Technical Report

NREL/TP-7A2-46065

November 2009

Definition of a "Zero Net Energy" 

Community

Nancy Carlisle, AIA

Otto Van Geet, PE

Shanti Pless
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Technical Report

NREL/TP-7A2-46065

November 2009

Definition of a "Zero Net Energy" 

Community

Nancy Carlisle, AIA

Otto Van Geet, PE

Shanti Pless

Prepared under Task No. FE09.3410
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iii

Executive Summary

A net zero-energy community (ZEC) is one that has greatly reduced energy needs through 

efficiency gains such that the balance of energy for vehicles, thermal, and electrical energy 

within the community is met by renewable energy. Past work resulted in a common zero-energy 

building (ZEB) definition system of “zero energy” and a classification system for ZEBs based 

on the renewable energy sources used by a building. This paper begins with a focus solely on 

buildings and expands the concept to define a zero-energy community, applying the ZEB 

hierarchical renewable classification system to the concept of community. A community that 

offsets all of its energy use from renewables available within the community’s built environment 

and unusable brownfield sites is at the top of the ZEC classification system at a ZEC of A. (A 

brownfield site is where the redevelopment or reuse may be complicated by the presence or 

potential presence of a hazardous substance, pollutant or contaminant.) A community that 

achieves a ZEC definition primarily through the purchase of new off-site, Renewable Energy 

Certificates (RECs) is placed at the lowest end of the ZEC classification but is still considered a 

good achievement .

The hierarchical approach emphasizes using all possible cost-effective, energy-efficiency and 

demand-avoidance strategies first and then using renewable energy sources and technologies that 

are located in three places: 

• First, on space within the built environment or unusable brownfield sites within the 

community 

• Second, on either green space within the community or located outside the community 

boundary but within the region and brought into the community 

• Third, through the purchase of new RECs.

Table 3 provides examples of each of these options.

In addition, we set up a series of intermediate milestones so a community can begin to plan 

toward the aspiration of net zero over time. The goal of this type of classification is to encourage 

developers and campus and community planners to develop a comprehensive multi-year plan to 

both understand the magnitude of change needed in community energy patterns to move toward 

the concept of a net-zero community and to lay out a multi-year scenario with defined milestones 

to drive the transition.
iv

Table of Contents

Executive Summary................................................................................................................................... iii

1 Background........................................................................................................................................... 1

2 Assumptions......................................................................................................................................... 2

2.1 Sanborn Principles of Sustainability....................................................................................3

3 Definition ............................................................................................................................................... 4

3.1 Applying a Classification System to Zero-Energy Communities........................................9

3.2 Intermediate Steps and Verification to Move to the Aspiration of Net Zero.....................11

4 Conclusions ........................................................................................................................................ 13

5 References .......................................................................................................................................... 14

List of Figures

Figure 1. Net-zero concept......................................................................................................................... 6

Figure 2. Intermediate milestones to Net Zero Communities............................................................... 12

List of Tables

Table 1. Community Efficiency and Renewable Supply Hierarchy........................................................ 5

Table 2. Pros and Cons of Options 2a versus 2b .................................................................................... 5

Table 3. ZEC Renewable Energy Supply Option Hierarchy (modified from Pless, et al.).................... 7

Table 4. Classifying ZEBs – Energy Supply Options ............................................................................ 10
1

1 Background

There are many ways to define net-zero energy in the context of a building. There are also many 

terms that have similar meanings to the term net zero or near net-zero energy communities

including carbon neutral, climate neutral, carbon advantage1

1. Net-zero site energy

or sustainable communities. We 

use the term net zero because it is a more narrow focus than some of the other terms and one can 

measure and determine if the goal has been achieved. Torcellini et al. (2009) have developed 

four well-documented definitions of Zero-Energy Buildings (ZEB) including:

2. Net-zero source energy

3. Net-zero energy costs

4. Net-zero energy emissions.

In a follow-on paper, Pless et al. (2009) go one step farther by proposing a classification grading 

system for ZEBs that establishes and ranks the renewable energy sources used by a building. For 

example, in the building-ranking system, building integrated photovoltaics (PV) rank higher than 

the burning of wood even though both are considered renewable energy. Generating renewable 

energy within a building’s footprint is defined as superior from an overall energy perspective to 

the importation of wood chips from a remote location to burn on-site for fuel. Pless et al. (2009) 

classify ZEBs based on the location of the renewable energy hardware with respect to the 

building. The goal of the building classification system is to encourage building owners and 

designers to first utilize all possible energy-efficient strategies and then use renewable energy 

sources and technologies located on the building.

