What Is HERS Rating (Solar)? Definition & Guide

Key Takeaways

HERS Index scores range from 0 (net-zero) to 150+ (inefficient), with lower scores indicating better efficiency
A standard new home scores around 100; existing homes typically score 130–150
Solar PV directly reduces the HERS score by offsetting energy consumption
Many state building codes now require HERS ratings for new construction
A HERS rating below 50 often qualifies homeowners for green mortgage programs and incentives
Solar designers must understand HERS scoring to size systems that meet builder and code requirements

What Is a HERS Rating?

A HERS rating (Home Energy Rating System) is a standardized score that measures a residential building’s energy efficiency. Developed by RESNET (Residential Energy Services Network), the HERS Index uses a scale where 100 represents a standard new home built to the 2006 International Energy Conservation Code (IECC). A score of 0 means the home produces as much energy as it consumes annually — a net-zero energy home.

When solar PV is added to a home, the system’s estimated annual production is subtracted from the home’s energy consumption in the HERS calculation. This means solar panels can significantly lower a HERS score, sometimes pushing it below zero for homes with high-efficiency envelopes and large PV systems. For solar designers, understanding the HERS rating is important because builders and homeowners increasingly specify target HERS scores that determine the required system size.

Every 1-point reduction in the HERS Index corresponds to roughly a 1% reduction in energy consumption compared to the reference home. A home scoring 50 uses approximately 50% less energy than the standard new construction baseline.

How HERS Rating Works with Solar

The HERS rating process integrates solar production into the overall energy performance calculation:

Home Energy Audit

A certified HERS rater evaluates the building envelope — insulation, windows, air sealing, HVAC efficiency, water heating, and lighting. Each component receives an efficiency rating.

Baseline Calculation

Energy modeling software (typically REM/Rate or EnergyGauge) calculates the home’s annual energy consumption without solar, establishing the pre-PV HERS score.

Solar Production Estimate

The proposed PV system’s annual production is estimated based on system size, orientation, tilt, shading, and local solar resource. This is entered into the HERS model.

Adjusted Score Calculation

Solar production offsets energy consumption in the model. The adjusted HERS score reflects the net energy position — consumption minus on-site generation.

Certification & Documentation

The HERS rater issues a certificate with the final score. This document is used for code compliance, green building certifications, mortgage applications, and incentive programs.

Core Formula

HERS Index = (Home’s Total Energy Use − Solar Production) / Reference Home Energy Use × 100

Types of HERS Rating Applications

HERS ratings serve different purposes depending on the context and stakeholder requirements.

Most Common

New Construction Compliance

Many state and local building codes require HERS ratings for new homes. Solar is increasingly specified to meet energy code targets, particularly in states like California (Title 24) where PV is mandatory on new residential construction.

Financial

Green Mortgage Qualification

Energy-efficient mortgages (EEMs) and green building programs offer better terms for homes with low HERS scores. A score below 50–60 often qualifies for reduced interest rates, higher loan amounts, or rebates.

Certification

Green Building Programs

ENERGY STAR, LEED for Homes, DOE Zero Energy Ready Homes, and other certification programs use HERS scores as a primary metric. Solar PV is often the most cost-effective way to achieve target scores.

Retrofit

Existing Home Upgrades

Homeowners adding solar to existing homes can request a HERS rating to document improved energy performance. This supports resale value claims and may qualify for additional incentives.

Designer’s Note

California’s Title 24 requires solar PV on all new residential construction. The required system size is calculated using the CEC’s prescriptive method, which correlates with HERS score targets. Understanding this relationship helps solar designers right-size systems for builder clients.

Key Metrics & Calculations

Several metrics connect HERS ratings to solar system design:

Metric	Unit	What It Measures
HERS Index Score	points	Overall home energy efficiency (lower = better)
Pre-Solar HERS Score	points	Home efficiency without PV offset
Post-Solar HERS Score	points	Home efficiency with PV offset
PV System Size Required	kW	Capacity needed to reach target HERS score
Annual Energy Offset	kWh/yr	Solar production credited against consumption
Net Energy Position	kWh/yr	Consumption minus solar production

Required System Size Formula

PV Size (kW) = (Pre-Solar Consumption − Target Consumption) / (Annual Solar Resource × System Efficiency)

Practical Guidance

HERS ratings create specific requirements for solar professionals working with builders and homeowners:

Coordinate with the HERS rater early. Get the pre-solar HERS score and target score before designing the PV system. This determines the minimum system size needed.
Account for shading in the HERS model. The HERS calculation uses the PV system’s estimated production. If your design has shading losses, the rater needs accurate production numbers — not nameplate capacity.
Use solar design software that exports HERS-compatible data. Production estimates from professional design tools carry more weight with raters than simple calculators.
Design for the target score, not maximum panels. Builders have budget constraints. Size the system to meet the HERS target with a small margin — not to fill every available roof surface.

