This article is based on a paper published in the 56th volume of the Research Group - Energy efficiency and renewable energy: Conflict of goals or synergy?. The original article by Tanja Schulz, is availabe in German in the proceedings, which can be accessed here.

In 2010, the European Union defined nearly zero-energy buildings (NZEB) in the Energy Performance of Buildings Directive [Directive 2010/31/EU] (“EPBD”) as follows: A “near-zero energy building” is a building that has a very high (…) overall energy efficiency. The near-zero or very low energy demand should be met through energy from renewable sources to a very significant extent, including energy from renewable sources generated on site or nearby. This deliberately vague wording raises the question of how an energy demand of “almost zero” and a “very significant proportion” of renewable energy supply are precisely defined. In Germany, the Building Energy Act (GEG) came into force on 1 November 2020. It stipulates that the standard for new buildings that had applied up till then (EnEV 2016) also represents the German definition of the NZEB standard. In addition, many buildings are being implemented to other energy efficiency standards, one of which is the Net Zero standard in particular, but also the Effizienzhaus 55 and 40 standards and last but not least, the Passive House standard.

This article will show which efficiency standards are suitable for achieving the goal of a climate-neutral building stock and what matters in this regard.

According to the German Government's statements in 2010, it is a “key goal (…) to reduce the heating demand of building stock in the long term with the aim of achieving a climate-neutral building stock by 2050. Climate-neutral means that the buildings only have a very low energy demand and the remaining energy demand is predominantly met through renewable energy sources. This requires a doubling of the energy retrofit rate from around 1 % to 2 % per year” [BMWI 2010].

This statement was substantiated in the Energy Efficiency Strategy for Buildings of 2015. “This means that the primary energy demand must be reduced by a combination of energy savings and the use of renewable energies by 2050 in the order of 80 per cent compared to 2008” [BMWI 2015].

In the context of several studies it was investigated whether, and how, the goal of a climate-neutral building stock can be achieved. This was already documented in [Feist 1999] more than 20 years ago and recently confirmed in [Schnieders et. al. 2021]. The 2010 study “Energy target 2050: 100 % electricity from renewable sources” [UBA 2010] also concluded that generation based entirely on renewable energies is technically possible and economically advantageous, provided that the existing savings potential is utilised. Final energy consumption in the building sector is to be reduced to 105 TWh by 2050, which means a reduction by 590 TWh compared to 2008 (Fig. 1).

However, the energy consumption of private households was still 644 TWh in 2018 (Fig. 2). Even though the balance boundaries of the two sources are not quite identical and the figures are therefore not directly comparable, it is clear that the efforts made to date are far from sufficient.

The same conclusion was reached in another recent study, which examined the expected reduction in greenhouse gas emissions based on various scenarios. The greenhouse gas reduction effect was estimated for the various sectors based on the 2030 Climate Protection Programme (January 2020). The authors concluded that, based on the current climate protection programme, the climate protection targets for both 2025 and 2030 will not be met.

“The shortfalls are particularly great in the transport and building sectors”. The fact that the reduction in emissions falls well short of expectations is primarily due to the fact that previous forecasts assumed a tightening of the EnEV standard to approximately the KfW 55 standard, which with the introduction of the GEG was not implemented. In addition, reconstruction activity and the effects of the decline in the heat demand due to increased replacement of old heating systems are lower than predicted. The current measures and above all CO₂ pricing are therefore not enough to achieve the target for 2030 [UBA 2020]

Fig. 1: Final energy consumption of private households (left) in 2005, 2008 and 2050 ¹⁾ [UBA 2010]

Fig. 2: Development of final energy consumption of private households from 1990 to 2018 [UBA / AG Energiebilanzen 2020]

Fig. 3: Greenhouse gas emissions of the building sector: target value and scenario of the “Climate Protection Programme (KSPr Jan 2020)”. The shortfalls for the year 2030 are almost 23 % [UBA 2020]

It follows from the cited studies that significantly greater efforts will be required in the building sector to achieve both the 2050 climate protection targets and the interim targets for 2025 and 2030, which extend far beyond the measures that have been adopted.

