, and from this, we can work out the num-ber of new builds that were additional to replace-ments, and the increases in national consumptionthey caused, i.e. N=G-B.

(36.198 million occupied dwellings)2011

(35.001 million occupied dwellings)2000

energy consumption (TWh)

A

B

I

F

retrofitsnew builds

D

H

Dwellings abandoned in 2000-2011

new boilers

C

G

non-technical factors

E

Fig. 1 Schematic diagram showing the possible contribution offive different factors to the fall in domestic heating consumption inGermany in 2000–2011: abandonment of dwellings, new builds,

thermal retrofits (insulation and windows), new boilers/heatingsystems, and non-technical influences

Energy Efficiency

In ‘Dwellings retrofitted in 2000–2011’, we considerthe contribution of thermal retrofits (insulation and newwindows; not counting heating system upgrades). To dothis, we first estimate the number of dwellings retrofittedin 2000–2011 and their pre-retrofit consumption, to givethe quantity C. We then estimate their post-retrofit con-sumption to give the quantity H. The fall in consumptionin this sector is given by K=C-H. Within this set ofcalculations, we include a factor for window replace-ments in dwellings that were not otherwise retrofitted

and therefore would not appear in national estimates ofretrofits. In ‘Replacement of boilers and heating systems’,we estimate the number of dwellings which had boiler orheating system replacements in 2000–2011 and the con-sequent average percentage energy efficiency increase.The dwellings this affects include both retrofitted andnon-retrofitted, so we display the fall in consumptiondue to boiler/heating system replacement as a separatewedge on the diagram. This consumption in 2000 thatwas eliminated by 2011 through these replacements isrepresented by the quantity D.

In Reductions due to nontechnical or unexplainedfactors, we bring all these figures together to estimatethe reductions in consumption in 2000–2011, if any, thatwere not due to technical factors, namely:

L ¼ E–I ¼ A–B–C−Dð Þ– F–G–Hð Þ ð1ÞWe draw conclusions from the results of this analysis

and make recommendations for policy and further re-search in ‘Discussion and conclusions’.

It should be noted that we are looking at an 11-yeartime span in this analysis. Our figures refer to the period1 July 2000 to 30 June 2011, or the 11-year periodclosest to these dates which each dataset used in theanalysis relates to.

New builds in 2000–2011

Destatis (2012) figures reveal that 2,689,965 new dwell-ings were completed in the 11 years from January 2000to December 2010 (see Table 3). These had an average‘useful’ area (Nutzfläche) of 114 m2 (Destatis 2010),giving a total new ‘useful’ area of 306,656,010 m2.‘Useful’ area includes floor area inside the front doorplus a portion of service stairwells, landings, basementsand lofts, and is on average 25 % larger than ‘livingarea’ (Wohnfläche), which only includes the floor areawithin the front door and excludes basements, loftsand internal access stairwells. Following Germanpractice for thermal standards, all calculations ofheating fuel consumption given in this paper arebased on ‘useful’ area.

We now estimate how much these dwellings wereconsuming in 2011. Prior to October 2002, the average1

1 The Energy Saving Regulations (EnEV) prescribe a range ofmaximum consumption figures for buildings depending on theirgeometry, size and connection to other buildings.

Table 1 Quantities of heating energy displayed in Fig. 1. Note thatthere will be overlap between these categories in some cases

Symbol Quantities of heating fuelconsumed in 2000 by

Quantities of heating fuelconsumed in 2011 by

A All dwellings in 2000

B Dwellings abandonedin 2000–2011

C Dwellings retrofitted(insulation & windows)in 2000–2011

D Dwellings with boiler/heating systemreplacement in2000–2011

E Dwellings not upgradedin 2000–2011

F All dwellings in 2011

G Dwellings constructedin 2000–2011

H Dwellings retrofitted(insulation & windows)in 2000–2011

I Dwellings notupgraded in 2000–2011

Table 2 Other symbols, and composites, used in the analysis

Symbol Quantity of heating fuel Equal to

K Savings through thermal retrofitsof existing dwellings

C-H

L Savings made throughnon-technical means

E-I

– Total reductions 2000–2009 A-F

M Savings through new buildsreplacing abandoned dwellings

N Additional consumption throughexcess of new builds overabandonments

G-B

P Net saving through new buildsand abandonments

Energy Efficiency

maximum permissible heating fuel consumption (i.e.theoretical, calculated consumption) for new buildswas 145 kWh/m2a; from October 2002 to September2009, it was 100 kWh/m2a; and thereafter, 70 kWh/m2a(Galvin 2012). Federal subsidies led to many homesbeing designed for higher thermal standards (DENA2012a), which would have lowered the average(theoretical) consumption in each of these periods.