This building’s classification scheme provides a good starting point for a definition of a zeroenergy community (ZEC). The United States and many other parts of the world have set 

aggressive goals for energy savings in many communities and states. Some of these goals can be 

realized best by looking at energy use in a community as an integrated system rather than

looking at strategies for buildings, transportation, and industry as discrete problems with discrete 

solutions. For example, community scenarios could link transportation, homes, and the electric 

grid as well as enable large quantities of renewable power onto the grid. Preliminary analyses 

show that the homeowner would gain a net cost advantage on a life-cycle cost basis by 

integrating a near-zero energy home with a plug-in hybrid vehicle (PHEV). By defining a zeroenergy community and offering a set of metrics to measure progress toward the aspiration of net 

zero, we aid in moving the dialog from building only zero-energy solutions to solutions that 

consider all uses of energy within a community for buildings, transportation, community 

infrastructure, industry, and others. A definition for a zero-energy community is different and 

more complex than that of a ZEB because a community uses energy not only for buildings but 

also for industry, vehicles, and community-based infrastructure, so the achievement of “zero

energy” needs to include all applications. A definition of this nature is also more complex 

because of all of the factors discussed in this report. 

 1 Term used in T. Friedman (2008), Hot, Flat and Crowded, is a better option to Carbon-Neutral because this term 

helps position these communities as models for new leadership in terms of attracting green businesses (jobs) and 

new ways of doing business as leaders, and “neutral” just doesn’t imply leadership.
2

2 Assumptions

The hierarchical definition for a zero-energy community includes both energy-demand reduction 

options and renewable energy supply-side options. It places emphasis on energy efficiency or 

energy-use avoidance as the highest priority strategy. The definition requires that a community 

energy-basecase (or carbon inventory) be calculated (for a new development) or measured (for 

an existing community) to determine if the net-zero goal is met.

This definition assumes that in order to establish the basecase, the developer, community, 

campus, or military base official needs to decide on both a community boundary and what uses 

of energy are included. Except in the case of an island, the boundary is arbitrary. The boundary 

can be geographically or politically defined or, as in the case of the U.S. Forest Service, defined 

as the boundary of an eco-system (Kandt and Mann 2009). The other assumption that needs to be 

made is what energy uses are included in the basecase. One assumption might be to include 

energy used for buildings, community infrastructure, industry, and transportation (both within 

the community and to and from work). For the Department of Defense (DOD) net-zero base pilot 

project at the Miramar Naval Air Station, the definition included the minimization of energy 

demand through conservation and efficiency. This includes all energy required for the 

functioning of the installation (regardless of what organization controls it) and transport fuel –

including fleet, commuting, business travel, and flying mission (Barnett 2009).

In the University Presidents Climate Change Commitment, (Dautremont-Smith 2009) “what to 

count” is defined as scope 1, 2, or 3. These are defined in terms of greenhouse gases (GHG) but 

could be defined in terms of energy instead. Scope 1 refers to direct GHG emissions occurring 

from sources that are owned or controlled by the institution, including on-campus stationary 

combustion of fossil fuels, mobile combustion of fossil fuels by institution-owned/controlled 

vehicles, and “fugitive” emissions. Fugitive emissions result from intentional or unintentional 

releases of GHGs, including the leakage of HFCs from refrigeration and air-conditioning 

equipment as well as the release of CH4 from institution-owned farm animals. Scope 2 refers to 

indirect emissions generated in the production of electricity consumed by the institution. Scope 3 

refers to all other indirect emissions—those that are a consequence of the activities of the 

institution, but occur from sources not owned or controlled by the institution. The signatories to 

this agreement need to account only for scope 1 and 2. EPA greenhouse gas accounting 

procedures also require only scopes 1 and 2. 

• The zero energy community definition assumes that the community has an electric grid. 