It can be deduced from this that the current energy efficiency standard for buildings (GEG), which also represents the German definition of NZEB in accordance with the EPBD, is unsuitable for initiating the urgently necessary CO₂ reduction measures for climate protection.

In addition to the currently applicable GEG (as of 2020), other efficiency standards, some of which are significantly more ambitious, are being implemented in the context of energy-saving construction. The definitions and a critical analysis of the standards relevant to this article are briefly described below.

The GEG is the currently applicable energy efficiency standard for buildings in Germany. It replaced the EnEV in autumn 2020, although the energy efficiency requirements were not tightened any further. The balancing procedure is based on a reference building method. The energy requirements for space heating and cooling, hot water generation and ventilation are determined using a reference building with the same geometry, useful area of the building and orientation as the building to be constructed. This can lead to significantly different primary energy demands for buildings with the same use and almost the same amount of space, as they differ in terms of orientation, arrangement and size of the window area and compactness. The focus of the EnEV is on limiting the primary energy demand. In addition, requirements are also placed on the quality of the building envelope (H‘T), which only ensure a minimum level of thermal protection. The moderate secondary requirement for the building envelope in particular is subject to criticism as it is neither technically nor economically sensible and wastes valuable potentials. For example, the reference building has U-values of 0.28 W/(m²K) for the outer wall and 1.3 W/(m²K) for the windows. In addition, limiting only the transmission heat losses makes further efficiency measures such as controlled ventilation with heat recovery, or even optimisation of the design in terms of compactness and solar gains far less attractive. A less energy efficient design even receives a bonus when this procedure is followed. [PHI 2016].

The primary energy demand can be met by combining am excellent building envelope with little effort for a renewable energy supply, or moderately efficient insulation of the building can be supplemented with building technology with a good primary energy assessment.

The quality of the building envelope and the ventilation system is decisive for the level of the heating demand. However, the primary energy assessment has the result that different heating demands are possible for the same building geometry - depending on the choice of energy source.

With the amendment in 2013, the permissible annual primary energy demand was reduced by 25 % (with regard to the requirements of EnEV 2009). This tightened requirement came into force in 2016, so it is generally referred to as EnEV 2016, which then became the GEG. This means that the GEG standard, i.e. the German implementation of the nearly zero-energy building, is actually the EnEV 2013.

The net-zero or zero-energy standard describes buildings in which the energy required for heating, hot water and household electricity throughout the year is produced by the buildings themselves. If this balance is positive, i.e. the building produces more energy than it consumes, this is referred to as a plus-energy building.

The so-called “Volume Deal”, a joint declaration of intent by the housing and construction industries, focuses on the net-zero or zero-energy standard for building renovation. This means that zero-energy or net-zero buildings are seen as an important component of energy-efficient construction in general. However, the net-zero or zero-energy standard is not an efficiency standard in the true sense of the word, as a minimum level of efficiency is not defined.

The fact that the balance boundaries and the calculation method are not standardised is also problematic. This applies especially to the quite relevant domestic electricity. There is also the question as to the extent to which annual balances are suitable for analysing nearly zero-energy buildings, especially when generation and demand differ greatly in terms of time. This is analysed in more detail in the following sections.

The subsidy programmes of the development loan corporation KfW in Germany have been successfully setting standards for a high level of energy efficiency for many years. The “Effizienzhaus 55”, “Effizienzhaus 40” and “Effizienzhaus 40 Plus” programmes (Efiizienzhaus: efficiency house) will also be continued as part of the new federal funding programme for efficient buildings (BEG) introduced in March 2021. Funding is provided for buildings with a primary energy demand that is lower ²⁾ by 27% (EH 55) or 47% (EH 40) in accordance with the GEG.