Install exactly what the HERS model specifies. Changing module count, orientation, or tilt after the HERS rating is issued may void the certificate. Any changes require re-rating.
Provide as-built documentation. The HERS rater needs to verify that the installed system matches the modeled system. Photos, serial numbers, and commissioning data support this verification.
Coordinate timing with inspections. HERS verification often occurs alongside final building inspection. Ensure the PV system is operational and producing before the rater arrives.
Keep module spec sheets on file. HERS raters reference module efficiency, temperature coefficients, and warranty specs. Have these documents accessible for verification.

Position solar as a HERS score optimizer. For builder clients, frame solar as the most cost-effective way to reduce HERS scores compared to envelope upgrades like triple-pane windows or spray foam insulation.
Highlight mortgage benefits. A low HERS score can qualify buyers for energy-efficient mortgages with better rates. This makes the home more attractive and the solar system easier to justify financially.
Quantify resale value impact. Studies show homes with low HERS scores sell for 3–5% more than comparable homes. Solar’s contribution to the HERS score is a measurable value-add.
Target production home builders. Builders constructing 50–500+ homes per year need standardized solar packages that hit HERS targets across floor plans. This is a high-volume sales opportunity.

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Real-World Examples

New Construction: California Title 24 Compliance

A production builder in Sacramento constructs 2,200 sq ft homes with a pre-solar HERS score of 58. California code requires compliance with Title 24 energy standards, which effectively mandate solar PV on new homes. Using solar design software, the designer determines that a 4.2 kW system (10 panels) reduces the HERS score to 42, meeting the builder’s target of sub-45. The standardized design is replicated across 120 homes in the development with minor orientation adjustments.

Green Building: DOE Zero Energy Ready Home

A custom home builder in Colorado targets DOE Zero Energy Ready Home certification, which requires a HERS score of 45 or below (before solar credits). The home’s efficient envelope achieves a pre-solar score of 48. A 6.8 kW rooftop system drops the score to 18, well below the threshold. The low HERS score qualifies the buyer for an energy-efficient mortgage saving $180/month compared to a conventional loan.

Retrofit: Existing Home Energy Upgrade

A homeowner in Virginia has an existing home rated at HERS 128. After adding attic insulation and sealing air leaks (dropping to HERS 105), a 7.5 kW solar system is installed. The post-solar HERS score drops to 62. This qualifies the home for a property tax exemption on the solar equipment and increases the appraised value by an estimated $15,000.

Impact on System Design

HERS rating requirements directly influence solar system design decisions:

Design Decision	HERS-Driven Design	Standard Solar Design
System Size	Sized to hit specific HERS target score	Sized to offset annual consumption or budget
Priority	Code compliance and certification	Maximum savings or ROI
Documentation	Must match HERS rater’s model exactly	Flexible based on field conditions
Module Selection	Higher efficiency may be needed for small roofs	Cost-per-watt optimization
Timeline	Must coordinate with building inspection schedule	Independent installation timeline

Pro Tip

When working with builders, create a HERS score lookup table for each floor plan: “For the 1,800 sq ft model, 3.6 kW hits HERS 45; for the 2,400 sq ft model, 4.8 kW.” This speeds up quoting and eliminates per-home design time on production builds.

Frequently Asked Questions

What is a good HERS rating for a home with solar?

A HERS score of 40–50 is considered very good for a new home with solar. Scores below 30 indicate near-net-zero performance. For ENERGY STAR certification, a home must score at or below the ENERGY STAR HERS target, which varies by climate zone but is typically 50–55. A score of 0 means the home produces as much energy as it consumes annually.

How much does solar reduce a HERS score?

The reduction depends on system size relative to home energy consumption. As a rough guide, each 1 kW of solar capacity reduces the HERS score by 5–10 points for a typical 2,000 sq ft home. A 5 kW system on a home with a pre-solar score of 65 might reduce it to 35–40. The exact impact varies by climate zone, home efficiency, and system orientation.

Is a HERS rating required for solar installation?

No. A HERS rating is not required for standard residential solar installations. However, it is often required for new construction to demonstrate code compliance. It may also be needed for green building certifications (ENERGY STAR, LEED), energy-efficient mortgage qualification, or certain state and utility incentive programs.

How much does a HERS rating cost?

A HERS rating typically costs $300–$800 for an existing home and $150–$400 for new construction (where the rater is already involved in the building process). Costs vary by region, home size, and rater. Some utility incentive programs cover the cost of the HERS rating as part of their solar or energy efficiency rebate package.