When on-site generation from renewable electrical energy is greater than the load, it can 

be sent to the grid. In a case where there is a large amount of renewable systems on the 

grid (called a high market-penetration scenario), the grid may not always need excess 

renewable energy generated at certain times of day. In this case, on-site energy storage 

may become necessary.

• By using the utility grid to account for the energy balance, excess production can offset 

later energy use. In accounting for net zero on the community scale, it is assumed that 

renewable electricity can be used to offset various fossil fuel energy use (such as natural 

gas or gasoline).

• The assumption is that over time the power grid will increasingly rely on renewable 

energy as more states implement renewable portfolio standards. This may reduce the total 
3

amount of renewable energy generation needed by a community to achieve the 

designation of near or net zero. Today the gap between the renewable energy supplied by 

the utility and that needed by a community to become net-zero is so large that energy 

efficiency, energy conservation, and renewable energy installed at the community level is 

needed.

There are many guiding principles for the development of a sustainable community. Below is 

one recognized set. The aspiration of a net-zero energy community is intertwined throughout 

these principles. The goal of net zero provides a means to quantify and to track a community’s 

progress toward sustainability by reducing its overall energy usage. Energy is the primary focus 

of this paper, but when developing a net-zero energy solution, one needs to be mindful that a 

successful solution must be one that works in terms of energy as well as other parameters of 

sustainability. For example, growing crops for energy within the boundary of the community 

would not be an acceptable solution if the water supply was not sustainable. Intentionally 

building low-density buildings to maximize roof area for PV, which results in increased traffic 

and limits access to community amenities for certain populations without access to cars, would 

not be in keeping with the principles of sustainability either. Sustainable and zero-energy design 

and planning are analogous to a three-legged stool: All legs need to be in balance for the system 

as a whole to function.

2.1 Sanborn Principles of Sustainability2

• Development is ecologically responsive and optimized for efficiency

• Residential and commercial buildings are efficient, healthy, and sensible and use 

renewable energy 

• Schools and public buildings are a focal point for community education

• The community thermal-power grid and water and waste water infrastructure are

efficient, planned, and installed.

• Some fraction of the heat and power source is distributed (on-buildings) and/or located 

within the community boundary and relies on renewable energy

• The transportation system is accessible and rich in alternative transportation options

• Water is sustainable

• The community minimizes waste

• The community is socially just

• The community promotes the growing and eating of local and sustainable food

• The community is culturally creative

• The community incorporates natural and man-made beauty

• The community is evolutionary.

 2 Adapted from the Sanborn principles (Harwood et al. 1994). 
4

3 Definition

If we apply the most general definition of zero-energy to the concept of community, a net-zero 

energy community is “one that has greatly reduced energy needs though efficiency gains such 

that the balance of energy for vehicles, thermal, and electrical energy within the community is 

met by renewable energy. 

The energy performance of a net-zero community can be accounted for in several ways. The 

definitions listed below are based on those by Torcellini (2006).

• Net-Zero Site Energy: As much renewable energy is produced in the community for 

buildings and infrastructure as is needed by buildings and infrastructure in a year when 

accounted for at the site. For transportation, site energy needs would be based on a 

calculated or measured vehicle miles traveled by community occupants regardless of 

whether they filled up their gas tank in the community or outside the boundary. As stated 

earlier, the definition allows for fuel switching so excess renewable electricity generation 

can offset various fossil fuel use. 

• Net-Zero Source Energy: A source ZEB produces at least as much energy as it uses in a 

year when accounted for at the source. Source energy refers to the primary energy used to 

generate and deliver the energy to the site. To calculate a building’s total source energy, 

imported and exported energy is multiplied by the appropriate site-to-source conversion 

multipliers based on the utility’s source energy type. For transportation fuel, source 

energy would include a multiplier to account for the energy required to transport the fuel 

to the fueling station. 

• Net-Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building 

owners and the community (for renewable energy generated on all residential and 

community buildings and infrastructure) 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. If you include transportation, the cost of the fossil-based fuels is 

offset by fuel generated from renewable sources. 