For the plus-energy variant, both an electricity-generating system and a ventilation system with heat recovery must be provided. In addition, generation and consumption must be visualised, which is a good quality assurance measure. The at first seemingly confusing designation of the Effizienzhaus standards is due to the fact that the permissible PE requirement according to the GEG has already been reduced by 25 % compared to that of the reference building. Subsidised efficiency houses are calculated according to the procedure in the GEG, so that the basic criticism of the procedure also applies here and has an even more relevant impact.

The annual heating requirement based on useful energy is limited to 15 kWh/(m²a) regardless of the building geometry and type. The useful energy, as energy that is available to the space for heating purposes, is a good indicator of the efficiency of the building. In addition to the heating demand, the primary energy demand is also limited, because the hot water demand and domestic electricity become more relevant when the heating demand is very low.

As energy efficiency and renewable energy go well together, this combination has been promoted by the introduction of Passive House classes: Passive House Plus and Passive House Premium. The building can be optimised either by further reducing the energy demand or by using renewable energy generation. The core criteria of Passive House efficiency for the building (heating and cooling) remain unchanged, true to the motto “Efficiency First”.

With regard to renewable energy production, the approach used in the Passive House classes expressly does not advocate the simple annual offsetting of energy demand and energy production on site (see article on primary energy assessment with the PER system in this Protocol Volume). Direct balancing of demand and production would neglect important aspects such as energy losses through storage and the availability of space for the production of renewable energy. Instead, energy production is related to the floor area occupied by the building. The aim is to optimally utilise the possibilities available in the specific project (see Section 4.2).

Fig. 4: Passive House Classic, Plus and Premium. The use of renewable energy sources is assessed on the basis of an excellent level of energy efficiency in each case.

The Passive House proves that a high level of efficiency and a renewable energy supply complement each other perfectly, because only when energy demands are very low does it become technically and economically viable to cover a significant proportion of the remaining energy demand with energy generated locally from renewable sources.  

The influence of the selected heating system on the heating demand is analysed below based on the example of a terraced house. A terraced house in Hanover-Kronsberg that was built to the Passive House standard was used as a model (Fig. 5) [PHI 2001]. The building was adapted for new builds in accordance with GEG (EH 55 and EH 40) in terms of the quality of the building envelope in accordance with the efficiency house criteria The building is adapted in terms of the quality of the building envelope in accordance with the efficiency house criteria for new builds based on GEG (EH 55 and EH 40). The minimum requirements (H‘T) according to EnEV 2016 are complied with. In the Passive House variant, the useful energy for heating is limited to 15 kWh/(m²a) in accordance with the Passive House criteria.

Fig. 5: Passive House building in Kronsberg: elevation and cross-section of the example building

Due to the GEG's focus on limiting the primary energy demand, identical buildings can have very different properties of the building envelope and thus different useful energy demands depending on the chosen energy source. A comparatively poor quality of the building envelope in terms of energy can be compensated for by a good supply of renewable energy using the assessment procedure in the GEG.

If the same variants are analysed on the basis of final energy, the differences become even more significant. It can be seen that the final energy demand can be very different for the EH-55 or EH-40 standard. The differences between the Passive House variants are small in comparison.

Fig. 6: Useful energy for the terraced house with the energy efficiency standards Effizienzhaus 55 (EH 55), Effizienzhaus 40 (EH 40) and Passive House. The useful energy demand for heating and hot water provision varies for the efficiency houses depending on the supply solution. The H’T requirement is met in all variants. The useful energy demand for space heating is limited to 15 kWh/(m²a) for Passive Houses and is therefore independent of the supply variant. The red line describes the useful energy demand of the Passive House variant for heating and hot water. Note: Calculation of the useful energy demand using the planning tool [PHPP], on the basis of the living area.

Fig. 7: Final energy for the terraced house with the energy efficiency standards Effizienzhaus 55 (EH 55), Effizienzhaus 40 (EH 40) and Passive House. The final energy demand for heating and hot water provision varies depending on the supply solution. The H’T requirement is met in all variants. The red line describes the final energy demand of the best Passive House variant for heating and hot water. Note: calculation of the final energy demand using the planning tool [PHPP], on the basis of the living area.

