Home Energy Rating System (HERS)

Summary: A home energy rating system (HERS) is a method of measuring and rating the efficiency of a home’s energy use with the goal of improving a home’s energy efficiency.

A home energy rating system (HERS) can be defined as a system for collecting, processing, and presenting the data needed to produce a home energy rating, which is a standardized determination (or measure) of a home’s energy efficiency plus its potential for improvement. It is a measurement used primarily in the United States.

The energy consumption related to building functions such as cooling, heating, powering of appliances, and lighting accounts for about 20 percent of the world’s total energy utilization. A home energy rating can be applied to gauge the current efficiency of a home’s energy use, or to project the efficiency of a home that is being improved or newly constructed. Hence, HERS are applied for both existing and new homes. In the case of existing homes, HERS help a homeowner to get a report listing options for upgrading the home’s energy efficiency. Based on the report, the homeowner can determine the most effective and efficient ways of upgrading the home’s energy efficiency. In the case of a new home, a HERS allows buyers to conduct a comparison of the energy costs of different houses they are considering buying. Energy assessments consider diverse climatic conditions in different parts of the world or a country and can be benchmarked according to households’ average energy consumption in a given climatic region.

Depending on the timing of ratings, there are basically two types of ratings: projected and confirmed.

• Projected ratings are ratings conducted before the construction of a home or before the installation of energy improvement initiatives at the existing home.

• Confirmed ratings are ratings performed using information or data gathered through an on-site inspection, which could incorporate performance testing of homes.

HERS Steps

As a “system,” home energy ratings are procedural endeavors constituting inputs, diagnostics, processes, and outputs. These HERS activities are accomplished by certified raters.

The initial step is to collect the required data on the house that is going to be rated. Although it is possible to rate houses based on their engineering drawings, a site visit is recommended to ensure reliable data input. The data required vary, depending on the particular HERS, yet generally they include the following information:

• utility providers and rating schedules

• location (climate zone), house age, and orientation (or front azimuth)

• dimensions of the house (wall heights and lengths, overhang sizes, and the like)

• thermal wrapping composition characteristics (basement types, insulation levels, roofs, window, wall, window, window shades, and so on)

• age, efficiency, size, and fuel type of heating system, water heater, and permanent air-conditioning system

• number of lamps, including their types and wattages, in permanent fixtures

• home appliances and the types of fuel they use (clothes dryers, hot tubs, cooking ranges, pools, and so forth)

Most of the time, it is not possible to collect certain pieces of information, so a rater will be forced to apply assumptions based on experience or on formulas. That is, there are frequently situations in which the rater either may consult a default table of average values based on the house’s age or may conduct an educated guess.

Instead of using standard assumptions for some house parameters, sometimes it is possible to use testing equipment to determine parameter values empirically. For instance, the accuracy of a rating can be enhanced through conducting tests such as air leakage tests of a house’s envelope as well as its canal system, applying blower-door testing equipment. Furthermore, diagnostic testing equipment can be used in order to ascertain the efficiency of home furnaces. Diagnostic testing usually improves the accuracy of ratings, yet it raises the cost of ratings by increasing the amount of time and the facilities required to conduct a rating.

Simulation Tools

Data collected as inputs are fed into a computer program. There are different software programs used for simulating building energy uses, for many types of buildings (such as residential and commercial) and for many different purposes (such as building design compliance). HERS software is a subset of this universe of building software. Most HERS simulation tools simulate energy use for only one or two years using standard assumptions for the information required that is not gathered by the professional rater. For the following, assumptions are always made:

• standard occupant behavior (hot water usage, personal appliance usage, and so on)

• based on climatic records, typical weather for the climatic zone under consideration

Output

Usually the HERS software produces three major types of outputs: rating score, energy and cost estimates, and recommendations.

• Rating score: A score is the level or measure of a house’s efficiency as compared to a reference house in a category. Rating scores are usually presented on a 0–100 scale or a 0–5 scale (with a higher score indicating a more energy-efficient house). Scores are often computed for a house’s holistic energy use and for many end uses, including cooling, heating, and water heating. Specifically, if a house receives a rating score of about 80, it can be labeled as a well-performing house.

• Energy and cost estimates: HERS gives rise to an estimated amount of annual energy use and cost of energy for a home as it currently exists, presuming typical occupant behavior. In practice, estimates are broken down into end uses. Comparably, end-use estimates are more relevant than estimates done on an overall basis, because they are highly dependent on the static nature of homes.

• Recommendations: Depending on the data generated and analyses made using available models, a HERS recommends energy efficiency improvements which are cost-effective on long-term and life-cycle basis. Different methods are available for estimating cost-effectiveness. Commonly, a measure is said to be cost-effective if its annual energy cost reductions (savings) are larger than the annual cost of the item if the incremental capital cost is financed throughout the lifetime of the item at a specific interest rate. Many of the recommendation sections of a HERS prominently show estimated incremental costs of installation for each item and at least one economic measurement of cost-effectiveness, such as payback period. The recommendations section of a HERS usually depicts the estimated incremental installation cost of each.

Accuracy of HERS

For HERS, accuracy is defined simply as how closely the HERS outputs match actual data. There are many variables that affect energy usage, and hence no one should consider a HERS as a perfectly accurate measure. There are two major sources of inaccuracy or errors in HERS: systematic errors and natural uncertainty. Systematic errors mainly arise from inaccuracies in the assumptions considered and rater mistakes, whereas natural uncertainty results from factors such as occupant behavior and weather conditions. For each of the three outputs of HERS, accuracy has different implications that require different methods of estimating accuracy for each type of output.

• Rating score accuracy: Broadly speaking, a scoring system is said to be accurate if and only if houses with high scores use less energy as compared to houses with low scores (especially if comparison is made among houses of similar floor areas).

• Energy use (cost) estimation accuracy: An error in this case is considered as the percentage that a rated amount is beyond the actual amount. HERS estimation accuracy can be judged by observing the bias and precision of sampled ratings, where precision is measured through averaging individual errors.

• Recommendation accuracy: A certain recommendation is labeled as accurate if it is cost-effective. To judge specific recommendations to be cost-effective, detailed end-user monitoring and installation of recommended features must be conducted.

Bibliography

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"Professional Home Energy Assessments." Energy Saver, www.energy.gov/energysaver/professional-home-energy-assessments#. Accessed 2 Aug 2024.

Singh, M. C., and S. N. Garg. “Energy Rating of Different Glazings for Indian Climates.” Journal of Energy 34 (2009).

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