• Net-Zero Energy Emissions: A net-zero emissions community produces and uses at least 

as much emissions-free renewable energy as it uses from emissions-producing energy 

sources annually. Carbon, NOx, and SOx are common emissions that ZEBs and 

transportation powered by renewable energy offset. To calculate a building’s and 

transportation (based on assumed or actual VMT) total emissions, imported and exported 

energy is multiplied by the appropriate emission multipliers based on the utility’s 

emissions and on-site generation emissions (if there are any).

A community may be designed or retrofitted to achieve one or more of the above definitions but 

may not achieve it in actual operations every year. If the community produces at least 75% of its 

required energy through the use of on-site renewable energy, it is considered a near-zero 

community. Off-grid communities are not considered in this paper.

Table 1 shows a hierarchy of options to move to zero-energy communities. The options are 

ranked in terms of desirable approaches to move toward the zero-energy definition. The goal of 

this type of classification scheme is to encourage community managers and developers to first 

utilize all possible cost-effective energy efficiency strategies and then evaluate options to move 
5

to net zero. Table 2 includes two approaches both with pros and cons. The strategy chosen will 

vary with the circumstances. 

Table 1. Community Efficiency and Renewable Supply Hierarchy

Option Number Option Name

0 Energy Efficiency and Energy Demand Reduction

1 Use Renewable Energy in the Built Environment & on 

Unusable Brownfield Sites

2a Use Renewable Energy on Community Greenfield Sites (A 

Greenfield site is a site that has not been previously 

developed or built on, and which could support open space, 

habitat or agriculture)

2b Use Renewable Energy Generated Off-site, On-site

3 Purchase New Off-site RECs3

Table 2. Pros and Cons of Options 2a versus 2b 

Option 2a

Renewable Energy Supply in the Community 

Greenfield

Option 2b

Renewable Energy Generated Off-site for use

On-site

Pros Pros

 Lower transmission line losses

 The community is more independent 

and self-sufficient

 This might create a new business 

opportunity—developing the 

infrastructure for distributed renewable 

energy within a defined community

 Community members will become more 

aware of where their energy comes 

from, which will lead to more energy 

conservation

 If there is a good renewable resource 

located within the community boundary

 Community has more control for nearand long-term energy decisions

 A central source of renewable energy serving 

a regional need rather than a community 

need might be more cost-efficient

 If there is a good renewable resource located 

outside the community boundary but located 

within the region (landfill gas, wind)

 A utility partner might have a business 

interest in working with a community or 

communities in a region to develop a 

regional renewable strategy, which could be 

a win-win for multiple parties

 For liquid fuels, this is the most viable means 

to provide renewable energy

 Off-site central station power may have 

higher efficiencies of scale

 This may facilitate greater coordination 

among regional communities (i.e. enhance 

regional solutions for transportation, land-use 

planning, district systems, etc.)

Cons Cons

 Use of community green space for 

energy generation where there might be 

a higher value use of the land

 The resource for energy generation 

might not be optimal

 The land use might not be compatible 

with other desired land uses

 There is an efficiency trade-off as the 

distance from the site of generation to the 

source of use increases

 The community has less control over its 

energy supplies

 3 RECs represent the attributes of electricity generated from renewable energy. These attributes can be unbundled 

from the electricity and bought and sold separately. 
6 

In Table 3, each of these options is defined more fully with examples provided in the categories

of buildings, transportation, and community infrastructure. Option 0, energy efficiency and 

energy demand reduction, is considered a prerequisite to the renewable energy supply side 

options. Option 0 includes a fourth category, behavior. From a community point of view, energy 

demand reduction can occur in all four categories, and it is important to place behavior changes 

on par with technological changes. For example, behavior can impact energy use. One recent 

example is an environmental competition held among 13 buildings at Harvard University. The 

result was $72,472 in energy dollar savings through changes in operations and maintenance 

practices (Bradt 2008). 

From a developer or community leader’s point of view, there are only certain things that they can 

do to influence energy use directly in the design of a new community or to retrofit an existing 

community. In the case of human behavior, a developer can influence energy use indirectly, and 

a community leader can have influence through policy or financial incentives/disincentives. If a 

developer wants to build a new zero-energy community, he or she needs to make an assumption 

about human behavior to establish the basecase for the purpose of measurement and verification 

and then decide “what counts” for the claim to a be a net-zero community. For example, the 

developer could define and quantify that he or she has designed a net-zero community “for only 

the aspects of energy use that he or she can directly control within a defined life-cycle cost.” (See 

discussion of intermediate milestones below.) 

