Home Energy Scores: What They Are and How They Work

A couple buying a home in South Portland last winter asked their realtor about the energy score listed on a property they were considering. The realtor was not sure what the number meant. The sellers were not sure either - their home inspector had included it in the report, and they had never paid attention to it. The buyers ended up calling us to ask if a score of 4 out of 10 was terrible or just average for a 1970's ranch in Maine.

Home energy scores and ratings are becoming more visible in the real estate market, in policy discussions, and in homeowner conversations. But most people do not understand what the numbers mean, how the scoring systems work, or what actions (if any) a low score should trigger. This guide breaks down the major scoring systems, explains what they measure, and puts them in context for Maine homeowners.

The DOE Home Energy Score

The U.S. Department of Energy (DOE) Home Energy Score is the most standardized rating system currently in use. It provides a simple 1-to-10 rating of a home's energy efficiency, where 1 is the least efficient and 10 is the most efficient.

How It Works

A qualified assessor visits the home and collects data on:

• Building dimensions and orientation
• Insulation levels (attic, walls, basement/crawlspace)
• Window types (single pane, double pane, low-E, etc.)
• Air leakage (estimated or measured via blower door)
• Heating and cooling systems (type, fuel, efficiency rating)
• Water heating (type, fuel, efficiency)

This data is entered into the DOE's scoring software, which calculates estimated energy use and assigns the 1-10 score. The score compares the home to the full range of U.S. housing stock - a 1 is in the bottom 10% nationally, a 10 is in the top 10%.

What the Scores Mean for Maine

Most older Maine homes score between 2 and 5. A 1970's ranch with original insulation, single-pane or older double-pane windows, and an oil boiler might score a 3. A home that has been weatherized and has modern heating equipment might score 5-7. New construction built to the current energy code typically scores 7-9.

A score in the 2-4 range does not mean the home is falling apart. It means the home was built to the standards of its era and has not been significantly upgraded for energy performance. That is the majority of homes in Greater Portland.

What a Low Score Tells You

A low Home Energy Score identifies opportunity, not crisis. It tells you that:

• The home likely has significant room for improvement in insulation, air sealing, or both
• Heating and cooling costs are probably higher than they need to be
• Targeted upgrades could meaningfully improve comfort and reduce energy bills
• Rebates and incentives are available to help fund improvements

A home scoring 3 out of 10 that gets comprehensive insulation, air sealing, and a cold-climate heat pump system might move to a 7 or 8. That is a significant improvement in both the number and the real-world performance.

HERS Index (Home Energy Rating System)

The HERS Index is the industry standard for rating energy efficiency of new homes and is sometimes used for existing homes as well. It is maintained by RESNET (Residential Energy Services Network) and is the most detailed and technically rigorous rating system available.

How It Works

A RESNET-certified HERS Rater conducts a thorough assessment of the home, including diagnostic testing (blower door, duct leakage). The home's energy performance is modeled in detail and compared to a reference home built exactly to the 2006 IECC standard.

The scoring scale:

• HERS 100 = Energy use equivalent to the 2006 IECC reference home
• HERS 0 = Net-zero energy home (produces as much energy as it consumes)
• Higher than 100 = Less efficient than the reference home
• Lower than 100 = More efficient than the reference home

Each point on the HERS Index represents approximately 1% more or less energy use than the reference home.

What Typical Numbers Look Like

• Older existing home (unimproved): HERS 130-200+
• Average existing home: HERS 100-130
• New home (current code): HERS 50-70
• High-performance new home: HERS 30-50
• Net-zero home: HERS 0 or below (with solar)

A lower number is better on the HERS Index. If your home rates HERS 150 and you bring it down to HERS 80 through improvements, you have reduced its energy use by roughly 47% compared to where you started.

When HERS Ratings Matter

HERS ratings are most commonly used in new construction, where builders seek a low score to differentiate their homes in the market. For existing homes, HERS ratings are less common but increasingly relevant in real estate transactions and energy improvement planning.

Some mortgage programs (like Energy Efficient Mortgages) use HERS ratings to qualify homes for favorable terms. A low HERS score can also support higher appraised values by documenting the home's reduced operating costs.

