How Solar Panels Actually Affect Your HERS Rating in Massachusetts
Solar panels and HERS ratings are often discussed together, but they don't work quite the way some homeowners and builders expect.
One of the most important things to understand is this:
You generally need to meet the applicable HERS threshold before counting on the benefit of on-site solar generation.
Solar can be an excellent part of a high-performance Massachusetts home. It can reduce operating costs, offset electricity use, and become particularly valuable in an all-electric home.
But solar shouldn't be viewed as a way to take a house that doesn't meet its required HERS target and simply bring the score into compliance.
The building needs to perform first.
How Solar Fits Into a HERS Rating
A HERS rating compares the energy performance of a proposed or completed home against a reference home.
The lower the HERS score, the better the modeled energy performance.
For Massachusetts projects using the HERS compliance path, the applicable maximum HERS score depends on the project and building type.
Under the current Massachusetts energy code, for example, new construction using the HERS pathway generally has a maximum score of 45 for an all-electric building and 42 for a mixed-fuel building. Accessory dwelling units and major alterations/additions have different thresholds.
But there is an important detail that can easily be missed:
The HERS score used for compliance is determined before credit for on-site renewable electric generation.
That means you can't simply add a large solar array to a home that is otherwise above the required HERS threshold and assume the solar will bring it into compliance.
Solar Doesn't Replace Building Efficiency
Consider a simplified example.
Suppose an all-electric new home has a required HERS target of 45.
The preliminary model comes back at:
HERS 49
The builder then proposes adding solar panels and assumes the project will now pass.
That's not the way the HERS compliance calculation works.
The project first needs to achieve the applicable HERS requirement without relying on the energy generated by the on-site solar system.
That means the focus may need to return to the building itself:
Air sealing
Insulation
Windows
HVAC efficiency
Hot-water efficiency
Hot-water distribution
Ventilation
Duct efficiency
Lighting
Other modeled energy uses
Once the building is performing where it needs to be, solar can become a very valuable addition.
Why Solar Still Makes a Lot of Sense
This doesn't mean solar isn't important.
In fact, solar can make a tremendous amount of sense for an efficient Massachusetts home.
The difference is that HERS compliance and long-term energy economics are two separate questions.
The HERS rating asks:
How efficiently does this home perform compared with the HERS reference home?
Solar then helps answer a different question:
How much of the home's remaining electricity use can be supplied by on-site renewable generation?
For an all-electric home, that distinction is especially important.
A home may use electricity for:
Heating
Cooling
Water heating
Cooking
Clothes drying
Lighting
Appliances
EV charging
A well-designed solar array can offset a significant portion of that electricity consumption.
All-Electric Homes Can Be a Particularly Good Match for Solar
Solar makes intuitive sense when a home's energy use is already heavily electrified.
Instead of having a mixture of electricity, natural gas, propane, and oil consumption, an all-electric home concentrates much more of its operating energy use into electricity.
That creates a larger opportunity for solar generation to offset the home's total energy costs.
This is one reason I particularly like the combination of:
High-performance envelope + heat pumps + heat-pump water heating + solar
The solar isn't being used to compensate for an inefficient building.
It is being used to offset the electricity required by a building that is already designed to use relatively little energy.
Roof Orientation Matters
Not every roof is equally well suited to solar.
A home with a large, relatively unshaded roof facing a favorable direction can have a much easier time producing useful solar energy than a home with:
Heavy tree shading
Small roof areas
Multiple roof planes
Significant obstructions
Poor orientation
Complicated roof geometry
Roof pitch also matters.
A steeper roof can sometimes provide a good opportunity for solar production in Massachusetts, particularly when the roof orientation and shading conditions are favorable.
That doesn't mean a particular roof pitch guarantees good solar production. The entire site needs to be evaluated.
For a new home, these decisions are best considered before the roof and building design are finalized.
Solar-Ready Isn't the Same as Solar
Massachusetts energy code also distinguishes between being prepared for solar and actually installing solar.
For example, current Specialized Code provisions require solar-ready considerations for certain buildings, but an all-electric home is not automatically required to have solar panels simply because it follows the all-electric pathway.
That's an important distinction.
A homeowner may choose to install solar because it makes financial and energy sense even when the HERS compliance pathway does not require the panels.
Conversely, a project shouldn't assume that being solar-ready means the solar array can be used to solve an underlying HERS compliance problem.
What About HERS Credit for Solar?
This is where the Massachusetts code becomes more nuanced.
The code includes provisions allowing certain clean-energy systems to affect the maximum allowable HERS score.
The exact treatment depends on the type of project and the applicable code provisions.
For example, the current Massachusetts guidance identifies different HERS allowances for new construction, ADUs, and major alterations/additions, and certain all-electric and solar configurations have different maximum scores.