The net-zero concept is shown graphically in Figure 1. 

Figure 1. Net-zero concept
7

Table 3. ZEC Renewable Energy Supply Option Hierarchy (modified) (Pless, et al. 2009)

Option Name and 

Number ZEC Supply-Side Options Examples

Option O

Demand reduction 

(a prerequisite for the 

renewable supply side 

options)

Buildings

Reduce site energy use through community design, setting 

targets for building energy efficiency and incorporating lowenergy building technologies for new construction; for 

existing communities, making energy savings retrofits to 

buildings.

Transportation

Reduce vehicle miles traveled for gasoline-powered 

passenger vehicles within the community and to and from the 

community. Provide convenient bus and/or rail stops for 

destinations to locations of employment outside the 

community.

Community Infrastructure

Assess loads for all sectors to define the biggest 

opportunities for energy savings and use energy-efficient 

strategies to minimize these loads. 

Behavior

Set community goals for energy and water use. Use policies, 

information and education, and incentives and disincentives 

within the community to achieve desired objectives.

Buildings

Influence energy demand reduction though urban 

design and lot layout, setting energy-efficiency 

targets and density targets including both jobs 

and dwellings per acre. 

Buildings can incorporate aggressive energy 

efficiency and use daylighting, passive solar 

design, high-efficiency HVAC equipment, natural 

ventilation, evaporative cooling, ground-source 

heat pumps, ocean water cooling, etc.

Transportation 

Community design can include a diversity of land 

uses, densities, and a mix of housing and retail and 

enhance walkability and connectivity within a 

community to minimize need for personal vehicles. 

Design can also create destinations within the 

community and minimize the distance to public 

transit. 

Community residents and visitors can use 

alternatively fueled transportation, advanced vehicles 

and fuels; maintain and operate vehicles to maximize 

efficiency; form car sharing clubs and other 

community-based initiatives; and provide and plan for 

alternative transportation and access to mass transit 

within the community.

Community Infrastructure

LED traffic lights, high-efficiency pumps (for water 

pumping). Utilities are installing “smart grid” to 

provide user feedback regarding energy use and 

impact on their energy costs.

Behavior 

Set aggressive energy-efficiency standards for all 

construction. Established policies and covenants can 

be used to incentivize building owners to use less 

energy and water. 

Through consumer education and feedback 

(metering, commissioning, retro-commissioning), 

consumers can be educated to turn lights and 

equipment off at night, metering, turn off vampire 

loads in buildings at night and when buildings are not 

occupied. 
8

Renewable Supply Options—Within Community 

Option 1 

Use renewable 

energy systems in 

the community or 

campus within the 

built environment 

and on unusable 

brownfield sites

Buildings and Community Infrastructure

Use renewable energy sources on sites available within the 

built environment or on site that are unbuildable such as 

brownfield sites. This includes using solar on residential and 

commercial rooftops, parking structures, and along roadways. 

Transportation 

Provide transportation options powered by renewable energy 

available to all destinations within the community.

Buildings

PV, solar hot water, ground-source heat pumps 

located on buildings, parking structures, along 

roadways and connected to building systems. 

For new construction, the design and layout of 

buildings should maximize rooftop area for renewable 

systems and the systems are either designed and 

installed on all new buildings or plumbed and wired to 

be added at a later date. 

For existing communities, maximize the amount of 

unshaded rooftop spaces with renewable systems 

that are determined to be cost-effective. 

This option would also include generating and using 

methane from a wastewater treatment plant or 

producing power from waste as long as the energy is 

generated from waste streams generated within the 

community and processed in the community. 

It would also include the use of PV, wind, solar 

located on a brownfield site within the community. 

Transportation 

This option includes the installation of electric plugs 

in homes, public parking so that people can plug in 

electric or electric/hybrid vehicles at home or in public 

parking and power vehicles from renewable sources 

on buildings. 

It includes using electric-powered buses and shuttles 

within the community powered by 

renewable energy generated in the community.

Community Infrastructure

Use renewable energy to power street lights, 

monitors, and meters. 