For the Passive House variant, five different supply solutions were evaluated in terms of their primary energy demand. As expected, the solutions with a renewable share perform significantly better than those with fossil fuels. The national climate protection plan for Germany recommends limitation of the primary energy demand for heating and hot water to 40 kWh/(m²a). This specification is easily met with the supply solutions using renewable energy as shown. This shows once again that a high level of energy efficiency allows leeway in the choice of the supply solution.

Fig. 8: Comparison of the primary energy demand (non-renewable) of the terraced Passive House in Hanover for five different supply solutions.

Building retrofits and new buildings are mostly designed for lower efficiency standards than the Passive House standard and are equipped with renewable energy generators (e.g. photovoltaics). This is not a viable solution for a sustainable climate-neutral building stock, as the following considerations will show. Two different efficiency standards are analysed in more detail using the terraced house already mentioned: the net-zero and Passive House standards.

The net-zero or zero-energy standard is deemed to have been achieved if the building energy balance over the year is zero. The electricity demand for space heating, hot water and household applications is compared with the electricity generated by the PV system installed on the roof. By definition, the net-zero standard does not include any requirements for the efficiency of the building. The statutory minimum standard (GEG 2020) is therefore applicable. This means that the net-zero or zero-energy building has a significant heating energy demand in the winter months. For the GEG standard building under consideration, the PV system on the roof on its own is not enough to meet the heating energy demand. Additional PV areas, e.g. on the façade etc. would have to be added to achieve net zero. On closer examination however, it becomes clear that the gap in coverage is actually much larger, because PV generation only covers a fraction of the demand in the main winter months (November to February). Due to the lower availability of solar radiation, the electricity demand arising in winter cannot be directly met by the PV system. Either storage systems must be installed to store the summer PV surplus for use in the winter, or other renewable energy sources must be utilised which are available in the winter (e.g. wind power).

For buildings with a high heating demand, the storage tanks would have to be correspondingly large, which would result in further investment costs. In addition to the electricity demand for heating, DHW and domestic uses, there are also storage losses that are not taken into account in the net-zero analysis. The actual amount of electricity that would have to be transferred to the winter is therefore greater by the amount of these storage losses than is shown in the balance.

Alternatively, this demand could also be met through other renewable energy sources. However, for the capacities to be sufficient, it is necessary to rely on buildings with a high level of energy efficiency.

Fig. 9: Electricity demand of an end-of-terrace house built according to the GEG standard, for heating and hot water and domestic applications (blue columns) and electricity generation via a PV system installed on the roof (green). Net zero is therefore not achieved despite the large-scale PV system: coverage gap of the annual balance 6.3 kWh/(m²a) or 748 kWh/a

The lower the average efficiency of the building stock, the higher the demand and the more renewable energy must be generated, e.g. using wind power. In the light of the currently hotly debated regulations for the expansion of wind power, it is clear that high efficiency will relieve the burden on supply systems and at the same time it will improve supply security.

The potentials of energy efficiency become apparent if the same building is considered, this time with the Passive House standard. Here too, the energy demand in the main winter months cannot be met solely by the energy generated by the building. Compared to the demand of the GEG building, however, the coverage gap is significantly smaller.

Fig. 10: Electricity demand of an end-of-terrace house with the Passive House standard for heating, hot water and domestic applications (blue columns) and electricity generation via a PV system installed in the roof (green). Net-zero is thus just about achieved: surplus +17.8 kWh/(m²a) or +2,100 kWh/a. The electricity requirement of the GEG variant is highlighted in red

The lower domestic electricity demand must be taken into account when calculating the balance. To achieve the Passive House standard, typical household appliances such as refrigerators and freezers with a high level of energy efficiency must be foreseen. Lighting must also be optimised. In practice, this leads to lower electricity consumption on average, as has been assumed here for the Passive House variant.