The most comprehensive peer-reviewed study of theenergy performance of new homes built within thisperiod is Greller et al. (2010), though this does notextend through to 2011. These authors analysed themetered consumption of 110,000 gas and oil heatedhomes built from 1977 to 2006, including 25,650 inthe period 2000–2006. They found an average heatingfuel consumption of 95 kWh/m2a for those built in theyears 2000–2006 inclusive (Greller et al. 2010) and afalling trend from 100 kWh/m2a to 90 kWh/m2a overthis period. Since minimum standards were tightened toan average of 100 kWh/m2a in 2002, this implies neg-ligible ‘energy performance gap’ (i.e. the gap betweendesign and performance; see Demanuele et al. 2010;Tronchin and Fabbri 2007—often mistakenly called‘rebound effect’, for a robust definition of which seeSorrell and Dimitropoulos 2008). Due to the furthertightening of standards in 2009 to 70 kWh/m2a wewould expect average consumption to have fallen to thisstandard or better by 2011. Hence, we accept Grelleret al.’s (2010) figure of 95 kWh/m2a for 2000–2006 andsuggest an average of 65 kWh/m2a for homes built in2007–2010.Weighting these according to the number ofhomes built in each of these years gives a total heatingconsumption in 2011 from all these homes of 26.7 TWh(see Table 3).

The three main sources of heating fuel not covered inthe analysis of Greller et al. (2010)—district heating,wood and electricity—make up 7, 7.5 and 3 % of

heating consumption, respectively (Schloman et al.2004). District heating and wood are associated withprimary energy consumption 10 % lower than averageand electricity with up to three times its end-energyconsumption. Hence the differences here tend to canceleach other out, so we stay with the figure of 26.7 TWh.This is variable G.

Dwellings abandoned in 2000–2011

The number of ‘dwellings abandoned in 2000–2011’, asdefined above, is equal to the difference between thenumber of new dwellings built in 2000–2011 and theincrease in the number of occupied dwellings in thisperiod. In 2000, there were 35,001,000 occupied dwell-ings, and in 2011, there were 36,198,000, an increase of1,197,000 (BMVBS 2012). Since 2,689,965 new dwell-ings were built in this period, the number of existingbuildings that became abandoned (to the nearest 1,000)was 2,689,965—1.197,000=1,493,000.

There are no existing studies on the heating consump-tion of German dwellings that become unoccupied, butthere are good reasons to assume their consumption washigher than the national average. Dwellings become un-occupied for two main reasons in Germany: internal mi-gration and poor quality of buildings. Migration is themajor factor in Germany as there have been large internalpopulation shifts in Germany over the last 20 years, most-ly from East toWest andNorth to South (Szymanska et al.2009). The housing stock in former East Germany wasmostly of pre-World War II thermal quality prior to reuni-fication, and the Communist era Plattenbau (prefabricatedslab) apartment blocks were of low thermal quality(Flockton 1998). In the western states, emigration occursmostly from old industrial areas, such as the Ruhr Valley.In regions of falling population such as these, relatively

Table 3 Numbers of new dwellings built in Germany in the years2000–2010 inclusive, giving estimated heating energy consump-tion based on an average ‘useful’ floor area of 114 m2 per

dwelling, to give an estimate of heating energy consumption in2011 of all dwellings built in this period. Source: Destatis (2012)

Year 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Total

No. 348340 290978 274117 296823 268679 240571 247793 182771 174691 177570 187632 2689965

Est. heatingconsumption(kWh/m2a)

95 95 95 95 95 95 95 65 65 65 65

Heating consumption(TWh)

3.773 3.151 2.969 3.215 2.910 2.605 2.684 1.354 1.294 1.316 1.390 26.661

Energy Efficiency

few homes have been built in recent decades. Greller et al.(2010) show that the average actual consumption of pre-SecondWorldWar dwellings is around 165 kWh/m2a andthat of dwellings built in 1946-1964 is around 160 kWh/m2a. Schröder et al. (2011), using similar methodologyand datasets, estimate total national average actual con-sumption at149 kWh/m2a. This would put the consump-tion of pre-Second-World War dwellings about 10 %higher than the national average, and as most abandoneddwellings are of older stock, it would seem reasonable toassume their average consumption was around 10 %above the national average. We note, however, that arecent French study found that abandoned dwellings inFrance have the same average consumption ratings asoccupied dwellings (Osso et al. 2011).