Option 2a

Renewable energy 

supply in the 

community 

Greenfield 

Buildings and Community Infrastructure 

Build renewable energy sources on Greenfield sites located 

within the community boundaries. The renewable systems are 

connected to the electrical or distribution grid. 

Transportation 

Include the use of renewably generated electricity for cars, 

busses, and shuttles. 

Buildings

PV and wind located within the community boundary 

and connected to the grid. 

Central solar hot water connected to a distribution 

grid. Biofuel applications only if the fuel were grown 

in the community.

Transportation 

Renewably generated electricity for cars, trucks, and 

busses. Ethanol or biofuels for transportation only in 

cases where the plants for the fuels are grown in the 

community. 

Community Infrastructure

Community scale micro-grid connects distributed and 

community-scale renewable systems (electric and or 

thermal energy) to buildings and utility grid. Storage 

may be added to the grid to power peak needs, nighttime loads, or seasonal loads. 
9

Off-Site Supply Options

2b

Off-site renewable 

energy used to 

generate on-site 

Buildings

Use renewable energy sources available off-site to generate 

energy on a campus in a community or neighborhood.

Transportation 

Use renewable-based fuels generated off-site for use on site. 

Buildings/Community Infrastructure

Biomass, wood pellets, ethanol, or biodiesel or 

landfill gas that can be imported from off site, 

which can be used on-site to generate electricity, 

heat or fuel.

A community could also negotiate with its power 

provider to install dedicated wind turbines or PV or 

solar panels at a site with good solar\wind resources 

outside the community. In this approach, the 

community would own the hardware and receive 

credits for the power. The power company or a 

contractor would maintain the hardware. 

Transportation

Ethanol and biodiesel fueling stations located on site 

are included in this option. 

3

Purchase new 

RECs

Buildings/Transportation/Community Infrastructure

Purchase new off-site RECs that result in additional generation 

added to the grid.

Buildings/Transportation/Community 

Infrastructure

Utility-based wind, PV, emissions credits, or other 

“green” purchasing options. Hydroelectric is 

sometimes considered. All off-site purchases must be 

shown to add new generation capacity to the grid.

RECs could be used as a strategy to meet a goal for 

an interim period of time or as a “top off” strategy to 

provide, for example, the last 10% of renewable 

energy. It is important that an REC purchase add 

new generation capacity to the grid. 

3.1 Applying a Classification System to Zero-Energy Communities

While all attempts to achieve a zero-energy community (ZEC) are valuable, we can classify a 

ZEC based on the renewable supply options used and definitions met. A community that meets 

the ZEC definition solely through the purchase of new off-site RECs is placed at the lowest end 

of the ZEC classification as shown in Table 4. A community that offsets all of its energy use 

from renewables available within the built environment is at the top of the ZEC classification 

system. (This also includes brownfield sites). In order to keep the nomenclature consistent 

between the zero energy buildings and community net zero definitions, the achievement of an 

“A” classification for community means that all power would need to be generated within the 

built environment (and assumes that energy efficiency is first maximized). From a community 

perspective, one would want to trade off building density versus available area within the built 

environment suitable for renewable production. As a community becomes denser, there is less 

rooftop and built-up areas suitable for renewable production. In a community, density allows for 

greater walk-ability and many other attributes desired in a sustainable community. In addition, 

today it would be very challenging for a community to generate 100% of its transportation fuels 

within its boundaries. For these reasons, the achievement of a ZEC “A” may be impossible in 

many locations. The ZEC “B” category includes renewable energy generation in both the built 

environment and within the community green space and/or with some aspect of renewable 

energy generated off-site and imported to the community for use or further refinement. It also 

allows a fraction of the renewable energy to come from new RECs. For a community, 
10

Classifications A, B, and C are all considered excellent. A fourth option where the community 

purchases all of it renewable energy from new RECs is considered a good achievement. 

