## 2 Zero Energy
### Site Energy  
Zero site energy buildings produce energy on site and ideally consume as much as they produce on site. In urban areas in particular, they may use the grid for back up, assuming that the grid is two-way where energy can be delivered to and received from (most grids are two-way). To understand energy use on site, monitoring is required; also, such buildings, if well designed, will make use of their passive designs to ensure that the building itself uses as little energy as possible for operations. If there is more than one building on site, it is important to measure the sub metering for each of the buildings separately, especially if different sources of fuel are used; for example, solar and geo thermal fuel sources will need to be separated. Another important factor to be considered in this case is also what constitutes site for the building: Is it just the footprint of the building or the entire property, including car parking? For example, parking areas covered with photovoltaics (PVs) were used in a ZEB in Singapore built by the Building and Construction Authority [9]. Energy uses are usually for heating, cooling, ventilation, hot water, lighting and plug load for various appliances. [PERSON] and others [10] expanded further from [PERSON] and others' [11] definitions to include categories from A to D in relation to zero energy buildings. They stated that a ZEB A uses renewable energy from the building footprint, ZEB B uses sources available on site, and ZEBs C and D consider offsite options. ZEB C uses energy sources off-site to generate energy on site. Basically, this would involve importing materials that can generate electricity and heat to the site. In a ZEB D, renewable energy is purchased off site; in other words, green energy is used on site. Other researchers [12; 13] have distinguished two units of the balance: Emissions and energy, though they have done so without specifying delivered or primary energy. To understand the impact of the building on the environment, [PERSON] [14] stated that the metric of the balance in the ZEB definition should address both the quantity as well as the quality of energy. In this context, his proposal is a definition for a ZEB where the building has zero exergy. He states that a net zero exergy building is one:  
Which has a total annual sum of zero exergy transfer across the building-district boundary in a district energy system during all electric and any other transfer that is taking place in a certain period of time. Exergy is defined as the'maximum available work that can be extracted from a system during a process that brings the system into equilibrium with its environment' [15; 16]. Exergy can be destroyedand, therefore, provides a better concept for understanding which components of a system can account for irreversibility's [16]. According to [PERSON], exergy is the available amount of energy to be used; if the exergy is zero, then, the building qualifies as a zero-exergy building. If the building draws exergy from the surroundings, then there are associated emissions that need to be considered. [PERSON] [17] also used exergy rather than energy for understanding how different components of a system use energy and where emissions occur in a system. His argument for the use of exergy hinged on understanding 'what is consumed in what manner, how much and where.' According to him, exergy may be used to understand how a system may work thermodynamically. [PERSON] [17] used exergy for understanding the environmental planning and design of the built environment, whereas [PERSON] and [PERSON] [16] used exergy for the dynamic understanding of systems at a building scale. The exchange between the building and its surroundings or the grid makes more sense when using green or renewable energy during peak demands. A net zero energy building connected to the grid when compared to an autonomous house [18] offers a better solution if renewable energy is used to capture energy.