Journal Article

NREL/JA-550-46382

September 2009

Getting to Net Zero

Drury Crawley

U.S. Department of Energy

Shanti Pless and Paul Torcellini

National Renewable Energy Laboratory

Published in ASHRAE Journal

September 2009
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This article w

Contents

Introduction .............................................................................................................................. 1

What Is a Net-Zero Energy Building? ..................................................................................... 2

From “Real Low” to Zero ........................................................................................................ 5

Calibrating Expectations .......................................................................................................... 6

The Future for Net-Zero Energy Buildings ............................................................................. 7

References ................................................................................................................................ 8
1 

Introduction

As the futurist Stewart Brand observed, “Every building is a forecast. Every forecast is wrong.” 

Making forecasts progressively less wrong over time—specifically, forecasts about highperformance buildings—is the purpose of the U.S. Department of Energy’s (DOE) Zero Energy 

Buildings Database. The intent of this article is to provide an overview of the DOE’s efforts 

toward realizing cost-effective net-zero energy buildings (NZEBs). 

Buildings have a significant impact on energy use and the environment. Commercial and 

residential buildings use almost 40% of the primary energy and approximately 70% of the 

electricity in the United States. The energy used by the building sector continues to increase, 

primarily because new buildings are constructed faster than old ones are retired. Electricity 

consumption in the commercial building sector doubled between 1980 and 2003 and is expected 

to increase another 50% by 2025 (EIA 2005). 

The vision of NZEBs is compelling. These highly 

energy-efficient buildings will use, over the course 

of a year, renewable technology to produce as much 

energy as they consume from the grid. Building 

owners and tenants stand to realize attractive returns 

on their NZEB investments while reducing carbon 

footprints. And, while today’s buildings are our 

nation’s highest energy-consuming and carbonemitting sector, with NZEBs, our nation can gain a 

network of clean domestic energy assets (see Figure 

1). 

Yet, how realistic is this vision? How close do 

NZEBs come to realizing their design goals? How 

much does it cost to design and build a net zero 

energy building? Thanks to data being provided 

voluntarily by building owners in the Zero Energy 

Buildings Database, we now have some early insight into these questions and into the drivers of 

net zero energy performance. 

Just as important, we now have an influential community of industry leaders who are committed 

to pushing the boundaries of building performance and sharing the results. As part of the NetZero Energy Commercial Building Initiative, authorized by Congress in the Energy 

Independence and Security Act of 2007 (EISA 2007), DOE hosts industry-led Commercial 

Building Energy Alliances focused on specific interests of key sectors. To date, DOE has 

launched three alliances representing substantial proportions of the square footage in their 

respective sectors: the Retailer Energy Alliance (17%), the Commercial Real Estate Energy 

Alliance (21%), and the Hospital Energy Alliance (17%) (DOE 2009a, 2009b, 2009c). EISA 

2007 also authorizes the Net-Zero Energy Commercial Building Initiative to support the goal of 

net zero energy for all new commercial buildings by 2030, and specifies a zero-energy target for 

50% of U.S. commercial buildings by 2040 and net zero for all U.S. commercial buildings by 

2050. 

Some members of the alliances take their commitment to high performance an additional step. 

With technical support from DOE national laboratories, each of these “National Account” 

companies pledges to design and construct one new building that is at least 50% more efficient 
2 

than ASHRAE Standard 90.1-2004 and to renovate a facility to achieve at least 30% savings 

over standard practices. With these investments—and the detailed measurement and verification 

that will accompany them—researchers will gain a richer understanding of the best practices and 

technologies for achieving high performance, along with the operational and cost data for a solid 

business case. By sharing this knowledge base with peers, industry leaders will spur further 

investment in high-performance buildings—and, over time, a critical mass of data will enable 

increasingly precise modeling of building energy performance and greater certainty in net zero 

energy design. 

Building design professional societies also have recognized the vision of net zero energy 

buildings. For example: 

• ASHRAE Vision 2020 report (ASHRAE 2008) sets out requirements for developing the 

tools by 2020 to enable commercially viable net zero energy buildings by 2030. 

ASHRAE’s recent conference on net zero energy buildings featured more than 25 posters 

(ASHRAE 2009) of NZEBs, some operating close to or at net zero and others in various 

stages of design or construction. 