Table 4. Classifying ZECs – Energy Supply Options

ZEC Classification Renewable Energy Supply Option Number

A

Option 0: Energy efficiency opportunities maximized

Option 1: 100% energy load met by renewables in the built 

environment and unbuildable brownfield sites within the 

community

B

Option 0: Energy-efficiency opportunities maximized

Option 1: A fraction of the energy load met by renewables in the 

built environment and unbuildable brownfield sites within the 

community 

Option 2:A fraction of the energy load met by renewable 

generation on community Greenfield sites or from off-site 

renewables used on site

C

Option 0: Energy efficiency opportunities maximized 

Option 1: Fraction of the energy load met by renewables in the 

built environment and unbuildable brownfield sites within the 

community 

Option 2: Fraction of the energy load met by renewable 

generation on community resources or from off-site renewables 

used on site

Option 3: Fraction of the energy load met through RECs that add 

new grid generation capacity

D

Option 0: Energy efficiency opportunities maximized

Option 3: The remainder of the load is met through RECs that 

add new grid generation capacity
11

3.2 Intermediate Steps and Verification to Move to the Aspiration of Net Zero

Achieving the net-zero definition will need to be time-phased with intermediate goals. Timephasing offers flexibility to develop an approach to a net-zero energy community, campus, or 

base that is very aggressive in terms of energy but yet recognizes that the energy solution needs 

to be balanced against budget. Time-phasing also extends to the annual cycle. For example, 

assuming the system is grid connected, net zero allows a community to go into a carbon 

electricity debt through the winter months and “break even” at the end of the summer season 

with excess PV generation. In addition, if a developer takes on a significant carbon debt during 

the construction phase, the process to achieve net zero could establish a target year for getting 

back to net zero through cumulative years of additional renewable energy generation in excess of 

the community needs.

In order to time-phase and set intermediate milestones, the most important task is to define and 

quantify the basecase. For a new development, the basecase represents what the development 

would have used in terms of energy if buildings were designed to code and if transportation 

systems were designed using conventional patterns. It is useful to break down the basecase by 

subcomponents such as: residential buildings, commercial buildings, public buildings, 

transportation within the community, transportation to and from the community, and 

infrastructure. Once the basecase is defined and quantified, intermediate targets can be set. The 

intermediate steps can be anything that community members can visualize and can be large or 

small. For example, “the community has verified that it has generated enough power from 

renewable sources to claim it is net-zero for the equivalent of the elementary school or all 

buildings in the community.” 

One set of intermediate targets could include:

• The equivalent energy offset to accomplish all the “Option 0” strategy in the renewable 

energy supply hierarchy. The Option 0 case represents the amount of energy (or carbon) 

needed to power the community under a scenario where all energy-efficiency measures 

and design improvements are included to maximize cost-effective energy savings.

• The equivalent energy to offset the energy used by buildings in the community.

• The equivalent energy to offset the energy used by transportation in the community.

• The equivalent energy to offset all other uses of energy in the community. 

• The equivalent energy to offset the added energy growth that will come over time as the 

community grows. 

If the community wanted to become a ZEC “C” for example, it could use a combination of 

Options 0, 1, 2, and 3 (from Table 3) to meet each intermediate milestone. Once the equivalent 

energy was offset to meet the intermediate milestone, the community could claim, for example, 

that is has generated enough energy from renewable sources to be net zero for the “equivalent 

of all energy used in the community for buildings.” By breaking down the overall goal into a 

series of smaller sub-goals, communities can measure and demonstrate very significant 

intermediate milestones over time. The intermediate milestones break the daunting task of 

achieving net zero into a series of manageable pieces that are planned, budgeted for, and 

achieved over time. Breaking this goal into a series of smaller, manageable and financeable 

pieces over time, might be the key for communities to move towards the aspiration of net-zero 

energy. 
12

One example set of intermediate milestone toward the aspiration of net zero is shown 

graphically in Figure 2.

Figure 2. Intermediate milestones to Net Zero Communities

When the community has implemented all energy efficiency and conservation opportunities and 

quantified these energy savings, the community can claim it fully met the achievement of a netzero community for milestone one. In this way, over time the community can show progress and 

claim success as it moves toward the “aspiration of net zero.”

Developers of a new community may decide that they cannot take the community to net zero 

because they do not control all aspects of energy use within the community. In that case, the 

suggestion is that they define the energy used in a typical community and then define the 

quantity of energy that represents their sphere of influence and control. When the developers can 

calculate and verify that they have reduced energy use in the community to the degree necessary 

to achieve the milestones they feel are within their control, then they can claim they have 

achieved net zero for “all measures within their control.” The measurement and verification 

would be important to substantiate the achievement of the goal.