As we noted above, the national domestic heatingenergy consumption in 2000 was 669 TWh and therewere 35,505,000 occupied dwellings, so that averageconsumption was 18,842 kWh/a per dwelling. If thebuildings that were later abandoned were consuming10 % above the average, their consumption would havebeen 20,727 kWh/a per dwelling. As there were1,493,000 of these dwellings, their total heating con-sumption in 2000 would have been 28.1 TWh. Hence,variable B=28.1 TWh.

While this is not essential to our analysis, we notealso that 1,493,000 of the new builds in 2000–2011 canbe seen as replacing abandoned dwellings. This is 56 %of all the new builds, hence their 2011 consumption was56 % of 26.7 TWh=15.0 TWh. Therefore, the heatingconsumption of new builds that were additional to thenumber of dwellings abandoned was the difference be-tween the total new build consumption of 26.7 TWh andthis figure, i.e. 26.7–15.0=11.7 TWh.

An interesting result here is that the fall in consump-tion from dwellings being abandoned, at 28.1 TWh, isclose to the consumption from new builds, at 26.7 TWh,giving a net reduction of only 1.4 TWh. The net effect ofthese two sectors on fuel savings is to almost canceleach other out, as there were almost twice as many newdwellings as abandoned dwellings.

Dwellings retrofitted in 2000–2011

General considerations

The most intricate part of our analysis is to assess thefuel consumption reductions due to retrofits. Not only

are the precise numbers of retrofitted dwellings in dis-pute, but there is a variety of degrees of retrofit, up to afull project including wall, roof and basement-ceilinginsulation; window and door replacement; addition ofventilation system (with or without heat recovery); boilerand radiator replacement (which is considered in‘Replacement of boilers and heating systems’); and in-stallation of solar collectors. Further, as there is no in-spection of thermal (or other) building standards inGermany, the authorities do not automatically have re-cords of what retrofitting has taken place, and we haveobserved a certain amount of ‘sub-standard’ retrofittingbeing carried out illegally.

We will consider national summary figures frompeer-reviewed papers and from research institutescommissioned by the Federal government. Since it isdifficult to get precision in this section, we will estimatethe range within which the true value is highly likely tofall, and carry these results forward to our final totals.Further, we will frequently be referring to ‘equivalentdwellings’ rather than ‘number of dwellings’ here, assome studies estimate the savings due to partial retrofitsin terms of their equivalent number of dwellings or floorarea fully retrofitted.We will also include an extra factorfor window replacements in dwellings that would nototherwise fall into the category of retrofitted dwellings.

Annual rate of thermal retrofits

The German Energy Agency (DENA) estimates theannual rate of thermal retrofits at 0.8 % of the residentialbuilding stock per year over the last decade (Stolte2011), i.e. 8.8 % of the residential building stock over2000–2011. Since the average number of dwellings(occupied and unoccupied) in 2000–2011 was 39.2million, this is equivalent to 3.45 million dwellings.

Diefenbach et al. (2010) found a retrofit rate of0.83 % per year in a survey of 7,500 building owners,equivalent to 9.1 % over 2000–2011, or 3.57 milliondwellings if extrapolated nationwide, though this is alimited sample size.

Friedrich et al. (2007) estimate the equivalent livingarea of existing homes retrofitted with insulation andnew windows in each of the years 2000–2006 asamounting to an accumulated total of 5.9 % of theresidential stock over these 7 years, which wouldamount to 9.3 % if it continued for the remainder ofthe 11 years, or 3.64 million dwellings.

Energy Efficiency

Weiss et al. (2012) estimate an annual retrofit rate forwalls at 0.8 % and roofs at 1.2 %. Weighting walls androofs according to their average thermal impact2 in theratio 3:1 would give an annual retrofit rate of 0.84 %,assuming windows were upgraded correspondingly.This would amount to 9.24 % of the residential stockover 11 years, or 3.62 million dwellings.