A second example will demonstrate clearly that the approach with the annual offsetting of the energy demand and energy generation on site is not expedient.

Fig. 11: Apartment block with 20 residential units. The sloping roof ideally faces south and is covered to maximum with PV modules

Multi-storey residential buildings provide living space for many people, have a comparatively low consumption of resources and space and with their very good SA/V ratio, they offer ideal conditions for energy-efficient construction. The net-zero standard systematically discriminates against this type of building, because achieving a good annual balance is much more challenging here as only a small area of the roof is available in relation to living space. While a small building with a low efficiency standard can easily generate enough electricity to meet its own annual requirements, multi-storey residential buildings would either have to develop additional PV areas with lower yields and therefore lower efficiency (e.g. on balcony parapets or wall surfaces), or they would have to be implemented with a much higher level of energy efficiency.

Fig. 12: Schematic illustration of the use of renewable energy sources in single-family houses and multi-storey buildings. The latter offer smaller roof areas compared to the living space. It is much more difficult to meet the demand with energy generated on site near the building.

It makes more sense to consider energy generation in relation to the building footprint, while the energy demand is related to the energy reference area. The aim of this approach is therefore deliberately not to compare generation with demand, but to evaluate the utilisation of the available possibilities on site.

The building will be considered in accordance with the GEG standard in the same way as the example of the terraced house. Fig. 13 compares the electricity demand for space heating, hot water generation and domestic uses with the PV generation. It is very clear here that the energy generated on site is nowhere near sufficient to meet the demand.

Fig. 13: Electricity demand of a multi-storey residential building with the GEG standard for heating and hot water and domestic applications (blue columns) and electricity generation via a PV system installed on the roof (green). Net-zero cannot be achieved with this standard: coverage gap 36 kWh/(m²a) or 59,000 kWh/a

In comparison with this, the consistent implementation of high energy efficiency leads to a significant reduction of the winter peak. The additional energy to be provided externally in winter is many times lower in the Passive House variant (Fig. 14).

Fig. 14: Electricity demand of a multi-storey residential building with the Passive House standard for heating and hot water and domestic applications (blue columns) and electricity generation via a PV system installed on the roof (green). The electricity demand of the GEG variant is highlighted in red.

Climate-neutral building stock with the Passive House standard

In principle, all efficiency standards that enable a fully renewable energy supply system based on a highly energy-efficient building are suitable for achieving the goal of a climate-neutral building stock. In contrast, standards that merely appear to achieve climate neutrality are unsuitable. In particular, the unevaluated offsetting of demand and generation on the basis of a net annual balance is misleading, as essential aspects of the energy supply and the question of energy storage are not taken into account.

Buildings with one to two storeys can be equipped with an appropriate photovoltaic area even with the minimum energy standard, so that in purely mathematical terms (net) electricity demand and generation are equal. Energy supply in the winter half-year must then be ensured either through large storage systems or other renewable generators. Here it becomes clear that only a high level of efficiency can ensure economically and ecologically sensible and socially acceptable energy supply in the winter months. In contrast, moderate efficiency requirements in accordance with the GEG would necessitate gigantic wind farms or expensive storage solutions that are associated with losses.

The influence of energy efficiency on a renewable energy supply system is apparent based on the example of a multi-storey residential building. The GEG standard (Fig. 15, above) and the Passive House standard (Fig. 15, below) are considered. In both cases, the demand cannot be covered solely by the PV system on the building. In the winter months, the demand of the GEG multi-storey residential building is more than twice as high as that of the Passive House building. While the demand in the Passive House building is 2.5 kWh/m² in January (with approximately 0.5 kWh/m² PV generation), the GEG building must cover almost 7 kWh/m² either via storage or other renewable energy systems (e.g. wind power). This comparison clearly demonstrates that the dimensions of the renewable solutions necessary for the “winter shortfall” depend very much on the energy efficiency of the building (this also applies for small buildings such as detached houses and may be even more pronounced there). A high level of efficiency allows room for manoeuvre and thus creates the conditions for a sustainable supply of renewable energy.

