In buildings, net zero is commonly defined as “producing as much energy onsite as is used onsite over one year.” A concern is that “net zero” is equated to “carbon neutral,” which it is not. Even in the arena of operational energy, there can still be a carbon footprint to a net zero building. For example, let’s say you are producing electricity during the day and feeding it into the grid (because you are not using all of it). Then you use the same amount of energy at night as you produced during the day. The energy you produced during the day was from a sustainable source, but the electricity you are consuming in the evening, during peak demand, is going to have a carbon footprint.
As the sun goes down, the supply of solar power is diminishing and demand is going up. To make up for this lack of supply, the utilities start up their fuel burning power plants. Many of the plants they use to fill this gap are fueled by burning coal. Finally, because of transmission losses (which can be up to 2/3 of total energy produced) the system is making more electricity than you fed in to resupply your house. That extra energy, and its carbon footprint, now belong to you.
Through Passive House strategies, the electricity demands of this home have been reduced by about 80% and the remaining power to operate it is produced on site with photovoltaic panels. The carbon footprint of this home can be further offset by the use of onsite battery storage.