Why does gas DHW score better than electric resistance in a HERS model?

HERS scores total source energy against a reference home that uses gas DHW. Electric resistance has an effective COP of roughly 1.0 at the appliance, plus upstream electric grid losses that HERS accounts for in source energy. Net result is that switching from gas to electric resistance often raises the HERS index by 6 to 8 points, which is what we saw on the project that moved from a 40 to a 48.

Is gas water heating still allowed under the Massachusetts Specialized Stretch Code?

Yes, on most multifamily projects, though the path requirements vary. The Specialized Stretch Code is performance-based through HERS or ERI targets, and gas DHW is a recognized fuel choice within those targets. Mass Save also continues to offer incentives for gas DHW on multifamily, which suggests the option is intended to remain viable for projects that need it.

How is electric resistance different from a heat pump water heater for HERS scoring?

Substantially. Electric resistance operates at a COP near 1.0 and tends to raise the HERS index 6 to 8 points compared to gas DHW. Heat pump water heaters operate at a COP near 3.0 and typically beat both gas and electric resistance in the model. The cost picture reverses too, with HPWHs carrying higher equipment and infrastructure costs but more compliance headroom.

Should we expect Mass Save to keep incentivizing gas DHW long term?

Program details change year to year, and the broader Massachusetts policy direction favors electrification. We treat gas DHW as a currently-valid compliance and incentive path on multifamily, but the right project answer depends on the project schedule, financing horizon, and the owner's electrification commitments. Confirm current incentive eligibility against the active program rules at the time of design.

Why Switching from Gas to Electric Water Heaters Cost This Project $627K

Switching multifamily gas water heaters to electric resistance pushed HERS from 40 to 48 and required $627,865 in envelope upgrades to recover.

May 09, 2026

On a multifamily project, the design team wanted to move from gas water heaters to electric resistance water heaters. The framing was that electric is more efficient. The swap moved the HERS score from 40 to 48. The compliance model needed envelope offsets to recover those eight points, and those offsets priced at $627,865. The more efficient choice was a more expensive choice once the energy model ran.

The Real Numbers

The starting design used gas water heating across the building. The HERS rating model returned a score of 40, comfortably under the project target. The team asked us to run the same building with electric resistance tank water heaters in place of the gas units.

The new HERS score came back at 48. An eight-point swing on a single equipment change. The building no longer cleared its compliance target on the existing envelope spec.

To bring the score back into compliance, the model needed envelope upgrades. Better windows on the heating-dominant facades. Upgraded exterior doors. A tighter air sealing target than code minimum. Stacked together, those upgrades priced at $627,865.

Net cost of the equipment swap: $627,865 in envelope work the project did not previously need. The water heater line item itself moved by far less.

The Gas-Versus-Electric-Resistance Math

The intuition that electric is more efficient than gas at the meter is technically correct on a per-fuel-unit basis, but HERS does not score what happens at the meter. It scores total source energy use against a reference building. The reference home uses gas DHW. Substituting electric resistance moves the proposed building further from that reference, not closer.

Electric resistance tanks operate at an effective COP of roughly 1.0. The energy that arrives at the tank is the energy that goes into the water, minus standby losses. Gas water heaters lose more at the appliance, but they avoid the source energy losses on the upstream electric grid that HERS accounts for. On a multifamily building, those grid losses get charged to every kilowatt-hour the building consumes.

Heat pump water heaters break the pattern entirely. An HPWH operates at a COP closer to 3.0, which beats both gas and electric resistance on the HERS index. The conversation about gas-versus-electric water heating is really a conversation about gas-versus-electric-resistance, with HPWHs sitting in a different cost and performance category.

Why Mass Save Still Incentivizes Gas DHW

Mass Save continues to offer incentives for multifamily buildings using gas water heating. That is not a contradiction with the broader electrification push in Massachusetts. It is an acknowledgment that many multifamily projects do not work without it.

The incentive structure recognizes that pulling gas DHW out of a building either forces an HPWH solution, which carries its own infrastructure cost, or forces electric resistance, which forces envelope offsets like the $627,865 above. For projects where neither path closes, gas DHW remains the route that makes the project financeable and code-compliant.

The lesson is not that gas water heating is preferred. It is that the energy model treats gas DHW better than developers expect, and the cost of substituting electric resistance is rarely recoverable through envelope alone.

When Electric Water Heaters Still Make Sense

Electric water heating can be the right call on the right project. The conditions matter.

Heat pump water heaters, not resistance tanks. HPWHs sit in a different performance band. On larger multifamily buildings or projects with envelope already specified to its ceiling, HPWHs can clear compliance where electric resistance cannot.

Code paths that prohibit gas. Some local jurisdictions, lender requirements, or owner-imposed electrification commitments rule out gas DHW regardless of the model output. In those cases the question shifts from gas-versus-electric to which electric configuration costs the least.

Buildings with onsite renewables. Solar PV or other production can absorb a portion of the electric resistance penalty before it lands in the index, depending on how the path treats production credits.

Projects with envelope room to give. On a smaller or tighter building where the envelope has not been fully optimized, the cost to recover an electric-resistance penalty through envelope upgrades may be acceptable. Larger or already-tight buildings rarely have that room.

The Decision Framework

The intuition that electric equipment is more efficient than gas equipment is correct in some contexts and reversed in others. HERS is one of the contexts where it can run backwards, because the index scores against a gas-DHW reference and counts grid losses on the electric path.

Gas water heating remains a valid compliance route on most Massachusetts multifamily projects, including under the Specialized Stretch Code. Mass Save continues to incentivize it for a reason. Pulling gas out of the design is a real choice with real cost consequences, and it is worth modeling before the design team commits.

The right water heating choice on any given project depends on the envelope, the compliance path, and what the building can realistically deliver on the electric side. There is no single answer that holds across projects. Look at gas water heating on your next multifamily project before assuming electric is the upgrade.