What is a low-energy construction and how is it defined?

01 July 2008 - Comments (2) Construction

What is a low-energy construction and how is it defined? The concept of low-energy is widely used for houses and buildings in many different ways. What does it means exactly and how is it defined? What energy is taken into account? What is primary and electric energy? In this article, we answer those questions and we explain further how is low-energy normalized and regulated across Europe.

What does low-energy means?

Low-energy is always linked to the amount of primary energy needed in a dwelling for providing comfortable living to its occupants. To be low-energy, that amount of energy must be below a given threshold. The maximum energy spending is defined by norms which vary from country to country and that likely evolve over time. What was considered low-energy yesterday is no more today, and what is low-energy today might not be tomorrow.

To measure energy spending uniformly, from one dwelling to the other, it is always given in term of primary energy spending per square meter and per year. The amount of primary energy is expressed in kilowatt hour of primary energy. The unit is therefore: kWhpe⁄m².year

What types of energy consumption are counted?

It depends of the energy label. There are several usages of energy in any dwelling:

  1. energy for heating
  2. energy for cooling and/or complementary heating
  3. energy for producing domestic hot water
  4. energy for ventilation
  5. energy for lighting
  6. energy for all electrical appliances (from fridge to TV)

To be properly defined, the notion of low-energy has to specify what type of energy consumption is taken into account. Most often, low-energy refers to the heating energy only or to the energy spent for heating and hot water. Sometimes, it is the energy spent for heating, cooling, hot water, ventilation and lighting. Of course, depending of what consumption is taken into account, the energy threshold will vary.

Monthly energy consumption by use in a regular house in the USA

Monthly energy consumption by use in a regular house in the USA (source Zero Energy Design)

Comparing low-energy labels is not easy for that reason. They usually do not take into account the same thing. For instance, in countries where cooling is not an issue, it may not be counted. For countries where cooling represents a sizeable amount of energy consumption (like in Serbia), it would be misleading not to include it into the equation.

What is primary energy?

As we said above, the energy spent in a dwelling is always measured in term of primary energy. For instance, when coal or natural gas is used for heating, that is primary energy from fossil fuel directly converted into heat.

Primary energy is energy before transformation. Primary energy essentially originates in fossil fuels (oil, natural gas, coal and uranium) or is captured from renewable sources (hydro, wind, solar, geothermal).

What is electric energy?

In any dwelling, it is usually a combination of primary energy and electric energy that provides all energy needs. Electricity is a secondary energy that is generated using primary energy, mainly coal, hydro-electric and nuclear power. Because of losses during the production and transport of electricity, not all primary energy used to produce electricity is converted into usable electric energy. In fact, it's quite the opposite. Less than 30% of the primary energy used to produce electricity is converted into useable electric energy.

US energy flow 2006

US energy flow 2006 in Exajoules (source Lawrence Livermore National Laboratory)
This Sankey diagram shows the sources and use of energy in the United States in 2006 in Exajoules. Electricity generation was mainly done using coal and nuclear. Out of 38 Exajoules of primary energy used to produce electricity, 25.54 Exajoules were wasted in losses, that is 69% of losses, mainly in heat energy. Globally, more than half of the energy used in the USA goes into useless losses. That figure is similar across all industrialized countries.

There is a lot of waste in the process of producing electricity. The only way not to loose so much energy, is to recuperate and use the heat produced during the generation. That is called cogeneration but it is not widely used yet.

How is electric energy counted in term of primary energy?

To count how much primary energy is used in a dwelling, it is necessary to convert the electric energy that is used into primary energy. The losses occuring during the production of electricity are taken into account with a conversion factor. That factor is applied when converting kilowatt-hour of electric energy into kilowatt-hour of primary energy. It varies from 2 to 3 depending of the country. It means that using 1kWh of electric energy in a dwelling, account for 2 to 3 kWh of primary energy in the energy balance.

For this very reason, using electricity for heating is very bad in term of primary energy consumption. Getting 10kWh of heating energy from an electric heater consumes 20 to 30kWh of primary energy. Getting 10kWh of heating energy from a gas boiler would only consume 11 to 12KWh of primary energy (gas), taking losses into account.

What is renewable energy?

Renewable energy is energy that cannot be depleted because the source of energy is not affected by its use or because the source is naturally replenished. The largest use of renewable energy in the world is hydroelectricity. For dwellings, the most common use of renewable energy is through geothermal, solar, wind and biomass energy.

Source of energy, primary and secondary, renewable and non-renewable

Source of energy, primary and secondary, renewable and non-renewable (source Energy Information Administration (EIA), Energy Kid's page)

Renewable energy can be used very efficiently to heat or cool dwellings or to produce hot water or electricity. Using renewable energy locally does account in primary energy use, but it cannot be counted the same way as using primary energy generated from fossil fuels. In fact, local use of renewable energy is mostly free, does not contribute to global warming and protects from the rising price of energy.