While the figures from each of these studies do notprecisely agree, they are sufficiently close to be used inour analysis. We note their average, of 3.57 milliondwellings (9.1 % of dwellings), with upper and lowerbounds 3.64 and 3.37 million dwellings, respectively.We will carry forward both upper and lower boundfigures in our analysis so as to find whether the degreeof imprecision defeats the hypothesis we are testing.

Pre-retrofit consumption in 2000

We now estimate the heating fuel savings achievedthrough these retrofits. To begin with, there is no precisedata as to their actual pre-retrofit heating fuel consump-tion. The German EnergyAgency targets dwellings witha calculated (i.e. based on the thermal quality of thebuilding) consumption of 225 kWh/m2a or higher forretrofitting (Stolte 2011), but notes that actual consump-tion in Germany is, on average, 30 % below the calcu-lated value (DENA 2012b: 43). This accords withSunikka-Blank and Galvin (2012) who find the samefigure, and who show that this gap between actual andcalculated consumption increases as the calculated con-sumption increases, i.e. as buildings’ thermal qualitydiminishes.

This would give an actual pre-retrofit consump-tion of around 160 kWh/m2a. Data from CO2online’s(www.co2online.de) database of over one milliondwellings indicates an average pre-retrofit actual(measured) consumption of 160 kWh/m2a for detachedand semidetached houses, and 150 kWh/m2a for multi-dwelling buildings. However, the self-selection bias ofCO2online’s data might make these actual consumptionfigures lower than national averages for pre-retrofitdwellings.

A further consideration is that many residential build-ings in Germany that are thermally retrofitted do not

necessarily have especially high pre-retrofit consump-tion, since thermal retrofitting is compulsory when thesubstance of the building is being upgraded, rather thanwhen the thermal quality is low (EnEV 2009). Further,large apartment blocks, which are among the most fre-quently retrofitted buildings, do not generally haveabove-average pre-retrofit consumption as the volumeto surface area of these buildings makes them inherentlymore thermally efficient than the average building.

Hence, it is likely that the pre-retrofit actual con-sumption of dwellings that were retrofitted in 2000–2011 was not far above the average (149 kWh/m2a),so we will estimate it at 15 % above this, at around170 kWh/m2a. As above, however, we will check at theend of our calculation whether the uncertainties in thisfigure defeat our working hypothesis.

Since the average useable area in 2000 was 110 m2

(living area 84m2), this gives a heating fuel consumptionof 18,700 kWh per year. For the lower bound of 3.37million dwellings retrofitted, this gives consumption in2000 of 59.6 TWh. For the upper bound of 3.64 milliondwellings, this gives 64.4 TWh. To obtain the upper andlower bounds for variableC, these figures will need to bemodified by the effect of window replacements in dwell-ings that were not otherwise retrofitted, as we do in‘Window replacements additional to these retrofits’below.

Consumption reduction per retrofitted dwelling

We now estimate the reduction in heating fuel consump-tion achieved through these thermal retrofits. Schröderet al. (2011) investigated the heating energy consump-tion of residential buildings of two or more dwellingsthroughout Germany for 2004–2008. Buildingsretrofitted or constructed since 1995 showed an averageconsumption of 110 kWh/m2a after retrofitting, whilethose constructed prior to 1995 and not subsequentlyretrofitted consumed an average of 145 kWh/m2a—adifference of 35 kWh/m2a, or 24 %.

A further nationwide empirical study (Schröder et al.2010) gives cumulative frequency distribution curvesfor the heating energy consumption throughoutGermany of retrofitted and non-retrofitted apartmentblocks of floor area larger than 700 m2. The mean is140 kWh/m2a for those completely non-retrofitted, and90 kWh/m2a for those comprehensively thermallyretrofitted, a fuel saving of 50 kWh/m2a, or 36 %.

2 The term ‘thermal impact’ means the relative contribution ofeach feature to the thermal quality of the building, based on thearea each contributes to the building envelope, and typical Uvalues of each.

Energy Efficiency

Regarding smaller buildings, Walberg et al. (2011)investigated the nationwide retrofit performance of fiveclasses of 1-2 dwelling buildings and found an averagemeasured consumption reduction of 26 %.

