How Energy Efficiency Of A Building Is Measured?

Energy efficiency is the practice of using less energy to achieve the same outcome from a building or space. The energy efficiency of a building measures how much energy is consumed for heating, cooling, lighting, and other energy needs compared to a typical or similar building.¹ Measuring a building’s energy efficiency is important because it allows building owners and operators to understand how their building performs compared to standards and benchmarks. Tracking energy efficiency over time also helps identify opportunities to implement upgrades and reduce energy waste.

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Energy Use Intensity

Energy Use Intensity (EUI) is a metric that represents the energy consumed by a building relative to its size. It is calculated by dividing the total energy consumed by the building in one year (measured in kBtu) by the total floor area of the building (measured in square feet).

The EUI is expressed as energy per square foot per year. A lower EUI indicates a more energy efficient building. According to the U.S. Energy Information Administration, the average EUI for office buildings is around 90-100 kBtu/sq ft/year (Energystar).

The main advantages of using EUI as a metric are:

It allows easy comparison of energy performance between different buildings, since it normalizes for building size.
It accounts for all energy used in a building, including electricity, gas, steam, etc.

The calculation is straightforward once total building energy use data is available.

Some limitations of EUI include:

It does not account for occupancy, so buildings with different occupancy levels may not be directly comparable.

It does not indicate savings potential or provide detail into how energy is used in a building.
EUIs can vary significantly by building type, climate, and operational schedule.

Building Energy Star Score

The Building Energy Star Score is a scoring system developed by the U.S. Environmental Protection Agency (EPA) that measures the energy efficiency of commercial buildings on a scale from 1 to 100. It compares the actual energy use of a building to what a typical building of similar size and function would use (EPA 2022).

A building’s Energy Star score is calculated using data from the building’s past 12 months of energy bills along with additional details about the building like square footage, number of workers, computers, operating hours, etc. This data is run through EPA’s Portfolio Manager tool which compares it to the energy use of similar buildings across the country (EPA 2022).

The advantages of the Energy Star score are that it provides an easy way to benchmark a building’s energy performance and identify opportunities for improvement. However, the scoring is based on a building’s past energy use rather than its efficiency potential. Two identical buildings could receive different scores if operated differently. The score also relies on self-reported data that may not always be accurate (Statista 2023).

Overall the Energy Star score serves as a simple metric for tracking and comparing a building’s energy use over time and to peers. But it has limitations and is best used alongside more robust efficiency measurement tools.

Sources:

[1] https://www.energystar.gov/buildings/benchmark/understand_metrics/energy_star_score

[2] https://www.statista.com/statistics/1356978/energy-star-score-of-certified-buildings-us-by-type/

Home Energy Rating System (HERS) Index

The Home Energy Rating System (HERS) Index is a measurement of a home’s energy efficiency. It was developed by the Residential Energy Services Network (RESNET), a nonprofit organization.

The HERS Index is calculated by a certified home energy rater during a home energy rating. The rater will conduct onsite inspections and diagnostic testing to evaluate the home’s energy-related features such as the insulation, windows, heating/cooling systems, water heater, and more. The home’s data is input into RESNET-accredited energy modeling software which then generates the home’s HERS Index Score.

The Score is indexed to a reference home which scores 100. Therefore, a home with a HERS Index Score of 70 is 30% more energy efficient than the reference home. The lower the score, the more energy efficient the home. A home with a Score of 0 is a zero net energy home that produces as much energy as it consumes over a year.

New homes can achieve Scores between 60-80. An existing home being retrofitted can achieve Scores of 90 or higher. According to the U.S. Department of Energy, a typical resale home scores 130 prior to any energy improvements.[1]

The major benefits of the HERS Index is that it allows homeowners to understand how efficient their home is compared to a standard home, and identify areas for efficiency upgrades. The score also serves as a consistent and unbiased standard for determining a home’s efficiency. However, a limitation is that the Score is calculated under specific test conditions which may not reflect actual occupant energy usage.[2]

Building Emission Rate

The Building Emission Rate (BER) is a measurement used to determine the energy efficiency and environmental impact of a building’s design. The BER calculates the amount of carbon dioxide emitted per square meter of floor area each year (IEA).

To calculate the BER, the amount of energy consumed by the building for heating, cooling, ventilation, lighting and hot water is estimated. This energy use is then converted into kilograms of CO2 emissions based on the carbon intensity of the local energy grid. The total annual emissions are divided by the floor area of the building to arrive at the final BER result (kgCO2/m2/year).

The lower the BER, the more energy efficient the building. A low BER indicates less energy is required for the building’s operations, resulting in lower carbon emissions. The BER provides a benchmark to assess and compare the environmental performance of different buildings (UNEP).

The advantage of using BER is that it accounts for both energy efficiency and carbon intensity of the energy supply. However, the BER is based on modeled energy use rather than actual metered energy consumption. So it may not fully reflect how energy is used once the building is occupied.