• The AIA 2030 Challenge (AIA 2009) calls for incrementally reducing energy use, starting 

with a 50% reduction over existing buildings’ energy use and increasing savings up to 

2030, when new buildings will be carbon neutral. Architecture firms, large and small, are 

beginning to make this voluntary commitment to adopt energy-saving targets in building 

design and implement steps to reach the carbon-neutral goal. 

Policymakers also are embracing net-zero energy buildings as a key strategy for meeting energy 

and carbon goals. The California Public Utilities Commission, for example, has an energy action 

plan to achieve net zero energy for all new residential construction by 2020 and net zero for all 

new commercial construction by 2030. This action plan will provide direction for future 

development of California’s Title 24 building energy codes, as well as incentives for net zero 

buildings. NZEB goals also were recently announced by the European Parliament in a March 

2009 press release (European Parliament 2009). All European Union Member States are to 

ensure that all newly constructed buildings produce as much energy as they consume on-site no 

later than December 31, 2018. 

It is still early in the collection and analysis of energy-performance data. But it is already clear 

that high-performance commercial buildings—some NZEBs, some “almost NZEBs”—can be 

constructed cost effectively, providing productive environments for occupants, reducing 

operating costs, and enhancing the competitiveness of commercial properties. Here is a synopsis 

of what we’ve learned so far. 

What Is a Net-Zero Energy Building?

Calibrating expectations is a foundation for success in any net-zero energy building (NZEB) 

project. 

In the broadest sense, an NZEB is a residential or commercial building with greatly reduced 

energy needs. In such a building, efficiency gains enable the balance of energy needs to be 

supplied with renewable energy technologies. 

But this broad definition leaves plenty of room for interpretation—and for misunderstanding 

among the owners, architects, and other players in an NZEB project. Agreeing to a common 

definition of NZEB boundaries and metrics is essential to developing design goals and strategies. 
3 

To improve clarity, the National Renewable Energy Laboratory has created two foundational 

references:

Zero Energy Buildings: A Critical Look at the Definition (August 2006) documents four NZEB 

definitions: net zero site energy, net zero source energy, net zero energy costs, and net zero 

energy emissions (Torcellini et al. 2006a). 

Zero Energy Buildings: A Classification System Based on Renewable Energy Supply Options

(July 2009) classifies NZEBs based on the renewable energy sources used. At the top of the 

classification system is the NZEB:A—a building that offsets all of its energy use from renewable 

energy resources available within the footprint. At the lowest end is the NZEB:D—a building 

that achieves an NZEB definition through a combination of on-site renewables and off-site 

purchases of renewable energy credits.

Table 1 and the sidebar “Net Zero Energy 

Building Definitions” summarize the four 

NZEB definitions and four energy-use 

classifications. All NZEBs must reduce site 

energy through energy efficiency and 

demand-side renewable energy technologies 

such as daylighting, insulation, passive solar 

heating, high-efficiency HVAC equipment, 

natural ventilation, evaporative cooling, 

ground-source heat pumps, and ocean water 

cooling. In addition, they must use supplyside renewable energy according to one of 

the four NZEB classifications. Column 3 of 

the table discusses the ability of each 

classification to meet the site, source, 

emissions, and cost definitions of NZEBs. 

There is no “best” definition or energy-use 

accounting method; each has merits and 

drawbacks, and the approach for each project 

should be selected to align with the owner’s 

goals. But across all NZEB definitions and 

classifications, one design rule remains 

constant: tackle demand first, then supply. 

Net-Zero Energy Building Definitions

• Net Zero Site Energy: A site NZEB produces 

at least as much renewable energy as it uses in 

a year, when accounted for at the site. 

• Net Zero Source Energy: A source NZEB 

produces (or purchases) at least as much 

renewable energy as it uses in a year, when 

accounted for at the source. Source energy 

refers to the primary energy used to extract, 

process, 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. 

• Net Zero Energy Costs: In a cost NZEB, the 

amount of money the utility pays the building 

owner for the renewable 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. 

• Net Zero Emissions: A net zero emissions 

building produces (or purchases) enough 

emissions-free renewable energy to offset 

emissions from all energy used in the building 

annually. Carbon, nitrogen oxides, and sulfur 

oxides are common emissions that ZEBs offset. 

To calculate a building’s 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). 
4 

Table 1 Classifying NZEBs by Renewable Energy Supply

ZEB 

Classification

ZEB Supply-Side Options ZEB Definitions

On-Site Supply Options

A 

Use renewable energy sources available within the 

building’s footprint and directly connected to the 

building’s electrical or hot/chilled water distribution 

system. 