Energy Star Certification for Homes

Energy Star certification for homes is a label, not a score. A home either meets the Energy Star criteria or it does not.

Requirements:

• Must be verified by a third-party HERS Rater
• Must meet specific insulation, air sealing, duct sealing, and equipment efficiency standards
• Typically achieves a HERS Index of 65 or lower (varies by climate zone)
• Includes mandatory thermal bypass inspection (checking for insulation defects and air leakage paths)

Energy Star certification is primarily a new construction designation, though deep-energy retrofits of existing homes can sometimes qualify. In Maine, Energy Star homes are uncommon in the existing housing stock but increasingly available in new developments.

Pearl Certification

Pearl Certification is a relatively new system designed specifically for existing homes. It verifies and documents energy improvements made to a home, creating a certified record that transfers with the property.

How It Works

After an eligible contractor completes energy improvements, a Pearl Certification Advisor reviews the work documentation and issues a certification. The certification records specific improvements - for example, R-49 attic insulation, 4.2 ACH50 air leakage, cold-climate heat pump installation - and makes this information available to future buyers.

Why It Matters for Resale

One of the challenges in the real estate market is that energy improvements are often invisible. A home with $15,000 worth of insulation, air sealing, and heat pump work may look identical to an unimproved home in photos and listing descriptions. Pearl Certification creates verified documentation that the improvements exist and meet quality standards.

Research by Pearl indicates that certified homes sell for 5-7% more than comparable uncertified homes. Whether that exact premium holds in the Greater Portland market is hard to verify independently, but the principle is sound: documented, verified improvements are worth more than invisible ones.

Maine-Specific Programs and Context

Maine does not currently have a mandatory home energy disclosure requirement for real estate transactions. Some states and cities require sellers to provide energy scores or utility bill data to buyers - Maine has not adopted this requirement, though it has been discussed.

However, there are Maine-specific contexts where energy performance matters:

Efficiency Maine Assessments

When we conduct an assessment as part of an Efficiency Maine rebate project, we document the home's current insulation levels, air leakage (blower door results), and heating system performance. This documentation goes to Efficiency Maine as part of the rebate application and provides the homeowner with a clear record of baseline conditions and improvements.

Real Estate Market Trends

In Greater Portland's competitive real estate market, energy-efficient homes are increasingly valued by buyers. A home with documented improvements - modern insulation, heat pumps, low air leakage - has a marketing advantage. Buyers understand that lower energy costs mean lower total cost of ownership.

Municipal Climate Plans

Portland, South Portland, and other Cumberland County municipalities have adopted climate action plans that include goals for improving residential energy efficiency. While these plans do not currently mandate home energy scores, they signal a direction where energy performance documentation will become more important.

What Should You Do With This Information

If you are a homeowner considering improvements: An energy score can be a useful starting point for understanding where your home stands, but it is not necessary before moving forward. A free energy assessment from a BPI-certified contractor gives you more actionable information than a score alone, because it identifies specific improvements and their expected impact.

If you are a home buyer: Ask about the home's energy features - insulation type and depth, heating system age and efficiency, air sealing work. If a Home Energy Score or HERS rating is available, use it as one data point in your evaluation. A low score is not a dealbreaker - it is an indicator of improvement opportunity, and Efficiency Maine rebates can help fund upgrades.

If you are a home seller: Documenting your home's energy improvements can help your listing stand out. If you have had insulation, air sealing, or heat pump work done, ask your contractor for a summary of the work and the before/after blower door numbers. This documentation has real value in the current market.

The Assessment That Matters Most

All of these scoring systems share a common starting point: someone needs to evaluate the home's actual performance. Numbers on paper or in a database are only useful if they reflect reality.

Our free energy assessment is a practical evaluation of your home's energy performance. We look at insulation levels, air leakage paths, heating system condition, and overall building performance. You get specific recommendations, estimated savings, and a clear path forward - whether or not you care about the formal score.

Want to know where your home stands? Schedule your free energy assessment or call (207) 221-3221. We serve Greater Portland, including South Portland, Portland, Scarborough, Falmouth, Gorham, and surrounding communities.