XG GROUP OF ENGINEERS

The aim of this initiative is to demonstrate that a typical Greek apartment building can use very little energy, offer a high level of comfort and, at the same time, lay the foundations for a different way of producing and using energy in our cities.

The approach begins with the building itself. First, the energy needed for heating and cooling is reduced as far as possible by applying Passive House principles. The remaining, much smaller energy needs are then met by efficient systems, while electricity generation from solar panels and the ability to store it can further reduce dependence on the grid.

But the ambition goes beyond a single apartment building. The building is seen as an energy “cell” that can be replicated. Each new building can become another unit that produces, stores and uses energy. At a later stage, several such buildings could be connected, allowing surplus energy from one to be used where it is needed elsewhere.

For example, a building with low electricity demand at a particular time could make the excess energy it generates available to neighbouring buildings instead of leaving it unused. In this way, the apartment building becomes the first cell in a wider network of cooperating buildings.

The long-term vision is to move beyond individual energy-efficient buildings towards energy neighbourhoods, where energy production, storage and consumption complement one another.

At the same time, this project shows that the transition can begin with a real residential building rather than an experimental prototype. With an additional construction cost of approximately 6% compared with a similar conventional building, the model was designed to be not only energy efficient but also practical and replicable in the Greek housing market.

The initiative begins with the people who live in our cities and a simple need: a home with low energy costs, consistently comfortable indoor conditions and high indoor environmental quality.

The same model is also directly relevant to those who shape the housing market: property owners and investors, builders, developers, architects and engineers. The project demonstrates in practice that a highly energy-efficient building does not have to be an unusual or unaffordable construction; it can be part of the mainstream housing market.

On a broader scale, the approach is relevant to municipalities, organisations and professionals planning the future of our cities. The possibility of replicating and eventually connecting such buildings opens the way to neighbourhoods that do more than consume energy: they could produce, store and use it collectively.

The initiative therefore reaches beyond an audience already familiar with green building. It speaks to everyone who lives, designs, builds or invests in the modern Greek city, based on the belief that sustainable housing need not remain the exception but can gradually become the norm.

18 months of implementation – a long-term vision. Construction is complete, but the building’s story has only just begun. It is a living energy “cell”, designed to operate, evolve and inspire new applications in the years ahead.

The initiative centres on a five-storey apartment building in Volos, Greece, with a total net floor area of 611 m² and seven independent apartments. The building has been completed and is occupied, providing a real-life application of Passive House principles in a densely built urban setting.

The central design idea is simple: the best energy is the energy we never need to use. The first step, therefore, was to reduce the building’s heating and cooling needs to an absolute minimum.

A compact design, appropriate orientation and effective shading work together with a highly insulated, airtight building envelope. High-performance windows with triple glazing and careful treatment of thermal bridges substantially reduce heat losses in winter and overheating in summer. At the same time, mechanical ventilation with heat recovery continuously supplies fresh indoor air while retaining much of the energy already used to heat or cool it.

Once the building’s energy needs have been minimised, the next step is to meet them as efficiently as possible. To achieve this, the building uses two interconnected heat pumps, solar energy to support domestic hot water production, a 15 kWp photovoltaic system and a 15 kWh battery for energy storage.

A distinctive feature is the use of two identical, interconnected heat pumps with separate roles: one provides heating and cooling for the apartments, while the other produces domestic hot water. This arrangement allows the two systems to operate independently and efficiently according to the building’s different needs.

Their hydraulic interconnection also provides an important advantage: if one unit breaks down, the other can take over its function through a simple valve adjustment, providing backup capacity and continuity of service for residents.

In this way, the building envelope and energy systems are not separate “green” additions but parts of a single, integrated approach. First, energy demand is reduced; then renewable energy is planned to meet that demand and, wherever possible, to be stored for later use. This is the foundation of the building’s energy ecosystem.

This apartment building is the first energy “cell” in that approach. The model can be replicated in other buildings and, at a later stage, these buildings could form a wider network. Surplus energy from one building could be used by another that needs it, gradually creating an energy neighbourhood where production, storage and consumption work together.

The initiative therefore proposes more than a more efficient apartment building. It treats the apartment building as the smallest unit of a larger, expandable energy ecosystem.

The initiative was implemented in Volos, Greece, in a dense urban setting. Its design principles can be adapted to different climates and planning conditions and applied to apartment buildings and other residential buildings across Greece.