The measurement and verification process is the key to defining and tracking the goal. The 

energy used by the community would be measured by comparing a baseline of energy needs for 

buildings, community infrastructure and transportation (as a minimum) to the energy supplied by 

Options 0-3 on an annual basis from renewable energy sources. Measuring can be done on a site 

or source energy basis, carbon basis, or energy cost basis in order to compare all renewable 

energy supplied (for electricity, thermal and transport fuels) to all loads to determine if the goal 

was achieved over a given time period. In this way, for example, excess renewable electricity 

production could be used to offset the use of conventional thermal energy or transportation fuels 

used. 
13

4 Conclusions

In order to realize deep energy savings it is important to look at energy use in a community as an 

integrated system rather than discrete problems and solutions in each energy end-use sector. By 

defining a zero-energy community and offering a set of metrics to measure progress toward the 

aspiration of net zero, we aid in moving the dialog from buildings-only zero-energy solutions to 

solutions that consider all uses of energy within a community for buildings, transportation, 

community infrastructure, industry, and other.

A definition for a zero-energy community is different and more complex than that of a ZEB 

because a community uses energy not only for buildings but also for industry, vehicles, and 

community-based infrastructure. This paper builds on work done to develop a zero-energy 

buildings definition and expands the concept to define a zero-energy community. The ZEB 

hierarchical renewable classification system is then applied to the concept of community. This 

approach emphasizes using all possible cost-effective energy-efficiency and demand-avoidance 

strategies first and then using renewable energy sources and technologies that are located in three 

places: first, on space within the built environment or unusable brownfield sites within the 

community; second, on either green space within the community or located outside the 

community boundary but within the region and brought into the community; and third, through 

the purchase of new RECs.

In addition, we set up a series of intermediate milestones so a community can begin to plan 

toward the aspiration of net zero over time. The goal of this type of classification is to encourage 

developers and campus and community planners to develop a comprehensive multi-year plan to 

understand the magnitude of change needed in community energy patterns to move toward a netzero community. Both the long-term understanding and multi-year implementation strategies 

with a series of intermediate milestones are essential to drive the transition towards greater 

emphasis on energy demand reduction and greater reliance on the use of renewable energy.
14

5 References

Barnett, J. (April 2009). “Net Zero Energy Plan Update.” Presented at the 2009 Miramar 

Marine Corps Air Station to the Net Zero Energy Installation Task Force

Bradt, S. (April 2008). “Going for the Green at Harvard,” Harvard University Gazette Online. 

Accessed on Oct. 5, 2009. http://news.harvard.edu/gazette/story/2008/04/going-for-the-green-atharvard/#

Dautremont-Smith, J. (2009). American College and University President’s Climate 

Commitment (ACUPCC) Implementation. P. 13. Accessed October 5, 2009. 

http://www.presidentsclimatecommitment.org/pdf/ACUPCC_IG_Final.pdf

Harwood, B. et al. (1994). “Sustainable Cities: The Sanborn Principles for Sustainable 

Development.” Presented at the 1994 NREL Technology Mapping Initiative. Accessed October 

11, 2009. http://www.donaldaitkenassociates.com/sanborn_daa.html

Kandt, A.; Mann, M. (Summer 2009). “Technical Support of Greater Yellowstone Area GHG 

Inventory Compilation and Emissions Reduction Strategies.” Presented at the Greater 

Yellowstone Coordinating Committee Monthly GHG Inventory Presentations. 

Pless, S.; Torcellini, P.; Van Geet, O. (NREL); Crawley, D. (U.S. DOE). (2009). Zero Energy 

Buildings: A Classification System Based on Renewable Energy Supply Options. NREL/TP-550-

44586. Golden, CO: National Renewable Energy Laboratory. 

Torcellini, P.; Pless, S.; Deru, M. (NREL); Crawley, D. (U.S. DOE). (2006). Zero Energy 

Buildings (ZEB): A Critical Look at the Definition. NREL/CP-550-39833. Golden, CO: National 

Renewable Energy Laboratory.
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This document provides a definition for a net zero-energy community: A community that offsets all of its energy use 

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