A comprehensive figure is offered by Tschimpkeet al. (2011), of an average of 38 % reductions achievedthrough retrofits of all building types, though this refersto calculated, rather than actual, pre-and post-retrofitconsumption figures. Clausnitzer et al. (2009, 2010)estimate the calculated saving in retrofits with Federalsubsidies from the German Development Bank (KfW)at an average of 33 %. If actual pre-retrofit consumptionis, on average, 30 % lower than calculated consumption(see above), the actual savings for both these studieswould be significantly lower.

The German Energy Agency also estimates actualsavings at 25 % (DENA 2012c).

Figures from CO2online indicate actual savings ofaround 33 %, though this could be higher than thenational average due to self-selection bias in the data.However, we cannot ignore Schröder et al.’s (2010)figure of 36 % for large apartment blocks. Hence, wewould suggest a mean of around 30 % savings forequivalent comprehensive retrofits carried out in2000–2011, though this could be as large as 35 % oras little as 25 %.

We now bring these percentages together with thepre-retrofit consumption range estimated above. Themaximum reduction would arise if the maximum pre-retrofit consumption were reduced by the highest ofthese percentages, i.e. 64.4 TWh reduced by 35 %, areduction of 22.5 TWh. The minimum reduction wouldarise if the minimum pre-retrofit consumption werereduced by the lowest of these percentages, i.e.59.6 TWh reduced by 25 %, a reduction of 14.9 TWh.The midpoint between these is 18.7 TWh.

Window replacements additional to these retrofits

We now consider the energy saving effect of dwellingsthat had their windows replaced each year but which donot fall into the 9.1 % of homes that were otherwiseretrofitted in 2000–2011, i.e. 0.83% per year. We offer aback-of-the-envelope calculation as follows:

We consider that these dwellings have an averagewindow area of 7.5 m2 (e.g. five windows of 1.5 m2

each, as typical in German apartments) and a totalbuilding envelope area of 100 m2 (three external wallsand either a roof or ground floor). The windows would

then make up 7.5 % of the building envelope area.We assume the U value of the windows is reducedby 1.5 W/m2K through these window replacements. InGermany, most windows are already double-glazed, sothe reduction in U value would not be as great as itwould in a replacement in the UK of a single-glazedwindow with a modern double or triple-glazed version.This gives an approximate average reduction in the Uvalue of the building envelope of 7.5 %×1.5=0.1125 W/m2K. If the average original U value of thebuilding envelope was 1.5, this represents a reduction to1.3875, or 7.5 %. This would reduce the theoreticalheating consumption by approximately 7.5%. The samefigure is obtained using the calculator offered byCO2online (2013) for replacing older double-glazedwindows with post-1995 models.

Allowing for an energy services rebound effect of32 % (= an energy rebound effect of −0.68, see discus-sion in ‘Replacement of boilers and heating systems’),the actual reduction would be 5.1 % for dwellings thathave their windows replaced.

CO2online (2013) estimates that 1.6 % of Germandwellings per year have their windows replaced. This ismore likely to be on the high side than the low side, dueto the self-selection bias in CO2online’s data. The first0.83 % of this figure has already been incorporated inour estimate of consumption reductions through thermalretrofits. The total heating energy reduction in Germanyachieved through window replacement over 11 years forthe remaining 0.77 % per year would be 5.25 %×0.77 %×11=0.445 %, or 2.9 TWh. We add a furtherfactor of 10 % for window replacements unaccountedfor in this group, such as though breakages, bringing thetotal to 3.2 TWh.

This estimate is approximate, and possibly high, butthe figures are considerably smaller than those for ret-rofits, so their accuracy will not greatly affect the out-come of the overall calculation.

We now incorporate these window replacement fig-ures in the estimates for thermal retrofits above. Firstly,we add 3.2 TWh to the high and low estimates of pre-retrofit consumption for this sector: 59.6 TWh and64.4 TWh are increased to 62.8 TWh and 67.6 TWh,respectively. This is the range for variable C, with amidpoint of 65.2 TWh.

Secondly, we add this 3.2 TWh to the figures forconsumption reductions for retrofits obtained in‘Consumption reduction per retrofitted dwelling’, to givea maximum reduction of Kmax=22.5+3.2=25.7 TWh, a

Energy Efficiency

minimum of Kmin=14.9+3.2=18.1 TWh, and a mid-point of Kmid=21.9 TWh.