(Examples: Photovoltaic, solar hot water, and wind located on the 

building.)

YES: Site, Source, Emissions 

Difficult: Cost 

If the source and emissions multipliers for a ZEB:A are 

high during times of utility energy use but low during 

times the ZEB is exporting to the grid, reaching a source 

or emissions ZEB position may be difficult. Qualifying as 

a cost ZEB may be difficult depending on the net-metering 

policies in the area.

B 

Use renewable energy sources as described in ZEB:A

 and

Use renewable energy sources available at the building 

site and directly connected to the building’s electrical or 

hot/chilled water distribution system. 

(Examples: Photovoltaic, solar hot water, low-impact 

hydroelectric, and wind located on parking lots, adjacent open 

space, but not physically mounted on the building.) 

YES: Site, Source, Cost, Emissions 

Difficult: Cost 

If the source and emissions multipliers for a ZEB:B are 

high during times of utility energy use but low during 

times the ZEB is exporting to the grid, reaching a source 

or emissions ZEB position may be difficult. Qualifying as 

a cost ZEB may be difficult depending on the net-metering 

policies in the area.

Off-Site Supply Options

C 

Use renewable energy sources as described in ZEB:A; 

ZEB:B, and ZEB:C

 and

Use renewable energy sources available off site to 

generate energy on site and directly connected to the 

building’s electrical or hot/chilled water distribution 

system. 

(Examples: Biomass, wood pellets, ethanol, or biodiesel that 

can be imported from off-site, or collected from waste streams 

from on-site processes that can be used on-site to generate 

electricity and heat.)

YES: Site, 

Difficult: Source, Cost, Emissions

A ZEB:C source and emission position may be difficult 

if carbon-neutral renewables such as wood chips are 

used or if ZEB has an unfavorable source and carbon 

multipliers. This can occur if a ZEB exports energy 

during times that the utility has low source and carbon 

impacts, but imports energy when the utility has high 

source and carbon impacts. ZEB:C buildings typically 

do not reach a cost ZEB position because renewable 

materials are purchased to bring on-site—it would be 

very difficult to recoup these expenses by any 

compensation received from the utility for renewable 

energy generation. 

D 

Use renewable energy sources as described in 

ZEB:A, ZEB:B, and ZEB:C

and

Purchase recently added off-site renewable energy

sources, as certified from Green-E (2009) or other 

equivalent renewable-energy certification programs. 

Continue to purchase the generation from this new 

resource to maintain ZEB status.

(Examples: Utility-based wind, photovoltaic, emissions 

credits, or other “green” purchasing options. All off-site 

purchases must be certified as recently added renewable 

energy (Green-E 2009). A building could also negotiate with 

its power provider to install dedicated wind turbines or PV 

panels at a site with good solar or wind resources off-site. In 

this approach, the building might own the hardware and 

receive credits for the power. The power company or a 

contractor would maintain the hardware.)

YES: Source, Emissions

NO: Site, Cost

ZEB:D buildings may qualify as source and emissions 

if they purchase enough renewable energy and have 

favorable source and emissions factors. They will not 

qualify as site and cost.
5 

NZEB owners and designers should first use all possible cost-effective energy efficiency 

strategies and then incorporate renewable energy, weighing the many possible supply options 

available and giving preference to sources available within the building footprint. Use of on-site 

renewable options over off-site options minimizes the overall environmental impact of an NZEB 

by reducing energy transportation, transmission, and conversion losses. 

In addition to efficiency measures, demand-side strategies may include renewable energy sources 

that cannot be commoditized, exported, or sold, such as passive solar heating, daylighting, solar 

ventilation air preheaters, and domestic solar water heaters. Typical supply-side generation 

options include photovoltaics, solar hot water connected to a district hot water system, wind, 

hydroelectricity, and biofuels. 

From “Real Low” to Zero

Our earliest experiences were with high-performance buildings that were not designed for net 

zero energy. DOE monitored the performance of six of these “real low energy buildings” 

(Torcellini et al. 2006b).

What we found is that all of these buildings delivered significant energy and cost savings under 

actual operating conditions—but that none of them reached the levels of savings modeled in the 

design. First, design teams were a bit too optimistic about the occupants’ behavior and 

acceptance of systems. Undoubtedly, some of the shortfall also rested with inherent uncertainty 

of designing buildings and forecasting their performance. Energy consumption was higher and 

photovoltaic (PV) energy production was lower than simulations predicted. In particular, 

daylighting contributed less than predicted, which meant more electrical lighting was used 

(Torcellini et al. 2006). 