There are also specific provisions for clean-energy trade-offs.
So the important takeaway isn't simply:
"Solar gives you X HERS points."
Instead:
The amount and type of HERS flexibility available from solar depends on the project and the specific Massachusetts code pathway being used.
That's why the HERS model should be evaluated early rather than assuming a solar array will provide a particular number of points.
Don't Wait Until the End to Add Solar
Solar is easiest to incorporate when it is considered during the design phase.
For a new home, the design team can evaluate:
Roof orientation
Roof pitch
Shading
Available roof area
Electrical service
Mechanical equipment
Expected electricity consumption
EV charging
Future battery storage
Solar equipment placement
This can be especially valuable for an all-electric home.
If the home is being designed around heat pumps, a heat-pump water heater, induction cooking, and EV charging, the expected electrical load can be quite different from a conventional home.
Understanding those loads early helps determine whether the available roof area can provide meaningful solar production.
Solar Can Help With Operating Costs
One of the biggest reasons I like solar on an efficient all-electric home has little to do with the HERS score.
It has to do with operating cost.
An efficient home reduces the amount of energy it needs.
Solar can then reduce how much electricity needs to be purchased from the utility.
That combination can provide a powerful long-term strategy:
Use less energy + generate some of your own electricity.
For homeowners concerned about rising utility costs, that can provide another layer of protection against future electricity-price increases.
Massachusetts homeowners also have access to state programs and utility-related solar resources, including the SMART program and net-metering provisions, although program rules and individual project eligibility should always be evaluated based on the current requirements.
What If the Home Is Already Close to the HERS Limit?
This is where solar planning becomes particularly useful.
Suppose a new all-electric home is modeled at HERS 46 when the applicable target is 45.
Adding solar isn't necessarily the first solution.
Instead, the project should look at what is keeping the building from reaching the target.
Maybe there is an opportunity to improve:
Window performance
Air leakage
HVAC efficiency
Water heating
Hot-water distribution
Insulation
Ventilation
Duct performance
A relatively small improvement in one or more of these areas may bring the home into compliance.
Then the solar array can be evaluated on its own merits.
This approach is much more reliable than designing a house that misses the HERS target and hoping solar will make up the difference.
Solar Is Most Valuable When the Building Is Already Efficient
A solar array produces electricity.
It doesn't improve:
A poorly insulated wall
An inefficient HVAC system
Excessive air leakage
Long hot-water runs
Poorly designed ductwork
An inefficient ventilation system
Those issues need to be addressed separately.
That's why I generally view solar as the next step after designing an efficient home, not a substitute for efficient building design.
The strongest combination is often:
Efficient building → efficient mechanical systems → good air sealing → efficient appliances and water heating → solar
Each piece addresses a different part of the home's energy use.
The Bottom Line
If you're building a new home in Massachusetts and considering solar, don't wait until the end of the HERS process to think about it.
But don't make the opposite mistake, either.
Solar should not be the strategy for getting an inefficient home to its required HERS threshold.
First, make sure the building itself can meet the applicable HERS requirement.
Then evaluate solar based on the home's electricity use, roof orientation, shading, available roof area, electrical system, and long-term operating goals.
For an efficient all-electric Massachusetts home with a good solar site, the combination can be particularly compelling.
The HERS rating measures how efficiently the home performs.
Solar helps determine how much of the electricity that home needs can be generated on site.
Those are two different pieces of the same energy strategy.
Frequently Asked Questions
Can solar panels lower my HERS score?
Solar can affect the HERS compliance calculation under specific Massachusetts code provisions, but the applicable HERS compliance score is determined prior to credit for on-site renewable electric generation. The exact treatment depends on the project and code pathway.
Can I use solar to make a HERS 49 home pass a HERS 45 requirement?
You should not assume that you can. Massachusetts HERS compliance is evaluated before credit for on-site renewable generation. The home needs to meet the applicable underlying requirement before relying on qualifying clean-energy provisions.
Is solar required for an all-electric home in Massachusetts?
Not necessarily. Under the current Specialized Code guidance, all-electric homes can follow the applicable HERS or Passive House pathway without automatically being required to install solar panels, although solar-ready requirements may apply.
Does solar make an all-electric home more economical?
It can. An efficient all-electric home uses electricity for most or all major building energy loads, creating a larger opportunity for on-site solar generation to offset purchased electricity.
Does roof orientation matter for solar?
Yes. Orientation, shading, roof pitch, available roof area, and obstructions all influence how useful a roof can be for solar generation.
Should I design the solar system before the HERS rating?
Solar and HERS should ideally be considered together during design, but they serve different purposes. The building should first be designed to meet its applicable HERS requirement, while the solar system can then be evaluated based on the home's expected electricity use and site conditions.
