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Net Zero- & Plus-energy building 

The concept of passive houses is based on very low energy consumption. The total energy demand of a passive house is close to zero. Only in a passive house it is technically feasible and profitable to meet energy need partially or entirely by the means of exploiting renewable energy sources (RES) on site.

The combination of a passive house and renewable energy sources on site can easily equal the buildings’ net annual energy consumption to zero.

Net zero-energy building means that the annual demand of primary energy for all needs equals the generated energy by the means of renewable sources. This is not meaning, though, the building is energy self-sufficient and independent, thus electricity grid connection is not required. Energy self-sufficiency is possible, but within much higher investment costs, compared to net zero-energy buildings.

Depending on the type of building and its use, different kinds of renewable energy sources can be used in order to meet partially or entirely the building’s needs for energy. These are: biomass burners on: hardwood, pellets, wood chips and other types of renewable bio materials; building integrated photovoltaic systems for generating solar electricity, solar collectors for domestic hot water; heat pump can be used in order to meet the heating energy need. Possible and efficient are solutions for combining heat pumps with geothermal sources.

Photovoltaic system, Plus Energy House in Kladnica, Studio APXE

Diagram of the monthly yield from roof PV array, online portal of SMA, Plus-energy house in Kladnica, Studio APXE


See the first plus-energy building in Bulgaria, designed and managed by Studio APXE



How much is the initial investment in order to achieve net zero-energy balance, in comparison between a low-energy and a passive house?

Low-energy house

If a low-energy building with 150 m2 heated area requires 150 kWh/m2 annual site energy* consumption to maintain a good indoor micro-climate, it comes to 22 500 kWh annual consumption. Photovoltaic panels will require installed capacity of 20,45 kWp when counting 1,100 kWh average annual production of electricity from one installed kWp per year. The average cost of building such a PV system is 4 €/Wp, or a total of 81 800 €.

Passive house

In case of "passive house" building which requires 40 kWh/m2 site energy* consumption annually for the same heated area of the building, it comes to 6 000 kWh total consumption per year. The required installed capacity of PV panels is 5,45 kWp at the same annual production of the system which counts to 21,800 € total price for the PV system. If the required space for installing this system is 36,3 m2, for the low-energy building, we will need 136,3 m2, which is much more difficult to implement.

* The site energy is a sum of the energy consumption for heating, lighting, ventilation, cooling, household auxiliaries etc. within the building. The site energy differs from the energy for space heating. The passive house standard requires energy consumption for heating to be not more than 15 kWh/m2 per annum.