Again, driving down this uncertainty—and accelerating investments in net zero energy 

buildings—is the motivation behind the Zero Energy Buildings Database (DOE 2009d). 

The database currently houses data on eight commercial buildings, voluntarily reported by 

owners and architects. Some data sets are modeled for yet-to-be-constructed facilities; others 

include both modeled and end-use operating data. Each case study provides details on key 

energy efficiency and renewable energy technologies, a discussion of the NZEB definition(s) 

used, cost and financing information, and images of the project. In general, buildings with 

measured data in this database are fairly small (less than 14,000 ft2

 [1300 m2]) one- or two-floor 

commercial buildings. The next generation of net zero energy buildings, as evident in the 

posters mentioned previously, will include larger commercial buildings such as K–12 schools 

and medium and large office buildings. 

One example under construction is the new Research Support Facilities (RSF) at DOE’s National 

Renewable Energy Laboratory (NREL) in Golden, Colorado (see Figure 2). This 218,000 ft2

(20,253 m2

) administration and executive office space and data center will rely solely on renewable energy available on the roof and in the parking lot (NZEB:B Classification). At the point of 

building turnover, the energy goals are whole building energy use intensity of 25 kBtu/ft2·yr (284 

MJ/m2 · yr), LEED Platinum, and 50% energy savings. By including energy performance in the 

design/build acquisition process, NREL is attempting to ensure the energy models represent what 

is actually built. For example, to ensure the as-installed wall insulation represents the R-value in 

the energy model, installation preconstruction meetings are held to ensure the insulation is 

installed to minimize thermal breaks. NREL also specified plug loads to be used in the energy 
6 

models, and encouraged the design team to identify efficiency strategies in the purchasing, 

control, and operations of the plug and process loads. 

Figure 2 Proposed orientation of the NREL Research Support Facilities

Calibrating Expectations

In the quest for ever-greater precision in energy-performance measurement, we’ve uncovered the 

need for greater precision in definitions. What do people mean by net zero energy performance? 

Are they talking the same language and making the same assumptions? Aligning goals and 

expectations—as well as measurements—requires specificity.

Conceptually, an NZEB is one that has no adverse energy or environmental impact associated 

with its operation. It is a building that is highly energy efficient and that is capable of fulfilling 

the balance of its energy needs using renewable energy technologies, producing at least as much 

energy over the course of a year as it draws from utility grids. It is also possible to have a gridindependent NZEB, with back-up energy needs supplied from renewable resources such as wood 

pellets or biodiesel. 

To hone this concept into a measurable and verifiable definition, the National Renewable Energy 

Laboratory has developed a series of papers drawing on real-world experiences, including data 

reported through the Zero Energy Buildings Database. 

To arrive at a shared definition, parties in an NZEB project must evaluate two interrelated 

questions: 

• How will we account for energy use? Some projects target net zero energy use at the 

site. Others allow for purchased renewable energy to supplement on-site renewables, 

with that energy use accounted for at the source. In other cases, energy cost is the 

predominant factor, with the goal being to offset any purchased energy with revenues 

NREL PIX 16277
7 

realized through the sale of on-site renewables. Still others target net zero emissions of 

carbon, nitrogen oxides, and sulfur dioxide. 

• What are our boundaries for choosing among renewable energy options? If a project 

targets net zero energy use at the site, that necessarily limits the renewable energy 

choices to sources and technologies available within the building footprint or at the site. 

Some projects use renewable energy sources from beyond the site (e.g., biomass) to 

produce power at the site, while others incorporate purchased renewable energy.

Clearly, getting to an agreement on energy-use accounting and renewables options is pivotal to 

determining NZEB design goals and strategies. 

The Future for Net-Zero Energy Buildings

Ongoing measurement and verification are essential in realizing the full benefits of a net zero 

energy design. DOE recommends that NZEB status be reviewed and tracked each year through 

utility bills or submetering.

Realistically, a building may be designed to achieve one or more NZEB definitions but may not 

achieve a net zero energy position in actual operations every year. Any NZEB may fall into the 

near-NZEB category in a given year, depending on weather, the condition of the building, 

operations, and other factors. A well-operating NZEB also may become a near NZEB during 

abnormal weather years that have above-average heating and cooling loads, with below-average 

solar and wind resources. 