Energy Performance Certificate

An Energy Performance Certificate (EPC) is a measure of the energy efficiency of a building. EPCs are required in the European Union by the Energy Performance of Buildings Directive.

EPCs include:

Energy efficiency rating from A (very efficient) to G (inefficient)
Recommendations to improve the building’s energy performance
Approximate energy use and carbon dioxide emissions

Pros of EPCs:

Inform buyers and renters about a building’s energy efficiency
Motivate upgrades to improve energy performance

Provide data to gauge progress in energy efficiency

Cons of EPCs:

Ratings based on computer models, not actual energy use

Recommendations may not fit owner’s budget or needs
Limited quality control and auditing in some areas

Energy Benchmarking

Energy benchmarking refers to the process of measuring a building’s energy performance and comparing it to itself over time, or to other similar buildings. It involves gathering energy usage data and normalizing it for relevant variables like building size, operations,occupancy and weather^[1]. The goal is to establish a baseline for evaluating energy efficiency efforts and identifying opportunities for improvement.

The key steps in energy benchmarking typically include^[2]:

Collecting energy usage data (e.g. utility bills) for a minimum of 12 consecutive months
Entering the data into Portfolio Manager or another benchmarking software

Normalizing the data for relevant variables like climate and operating hours
Generating benchmarking metrics like Energy Use Intensity (EUI) and ENERGY STAR score
Comparing metrics to past performance and/or peers

Setting energy efficiency goals and identifying upgrades

Major benefits of energy benchmarking include^[3]:

Establishing an energy usage baseline

Identifying underperforming buildings
Setting energy reduction goals
Motivating continuous energy management

Assessing effectiveness of efficiency projects
Comparing performance across real estate portfolios

Potential limitations include^[3]:

Requires time and effort to collect robust energy data
May not account for all variables impacting energy use
Limits comparability across less similar building types

Does not prescribe specific efficiency solutions

Overall, energy benchmarking delivers valuable insights into building energy performance at moderate effort. Many cities now mandate it for larger buildings.

ASHRAE Levels

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has established standardized levels for building energy audits that are widely used in the industry (ASHRAE Energy Efficiency). There are 3 levels of ASHRAE energy audits:

Level 1 – Walkthrough Assessment: This is a basic energy audit that involves a walkthrough of the building to identify energy conservation measures (ECMs). The auditor will review utility bills and conduct limited equipment performance measurements. Pros: Quick and inexpensive. Cons: Provides only a rough estimate of potential savings.

Level 2 – Energy Survey and Engineering Analysis: This audit involves more detailed data collection including energy use data, systems and equipment measurement, and utility rate analysis. Advanced energy modeling provides investment-grade savings calculations. Pros: Provides solid analysis of ECM options and savings. Cons: More expensive than a Level 1 audit.

Level 3 – Detailed Analysis and Financial Grade Audit: The most comprehensive audit, involving detailed field data collection and complex energy modeling for the entire facility. All ECMs are analyzed for costs and benefits. Pros: Provides highly reliable projections for major capital projects. Cons: Requires significant time and expense.

Overall, ASHRAE audits can provide critical data to building owners and managers looking to improve energy efficiency and reduce costs. The level of audit should match the goals and budget of the project.

LEED Certification

LEED, which stands for Leadership in Energy and Environmental Design, is a green building certification program that rates buildings on their sustainability and energy efficiency. LEED certification is administered by the U.S. Green Building Council (USGBC).

There are four levels of LEED certification that a building can achieve based on the number of points it earns across several categories like energy savings, water efficiency, materials selection, and indoor environmental quality. From lowest to highest, the levels are Certified, Silver, Gold, and Platinum.

Some of the pros of LEED certification are that it can lower operating costs through energy and water savings, boost asset value, reduce waste, improve air quality and acoustics, and demonstrate a commitment to sustainability. However, there are also some cons such as the complex documentation and submittal requirements, high upfront costs for certification and design, and the need to stay current with evolving LEED standards [1].

Conclusion

There are various metrics used to measure a building’s energy efficiency and performance. The most common ones include Energy Use Intensity (EUI), Building Energy Star Score, Home Energy Rating System (HERS) Index, Building Emission Rate, Energy Performance Certificate, Energy Benchmarking, ASHRAE Levels, and LEED Certification.

EUI measures energy use per square foot, allowing comparison between buildings. Energy Star Score rates efficiency on a 1-100 scale. HERS Index is commonly used for homes. Emission rate calculates greenhouse gas emissions. Benchmarking compares against similar buildings. ASHRAE Levels use energy modeling simulations. LEED Certification is a holistic green building standard.

When choosing a metric, consider the building type, location, system boundaries, and reporting needs. Simpler metrics like EUI and Energy Star provide easy benchmarking. More complex methods like energy modeling offer deeper analysis. Select metrics aligned with efficiency goals and required by local regulations. Use multiple metrics for a comprehensive view of performance.