Fig. 15: Electricity demand for space heating, hot water generation and domestic electricity of a multi-storey residential building (20 apartments) with the GEG standard (top) and the Passive House standard (bottom). Heat supply is ensured monovalently via a heat pump with a coefficient of performance of 3.

In addition to the question of the dimensions that renewables must achieve to cover the energy demand for the building sector, the latest studies make it clear that the efforts made so far to reduce CO₂ emissions are not sufficient to achieve the climate protection targets ([UBA 2020]). It is still the case that “generation based entirely on renewable energies is technically possible and economically advantageous, provided that the existing savings potentials are utilised” ([UBA 2010], see also [Feist 1999] and [Schnieders et al. 2021]). Two measures can be derived from this: in the future buildings should be implemented with at least the economically necessary high level of energy efficiency. Only then will it be possible to supply them with locally usable renewable energy. 

[UBA 2010] Energieziel 2050: 100% Strom aus erneuerbaren Quellen (Energy target 2050: 100% electricity from renewable sources); Thomas Klaus, Carla Vollmer, Kathrin Werner, Harry Lehmann, Klaus Müschen; published by: UBA, July 2010, (https://www.umweltbundesamt.de/publikationen/energieziel-2050)

[Feist 1999]: Energieeffizienz Wohlstand Lebensqualität (Energy efficiency, prosperity, quality of life). In: Conference Proceedings of the 3rd International Passive House Conference Dornbirn / Darmstadt 1999.

[Schnieders et al. 2021] Schnieders, J.; Feist, W.; Grove-Smith, J.; Krick, B.: Der Weg zu einem klimaverträglichen Gebäudebestand. Einführungsvortrag (The path towards a climate-friendly building stock, keynote speech) at the International Passive House Conference 2021. Passive House Institute, Darmstadt, 2021.

[UBA / AG Energiebilanzen 2020] Energy consumption data for Germany: https://www.umweltbundesamt.de/daten/private-haushalte-konsum/wohnen/energieverbrauch-privater-haushalte#endenergieverbrauch-der-privaten-haushalte

[UBA 2020] Treibhausgasminderungswirkung des Klimaschutzprogramms 2030 (Kurzbericht) (Greenhouse gas reduction potential of the climate protection programme 2030, brief report), Ralph O. Harthan et al.; published by: UBA, March 2020

[PHI 2016] Schnellere Etablierung neuer technischer Standards inkl. Entwicklung von Systemkomponenten für Bau- und Anlagentechnik (Faster establishment of new technical standards and development of system components for construction and systems engineering), Oliver Kah, Tanja Schulz Kristin Bräunlich, Andreas Grill, Adrian Muskatewitz, Dr. Jürgen Schnieders, PHI, Darmstadt 2016

[PHI 2001] Klimaneutrale Passivhaus-Reihenhaussiedlung Hannover-Kronsberg (Climate-neutral Passive House estate Hanover-Kronsberg); Wolfgang Feist, Søren Peper, Matthias von Oesen; Hanover 2001

[BMWI 2010] Energiekonzept für eine umweltschonende, zuverlässige und bezahlbare Energieversorgung (Energy concept for an environmentally-friendly, reliable and affordable energy supply); published by the German Federal Ministry of Economic Affairs and Technology (BMWi) 2010; page 22.

[BMWI 2015] Energieeffizienzstrategie Gebäude (Energy efficiency strategy for buildings); published by the German Federal Ministry of Economic Affairs and Energy (BMWi) 2015; page. 9.

[Richtlinie 2010/31/EU] European Commission: Directive 2010/31/EU Energy Performance of Buildings Directive (EPBD) recast, Brussels, 2010

Energy efficiency and renewable energy – past and present

Energy efficiency and renewable energy