Depending of the low-energy label, the use of renewable energy is either not counted at all or partially counted in the energy assessment of one dwelling. Therefore, using renewable energy is a way to lower energy consumption.

What are the European norms defining low-energy labels?

There are many labels defining what is a low-energy construction. Most of the european countries have basically defined one, although some labels such as Passivhaus have found application in more than one country. Below we give 4 examples. We see that the type of consumption taken into account and the energy consumption limit vary from one label to the next.

Country FranceGermanyItalySwitzerland
Norm Effinergie logo
Passivhaus logo
CasaClima logo
Minergie logo
Energy consumption taken into account Heat
Hot water
Auxiliaries (cooling/complementary heating)
Hot water
Auxiliaries (cooling/complementary heating)
Heat Heat
Hot water
Maximum spending of Primary Energy in kWhpe⁄m².year 50 120
<15 for heat alone
50 42
Conversion factor electric energy to primary energy (multiply by) 2,58 2,70 - 2,00

Example of 4 European low-energy labels and their definition of low-energy dwellings.

Below a given threshold of energy needed for heating, the demand for heating energy is so small that it can be provided by the ventilation system, therefore suppressing the need of an independent heating system. Such a dwelling makes use of passive solar energy combined with energy conservation. That is the case of dwellings passing the norm Passivhaus and they are logically called passive. In that respect, passive can be seen as a restrictive specialization of low-energy.

Classes of energy rating have been created for dwellings in a similar way as what exists for domestic appliances. Below are the French and Italian one. It is a quick visual way to classify a dwelling based on its energy consumption.

Classes of energy rating in France and Italy

Classes of energy rating in France and Italy (source Effinergie and CasaClima)
Note that, in the French norm, heat, hot water, ventilation, light and auxiliaries are included while in the Italian norm only heat energy is included.

Note that, in the USA and in Canada, a norm called Leadership in Energy and Environmental Design (LEED) exists as well. It is even used by some countries in Europe.

What is the situation in Serbia?

In Serbia, as far as we know, there isn't such a thing as a low-energy norm for dwellings. That means that for people like us, developing the concept of low-energy in Serbia, we have to refer to foreign norms to define what we call low-energy. In our case, we used the French norm as a reference because it is the one we know best. But without clear signs from the Serbian government or from its agency in charge, the Energy Efficiency Agency of the Republic of Serbia (SEEA), one may wonder, rightly, how to define a low-energy building in Serbia.

In addition, just having a norm does little for its application. Everywhere, the path toward low-energy goes using 2 major complementary tools: passing laws and giving incentives. Passing laws put a given norm into force for all new constructions. That sets the minimum everybody has to comply to for new constructions. That goes together with giving tax breaks and/or subventions to those who go further than the norm set by law.

As of today, in Serbia, there is only one fund in place, the fund for the protection of the environment, run by the ministry of ecology, that is not suitable for low-energy dwellings. Defining new norms for dwellings, together with giving the Energy Efficiency Agency power and funding to act should be a priority of the new government.

How to achieve a low-energy construction?

There is certainly a lot to be said on the topic of achieving a low-energy construction, but it comes down to 2 main aspects:

  • Energy conservation: from architectural shape, orientation, thermal insulation, prevention of thermal bridges, using energy efficient lights and appliances, everything has to be done to spend less energy.
  • Use of renewable energy: starting by using passive solar energy to producing sanitary hot water or electricity. The use of geothermal, solar and wind energy reduce the ecological footprint of the construction and save energy.

By combining energy conservation and the use of renewable energy, it is possible to fulfill the strongest low-energy requirement. But from early planning, it is necessary to set what are the objectives and the budget, in order to decide on the best suitable solutions.

Why to achieve low-energy constructions?

Is it actually an important question. Why not continuing what has been done so far? In our opinion, the 2 strongest reasons are the following:

  • Energy is becoming increasingly expensive: energy has a cost and that cost is ineluctably rising. Peak oil and resource scarcity in general make the days of abundance history. Low-energy means therefore much lower cost in the long run and a better chance to maintain living comfort.
  • It is vital to reduce our ecological footprint: we've been living unsustainably for too long. We are drawing resources from Earth much faster than those resources can renew if they can renew at all. There are too many warning signs that we are in an ecological overshoot. Low-energy dwellings are not the answer to the problem but they are, undoubtedly, part of the solution.

Ecological debtor and creditor countries - 2003

Ecological debtor and creditor countries - 2003 (source Global Footprint Network)


Regardless of the norm used to define them, low-energy buildings share the same fundamental characteristics. They apply energy conservation solutions to use less energy, they use renewable energy (at least passive solar energy) to further decrease their ecological footprint and they provide fantastic comfort and well-being. It is not anymore the time to ask if we should build low-energy buildings but how can we make it happens.

Low-energy is not a final destination but rather a milestone. Ultra-low-energy buildings and positive buildings are already happening in several places. The path toward sustainability is the only rational way forward.

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