Misconceptions and calculations for
NET and NEARLY Zero Energy Buildings
In very general terms “Net Zero Energy Building” is a house that is connected to the grid and produces as much clean, renewable energy onsite as it uses over the course of a year.
Misconceptions of NZEBs
- “Net zero energy building” (NZEB) is a “zero energy building” or “off-grid” building
- NZEB is always energy efficient
- NZEB is calculated only by taking the amount of energy used vs the energy produced into account
Let's explain
A “net zero energy building” (NZEB) is connected to the grid (therefore, NZEB is not off-grid) and it produces renewable energy. Their excess energy produced is fed into the electricity distribution grid, which in turn supplies the house with energy required at times of insufficient self-production. This would be, for example, at night when the sun is not shining for solar panels to produce energy or whenever the wind is not blowing for wind turbines to produce energy.
NZEB is also not a “zero-energy building” (ZEB) which indicates a category of buildings with very high energy performance that is facilitated by sustainable design and the use of renewable energy sources on-grid as well as off-grid.
A NZEB can be a ZEB, but the concepts are not the same. In most cases, NZEBs are not ZEBs.
Why is NZEB typically not a ZEB?
The general definition of NZEB does not take into account the different energy sources (a NZEB would typically use energy from the grid that has mixed sources, such as gas and oil). The NZEB concept also does not consider energy efficiency in general – a house is a NZEB as long as the renewable energy fed to the grid equals or is larger than the energy required from the grid.
Different ways to measure NZEB
Site nZEB: A site nZEB produces at least as much energy as it uses within a year. To give an example:
A house uses 10,000 kWh of energy per year for heating, lighting, ventilation etc and produces at least 10,000 kWh of energy from renewable resources (solar, wind, geothermal, hydro and some others). If it buys 4000 kWh from the grid at times when renewable energy is not produced, it doesn’t matter whether this 4000 kWh comes from gas, oil or other non-renewable source.
Source nZEB: Source energy refers to the primary energy used to generate and deliver the energy to the site.
Different energy sources have different source energy values. A list of sources can be found here. To give an example:
If 100 kWh of grid electricity is used (ratio example from the US is 2.8), then 280 kWh of renewable energy (ratio example 1) should be produced for the grid to be considered nZEB.
Cost nZEB: In a cost nZEB, the amount of money the utility pays the building owner for the energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.
This refers to a €0 electricity bill. The energy company always pays less for the electricity they buy in comparison to the electricity they well. To give an example:
- Total energy need: 10,000 kWh per year
- Need for electricity from the grid: 4000 kWh per year
- Predicted cost of 1 kWh = 20 cents
- Predicted sale price of 1 kWh of renewable energy = 10 cents
When the sale price is 50% lower than the price of exporting electricity, the building needs to produce a total of 12,000 kWh of renewable energy per year in comparison to the actual total use of the house of 10,000 kWh to be considered a nZEB.
None of the concepts take into account the embodied energy or emissions, which is the amount of carbon emitted during the construction of a building. The extraction of raw materials, the manufacturing and refinement of materials, transportation, installation and disposal of old supplies can all produce embodied carbon emissions.
Which method is the best to use and which variables should be added to the NZEB concept is up for debate.
How do I know if my house is nearlyZEB?
The European Performance of Building Directive (EPBD) presents the ZEB definition framework as “nearly zero energy building” (nearlyZEB). According to this definition, a nearlyZEB is a very highly efficient building in which a large portion of the required annual consumed energy is produced on-site or nearby, mostly based on renewable energy sources1.
In the nearlyZEB definition framework from the EU, cost-optimality is assumed to be a consumption of < 30 kWh/m2 year of energy for covering the heating and cooling demand. This figure was the target for 2021. It is important to note that this is only heating (including water) and cooling, which indeed is the major contributor, contributing approximately 77% of all energy use in the EU. We would still also need to consider the rest.
Following this goal set by the EU, a net-ZEB is a nearly-ZEB when it consumes a maximum of 38,95 kWh/m2. Let’s say an average house is 100m2, this makes the annual energy use of this house to be 3895 kWh.