Replacement of boilers and heating systems

To calculate the reductions in heating energy consump-tions due to replacement of boilers/heating systems, webegin by taking the average rate of boiler replacementover the 11 years 2000–2011 (in this case 2000–2010inclusive). Diefenbach et al. (2010: 89ff) report a ‘heatingsystemmodernisation’ (Heizungsmodernisierung) rate of3.5 % for 2000–2004 inclusive and 2.8 % for 2005–2009inclusive (see also discussion in Simons 2012: 28ff). Ifwe assume that the rate of 2.8 % per year continued into2010, this gives an average annual replacement rate of3.1 % for the 11 years, i.e. 34.1 % of German dwellingswould have had their boilers replaced in this period.

The average energy efficiency improvement of newheating units in this period is estimated at 18 % byCO2online (Heimann 2012), and this accords with fig-ures from UBA (2011: 97). We note that this is anumerical increase of 18 % (e.g. if the previous boilerwas 70 % efficient, the new one is 88 % efficient) ratherthan a relative increase of 18 % (e.g. if the previousboiler was 70 % efficient, the new one is 1.18×70=82.6 % efficient). Consumption is inversely proportion-al to efficiency (Sorrell and Dimitropoulos 2008), so thereduction in consumption (excluding rebound effects) is(100/70—100/88)/(100/70)=20.45 %.3

This is a theoretical figure, and to work out the actualconsumption reduction due to this energy efficiencyincrease, we must consider the rebound effect.4 The‘rebound effect’may be defined as the energy efficiencyelasticity of demand for heating consumption (Madlenerand Hauertmann 2011; Milne and Boardman 2000;Sorrell and Dimitropoulos 2008; cf. Haas and

Biermeyer 2000). Studies consistently show that whenthe energy efficiency of domestic heating increases, aportion of the increase goes to energy consumptionreductions, while the remainder goes to increasing ‘en-ergy services’, such as indoor temperature or ventilationrate. Studies for Germany show energy rebound effectsof around −0.68 which means that 68 % of the energyefficiency improvement leads to energy consumptionreduction while the remaining 32 % goes to increasethe take of energy services (Galvin 2013a;Madlener andHauertmann 2011; cf. Rehdanz 2007). This would im-ply that the 20.45% theoretical decrease in consumptionled to actual consumption reductions of 13.9 % for thedwellings that had boiler replacements.

Hence, the reduction in consumption in the Germandomestic housing stock due to boiler replacementswould have been 13.9 % of 34.1 %=4.74 %, or32.8 TWh. Hence, we set the variable D=32.8 TWh.

Reductions due to nontechnical or unexplainedfactors

We now have sufficient information to estimate thesavings in 2000–2011 that are not explained by thermalretrofits or new builds. These are represented by thevariable L, where (recalling Eq. 1):

L ¼ E–I ¼ A–B–C−Dð Þ– F–G–Hð Þ¼ A–B–D–F þ Gð Þ – C–Hð Þ¼ A–B–D–F þ Gð Þ – K

The values for A, B, D, F and G are known to a highdegree of certainty. For K, we have estimated upper andlower bounds and a middle, or most likely value. Thehighest value of K will give the lowest estimate for L:

Lmin ¼ 669–28:1–32:8–564þ 26:7ð Þ–25:7¼ 45:194:1 TWh :

The lowest value of K will give the highest estimatefor L:

Lmax ¼ 669–28:1–32:8–564þ 26:7ð Þ–18:1¼ 52:7 TWh :

This would suggest that heating fuel reductions in2000–2011 that are not attributable to retrofits, windowreplacements, heater/boiler upgrades or new builds arelikely to be in the range 45.1–52.7 TWh. This wouldrepresent 43.0–50.2 % of the total reductions, or a

3 A relative energy efficiency increase of 18 % does not lead to an18% reduction in energy consumption. This can be illustrated by asimple example: a boiler with an efficiency of 45 % is replaced byone with 90 % efficiency. This is a doubling of efficiency, i.e. anefficiency increase of 100 %. But the heating consumption of thedwelling does not decrease by 100 %, i.e. to zero.4 For thermal upgrade measures on the building envelope(‘Consumption reduction per retrofitted dwelling’), the reboundeffect is taken into account implicitly in that actual consumption,rather than theoretical, calculated consumption, is estimated.