The initiative’s most important achievement is that the idea of a building with exceptionally low energy needs has become a reality. The five-storey apartment building in Volos has been completed, and all seven apartments have been occupied since June 2026. The project has moved from design into everyday use, allowing us to assess its energy performance under real living conditions.

The energy studies demonstrate the high standards of the design. According to the Passive House Planning Package (PHPP), the building’s calculated annual energy demand is just 1.89 kWh/m² for heating and 16.03 kWh/m² for cooling and dehumidification. Assuming an average heat pump efficiency factor of 4, these figures correspond to estimated electricity consumption of approximately 0.5 kWh/m² per year for heating and 4 kWh/m² for cooling. The PHPP assessment meets the calculated criteria for the Passive House Plus standard.

In parallel, the Energy Performance Study (MEA) calculates a reduction of approximately 88.5% in primary energy consumption compared with the reference building under Greece’s KENAK energy regulations. Following completion, the Energy Performance Certificate (EPC) placed the building in the highest energy class, A+, with calculated annual CO₂ emissions of 0.0 kg/m² and an energy balance showing a surplus of 1.8 kWh/m² per year. These are energy modelling and certification results, providing a baseline against which the building’s actual operation can be assessed.

The first measurements are particularly encouraging. Between 23 June and 22 September 2026, when the building was fully occupied, total electricity consumption for all seven apartments and the shared facilities was 4,289 kWh, averaging approximately 46.6 kWh per day for the entire building.

Data source: DEI electricity bills for 23/06–24/07, 25/07–22/08 and 23/08–22/09/2026.

The actual energy performance of the building systems is monitored using separate electricity meters. Since monitoring began in June 2026, the heat pump serving heating and cooling has recorded 2,350.12 kWh, while the second heat pump, dedicated to domestic hot water, has recorded 544.38 kWh. The combined consumption of the two systems is therefore 2,894.50 kWh, or just 4.74 kWh/m² of PHPP reference floor area, during this initial period of operation. These readings provide an early picture of how the systems perform in practice, while monitoring continues to assess their performance over a full annual cycle.

Data source: Photographic record of the separate electricity meters for the two heat pumps, 09/10/2026.

Importantly, all the actual consumption figures above were recorded before the building’s photovoltaic system had been brought into operation. The apartment building is still supplied through a temporary construction electricity connection, pending installation of the required meters by the Greek distribution network operator (DEDDIE). The results to date therefore reflect the building’s energy use without any contribution from on-site photovoltaic electricity generation. Once the 15 kWp photovoltaic system is activated and the 15 kWh storage battery is put to use, the amount of electricity the building needs to purchase from the grid is expected to decrease further. The next phase of operation will therefore allow the complete energy ecosystem to be evaluated, combining low energy demand with renewable electricity generation and storage.

Beyond energy performance, another important outcome is the economic feasibility of the model. The apartment building was constructed at an additional cost of approximately 6% compared with a similar conventional building. This shows that very high energy performance can be incorporated into a real residential project without requiring an unaffordable or experimental construction.

Above all, however, the project is not simply a technical demonstration. It is a building where people live every day, with real needs, real electricity consumption and the opportunity for continuous monitoring. It therefore provides a practical example of how the energy performance of Greek apartment buildings can change.

Its completion also marks the beginning of a broader vision: each similar building could operate as an energy “cell” and, in the future, many such buildings could work together, making use of the energy they produce and store. The initiative’s results therefore extend beyond the performance of one apartment building, laying the groundwork for energy neighbourhoods.

For XG Group of Engineers, delivering this apartment building means more than completing another construction project. It has provided an opportunity to put Passive House principles into practice in a real residential building and to assess the outcome not only through energy studies but also through everyday operation.

The experience gained at every stage, from architectural design and construction of the building envelope to the installation and integration of energy systems, strengthens the company’s expertise. It also provides a valuable foundation for improving the solutions used in future projects.

The ability to monitor the building’s actual energy performance is particularly important. Electricity bills and separate heat pump meters allow the company to compare the predictions made during design with real operating data. In this way, learning does not end when the building is handed over; it continues to grow through everyday use.

Another significant benefit is that the company now has a completed example of the model it advocates. With an additional construction cost of approximately 6% compared with a similar conventional building, the project provides a concrete basis for designing and evaluating the economics of comparable buildings in the future.

Finally, the apartment building represents the first step towards the wider idea of energy “cells”. For the company, the initiative’s value goes beyond gaining expertise or showcasing a successful project. It also lies in the opportunity to develop the model further, apply it to more buildings and gradually contribute to the creation of energy neighbourhoods.