An area under study at DOE and the national laboratories is the potential for net zero energy use 

at a community or campus level. Clearly, buildings do not stand alone, and more data is needed 

to determine optimal ways to extend the boundaries of net zero energy to most effectively use the 

renewable energy sources available within a broader footprint. 

Another area of study, initiated in partnership with utilities, is the longer-term impact of NZEBs

on energy production. Utilities are concerned about how large-scale application of NZEBs will 

affect grid stability. A primary lesson learned from some of the initial NZEBs is that peak 

demand issues are even more pronounced than in typical buildings (Torcellini et al. 2006b). 

While NZEBs have large PV systems that offset significant energy use, their demand for grid 

power peaks in the evening, during morning warm-up, or when a cloud blocks the sun. As a 

result, NZEBs often do not offset peak demand. 

To address the potential poor load factors of NZEBs, more research is needed on energy storage 

and its effect on large-scale NZEB penetration scenarios. With energy-storage integration, flatter 

load profiles, and better load factors, utilities will be much more agreeable to the concept of large 

numbers of buildings with significant on-site generation capacity.

Energy consumption in our nation’s commercial building sector will continue to increase until 

buildings can be designed to use energy efficiently and produce enough energy to offset their 

growing energy demand. DOE has set an aggressive goal to create the technology and 

knowledge base for cost-effective net zero energy commercial buildings by 2025, and we are 

gratified to have many industry leaders joining us in this quest. 

We invite commercial architects, engineers, and owners to use the Zero Energy Buildings 

Database and to contribute their own data to make it an increasingly useful tool. 
8 

References

AIA. (2009). AIA 2030 Commitment. www.aia.org/about/initiatives/AIAB079458. Last 

accessed September 2009.

ASHRAE (2008). “ASHRAE Vision 2020.” www.ashrae.org/doclib/20080226_ 

ashraevision2020.pdf. Last accessed September 2009.

ASHRAE. (2009). Net Zero Conference. Poster Presentations. 

www.ashrae.org/events/page/2198. Last accessed September 2009.

DOE. (2009a). Retailer Energy Alliance. www.eere.energy.gov/buildings/retailer. Last 

accessed September 2009.

DOE. (2009b). Commercial Real Estate Energy Alliance. www.eere.energy.gov/buildings/ 

real_estate/. Last accessed September 2009.

DOE. (2009c). Hospital Energy Alliance. www.eere.energy.gov/buildings/hospital/. Last 

accessed September 2009.

DOE. (2009d). Zero Energy Buildings Database. http://zeb.buildinggreen.com. Last accessed 

September 2009. 

EIA. (2005). Annual Energy Review 2004. Washington: U.S.Department of Energy, Energy 

Information Administration. www.eia.doe.gov/emeu/aer/contents.html. 

European Parliament. (2009). Press Release. www.europarl.europa.eu/sides/getDoc.do? 

pubRef=-//EP//TEXT+IM-PRESS+20090330IPR52892+0+DOC+XML+V0// 

EN&language=EN. Last accessed September 2009.

Green-E. (2009). www.green-e.org. (last accessed March 2009).

Torcellini, P.; Pless, S.; Deru, M.; Crawley, D. (2006a). Zero Energy Buildings: A Critical 

Look at the Definition. ACEEE Summer Study on Energy Efficiency in Buildings. Golden, CO: 

National Renewable Energy Laboratory. www.nrel.gov/docs/fy06osti/39833.pdf. Last accessed 

September 2009. 

Torcellini, P.; Pless, S.; Deru, M.; Griffith, B.; Long, N.; Judkoff, R. (2006b). Lessons Learned 

from Case Studies of Six High-Performance Buildings. NREL Report No. TP-550-37542. 

Golden, CO: National Renewable Energy Laboratory. www.nrel.gov/docs/fy06osti/ 37542.pdf. 

Last accessed September 2009.

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Buildings have a significant impact on energy use and the environment. Commercial and residential buildings use

almost 40% of the primary energy and approximately 70% of the electricity in the United States. The energy used by 

the building sector continues to increase, primarily because new buildings are constructed faster than old ones are 

retired. Electricity consumption in the commercial building sector doubled between 1980 and 200 and is expected to 

increase another 50% by 2025. 

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building; energy; net-zero energy bulding; nzeb

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