Energy Efficiency

middle value of 46.6 %. The percentage fall in con-sumption within this sector would be:

Q ¼100� E–Ið Þ =E¼ 100� L= A–B–C–Dð Þ¼ 100� 45:1=540:5 ¼ 8:3% lowestð Þ or 100� 52:7=535:7 ¼ 9:8% highestð Þ

The lowest estimate would result from the highestvalue of K and the highest of C, i.e. 8.3 %. The highestestimate would result from the lowest value ofK and thelowest ofC, i.e. 9.8 %. Table 4 gives all the quantities ofheating energy calculated.

A cross-check on our results is possible by con-sidering another body of research that tracks house-hold consumption responses to fuel price changes.The price of heating fuel rose by 58.9 % during theperiod 2000–2011 (BMWi, 2011a; b). The reduc-tions that are not due to technical factors, i.e. 8.3–9.8 % of their 2000 value, would therefore repre-sent a long-run fuel price elasticity of demand of−0.14 to–0.17 in respect of heating consumptionreductions that were not due to technical measures.This is lower than the likely band of fuel priceelasticities reported in Rehdanz (2007) and at thelower end of their equivalent as energy efficiencyelasticities (Sorrell and Dimitropoulos 2008) report-ed in Madlener and Hauertmann (2011). In otherwords, using fuel price elasticity as a guide to findthe percentage of reductions due to nontechnicalfactors would lead to the suggestion of significantlygreater reductions due to nontechnical factors thanhave been found in this analysis. This would sug-gest that the analysis has not overestimated thisportion of the total reductions.

Discussion and conclusions

There is a wide range of uncertainty in two of ourvariables: the pre- and post-retrofit consumption ofhomes that benefitted from thermal retrofits in the 11-year period. This leads to a wide range of uncertainty inthe quantity of energy saved through thermal retrofits(and other window replacements) carried out in the 11-year period, i.e. 18.1–27.5 TWh. However, this range isnot so great as to swamp the likely range of values ofresidual consumption reductions that do not seem to

have come from technical changes.5 As we have seen,allowing for the highest estimate of savings throughretrofits still leaves a gap of 46.3 TWh or 44.1 % ofthe total reductions, that does not seem to be explainedby any of the technical measures considered. This gapcould be as large as 52.7 TWh, or 50.2 % of the totalreductions, if the reductions though retrofits were at thesmaller end of the range.

Hence, we can conclude that our hypothesis stands,namely, that not all the reductions in domestic heatingenergy consumption in 2000-2011 can be explained bytechnical factors. It appears that a considerable por-tion—possibly around 45–50 %—are not explained bythe effects of thermal retrofits, replacement of boilers/heating systems, or the replacement of abandoneddwellings with new builds.

This does not necessarily imply that all the unex-plained reductions took place in homes that had noretrofits or boiler replacements. Some of the nontechni-cal reductions could have taken place in homes whichalso produced reductions from technical upgrades, andsome could have taken place in new builds in the yearssince their completion. The figures merely imply that alarge proportion of the reductions seem to have occurredas a result of nontechnical factors, without prejudice towhich sort of homes these factors pertained in.Nevertheless, since homes that had little or no technicalimprovement consumed the largest quantity of heatingenergy, these were most likely the homes in which thelargest reductions through nontechnical means tookplace, even if every home made the same proportionalcontribution to nontechnical reductions. An 8–10 %reduction in energy consumption in a 1950s home con-suming 20,000 kWh per year is much greater than an 8–10 % reduction in a low-energy dwelling consuming4,000 kWh per year.

A further interesting point is that the reductions dueto replacement of abandoned dwellings with new buildsappear to be very small, at 1.4 TWh, or 1.3 % of the totalreductions. This is because nearly twice as many newhomes were built in 2000–2011 as the number that wereabandoned, even though the population fell in 2000–2011 from 82.8 to 80.2 million (BMVBS, 2012;

5 An anonymous reviewer has pointed out that: ‘the methodologyhas some strong limitations, however is acceptable given the lackof detailed information required for such an assessment. Even withthe limitations, the analysis gives a good idea about the directionsand magnitudes of different components of decline of heating inGerman buildings’.

Energy Efficiency

Destatis 2013b). This accords with the demographictrend of smaller households. Average household sizefell during this period from 2.16 to 2.02 persons perhousehold (BMVBS, 2012). This trend should lead tocaution in predicting that significant nationwide reduc-tions in domestic heating energy consumption are likelyto result from increasing the rate of construction of newbuilds (though there may be other good reasons toincrease this rate). Our analysis has shown that signifi-cantly greater reductions have come from both thermalretrofits and heating system upgrades.

It remains to be asked what factors might be causingthe reductions that are not explained by technical fac-tors. To begin with, demographic trends seem to play arole. The reduction in household size implies that manyexisting homes are becoming emptier: the number ofbedrooms in Germany is increasing as the population isdecreasing. This suggests that many homes had moreempty rooms in 2011 than they had in 2000. It wouldseem to follow that many of these householders wouldnow heat less of their living area, and possibly also forless time per week, because with fewer occupants, ahouse is likely to be more empty more often and forlonger periods. In a sense, many old homes areexporting some of their heating consumption to newbuilds, as former household members take up residencein them.

A further factor is price elasticity of demand forheating fuel. With studies consistently showing elastic-ities of between −0.2 and −0.5, this implies that house-holds, on average, reduce their consumption in responseto fuel price rises. One concern here is that this could be

leading to fuel poverty. The German Energy Agency’sanalysis of the heating consumption of a sample of35,000 German homes shows that actual consumptionis, on average, 30 % lower than dwellings’ calculatedenergy rating (DENA 2012b). This accords withSunikka-Blank and Galvin’s (2012) analysis, whichadds that the percentage gap increases considerably forthe less energy-efficient homes. This lends weight to thesuggestion that households take less energy serviceswhen the cost is high. Some of the datasets used inSunikka-Blank and Galvin’s (2012) analysis and inSimons (2012) show very low consumption for somehomes with very high energy ratings, indicating thatmany people could be failing to heat their homesadequately.

However, the nontechnically induced reductionsmight also indicate that some households are becomingmore skilled at getting the level of energy services theywant, without consuming a great amount of fuel. Thesehouseholds may be heating fewer rooms, or for shorterperiods, or to lower temperatures, or some combinationof these, or ventilating less adequately, a possibilityexplored in Galvin (2013a). There is anecdotal evidenceof some households making greater use of curtains andusing off-the-shelf measures such as draft-stripping orlaying insulation materials on the loft floor. These smallthermal improvements do not generally show up instatistical studies.

Some households may be motivated by economicthrift, environmental concern, or other factors aboutwhich we do not yet have clear knowledge. It might bepossible to learn from such people, as some of the works

Table 4 Estimates of quantities of heating energy for each variable in the analysis

Symbol Estimate (TWh) Quantities of heating fuel consumed in 2000 by Quantities of heating fuel consumed in 2011 by

A 669.0 All dwellings

B 28.1 Dwellings abandoned in 2000–2011

C 62.1–66.9 Dwellings retrofitted in 2000–2011 (insulation & windows)

D 32.8 Dwellings with new boilers/heating systems in 2000–2011

F 564.0 All dwellings

G 26.7 Dwellings constructed in 2000–2011

Changes in quantities of heating fuel 2000–2011 due to

M 15.0 New builds replacing abandoned dwellings

K 18.1–25.7 Dwellings retrofitted in 2000–2011 (insulation & windows)

N 11.7 (additional) New builds additional to replacements

P 1.4 Net saving through new builds and abandonments

L 45.1–52.7 Reductions due to nontechnical factors

Energy Efficiency

cited above appear to have found that user behaviourplays just as important a role in heating energy con-sumption as technical measures play. More research isneeded to identify the reasons for this steady reductionin heating energy consumption, particularly among lessenergy-efficient homes, and whether it indicates increas-ing fuel poverty, smart heating skills, growing ecologi-cal awareness, or some combination of these, inGermany.

This paper also suggests that German householdersare saving considerable sums of money through non-technical means of heating fuel consumption reduction.The 45.1–52.7 TWh saved in this way equate to a savingof around 3.8–4.4 billion euros in 2011 (assuming€0.08/kWh), and around 10 million tonnes of avoidedCO2 emissions (assuming 0.0002 t/kWh). These con-siderable savings would tend to support the suggestionthat the specific causes of nontechnically inducedheating fuel reductions should be systematicallyinvestigated.

Acknowledgments The authors wish to thank six anonymousreviewers for their careful and detailed assistance in enabling thisstudy to develop to its full extent

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