WSEC Compliance · Builder Resource
Solar vs. Heat Pump Upgrades for WSEC Energy Credits
This solar vs. heat pump WSEC credits comparison is for builders whose plan set comes up short. There are usually two fixes: add rooftop solar, or upgrade the heat pump. Here is how the two paths compare under Table R406.3 in WAC 51-11R-40621. Use this to pick the option that closes your credit gap.
4.5 credits
Maximum available from solar under Option 6.1, Renewable Electric Energy
2.0 credits
Highest single-family credit available from any one HVAC Equipment option (Category 3)
10 options, 1 pick
Only one HVAC equipment option (3.1–3.10) may be claimed per home, versus solar’s single PVWatts-documented path
Phase 1
How the WSEC Credit Categories Actually Work
The 2021 WSEC requires new single-family homes to earn energy credits above the prescriptive baseline. These credits come from Table R406.3 in WAC 51-11R-40621. The table has seven categories: building envelope, air leakage and ventilation, HVAC equipment, HVAC distribution, water heating, renewable electric energy, and an appliance package. Inside most categories, a home may only claim one option. Stacking credits means picking one winner per category, not collecting every credit that looks appealing.
Some builders don’t want to redesign the envelope or add a heat recovery ventilator. For them, two categories close a credit gap fastest: HVAC Equipment (Category 3) and Renewable Electric Energy (Category 6, Option 6.1). Both can be added late in design. Neither touches insulation values or window schedules. That’s why builders compare these two paths most often.
Phase 2
The Solar Path: Renewable Electric Energy, Option 6.1
Option 6.1 credits on-site solar or wind generation at 0.5 credit for every 600 kWh produced per year, capped at 4.5 credits. Builders must document generation using the National Renewable Energy Laboratory’s PVWatts calculator, or an approved alternate. Plans must also note solar access.
In practice, this means one line item on the permit set: a PVWatts run tied to the array size, tilt, and roof orientation for that lot. Solphos runs this calculation as part of every builder solar package and documents the credit once. It doesn’t depend on which mechanical contractor ends up on the job, or whether equipment gets substituted mid-build.
Phase 3
The Heat Pump Path: High-Efficiency HVAC Equipment, Category 3
Category 3 lists ten heating equipment options. A home may only claim one of them (Items 3.1 through 3.10). For a typical single-family home, the options that show up most on builder plan sets are:
- Option 3.6: centrally ducted air-source heat pump, minimum HSPF2 9.4 (HSPF 11.0), earns 1.0 credit
- Option 3.4: closed-loop ground-source heat pump, minimum COP 3.3, or open-loop water-source heat pump, minimum COP 3.6, earns 1.5 credits
- Option 3.7: ductless split-system heat pump with no electric-resistance heat in the primary living areas, minimum HSPF2 9 (HSPF 10), sized to heat the entire dwelling, earns 2.0 credits
Option 3.7 is the highest single-family credit available anywhere in this category. A connected thermostat (Option 3.11) can add another 0.5 credit. But it only pairs with Options 3.1 or 3.3, not 3.7. It doesn’t stack on top of the ductless whole-house path.
Each option carries its own documentation requirement. The permit drawings must specify the exact equipment type and the minimum HSPF2 or COP rating. Option 3.7 also needs the heated floor area calculation and total installed heat capacity. If the mechanical contractor substitutes a different unit after permit, the credit documentation may need a second look.
Phase 4
Solar vs. Heat Pump: Side by Side on a Typical Plan Set
In a solar vs. heat pump comparison, the two options sit in different WSEC categories. A home is free to claim both. The real question most builders ask is which one to lean on first, when a plan set is short by a few credits.
Credit ceiling: solar tops out at 4.5 credits from one option. The best single heat pump option tops out at 2.0 credits.
Documentation: solar needs one PVWatts run per elevation. Heat pump options need the specific HSPF2 or COP rating for the equipment model actually installed.
Trade coordination: solar is scoped and inspected independently of the mechanical trade. Heat pump credits depend on what the HVAC contractor actually installs, and that can change after permit.
Repeatability across a plan book: a PVWatts-documented array scales predictably across elevations, with a simple re-run per roof orientation. HSPF2-dependent equipment credits can shift if a supplier substitutes a model between plan approval and rough-in.
For a plan set that’s short by three or more credits, solar alone usually closes the gap. It does this without touching the mechanical scope. For a one- or two-credit shortfall on a home that’s already speccing a heat pump, claiming the HVAC equipment credit can be the simpler fix. Many builders end up using both: a heat pump upgrade already in the mechanical budget, plus a smaller array sized to whatever credits remain.
Phase 5
Solar vs. Heat Pump: Which Path Fits Your Plan Set
The right answer depends on the specific gap on the specific plan, not a blanket rule. A few starting points:
- If the credit gap is large relative to what the mechanical scope can realistically close, solar’s 4.5-credit ceiling gives the most room to work with from a single option.
- If the mechanical plan already includes a heat pump upgrade for comfort or all-electric reasons, check whether it qualifies for Option 3.4, 3.6, or 3.7 first. That can capture a credit that’s effectively already paid for.
- If the same plan set gets built repeatedly across multiple lots with different roof orientations, a PVWatts-documented solar credit tends to be easier to keep consistent. An HVAC equipment credit depends on what a subcontractor actually installs, which varies more.
Solphos reviews the specific credit gap on a plan set. Solphos then returns a PVWatts-backed recommendation for how much solar, if any, is needed alongside what the mechanical scope already provides.
A note on scope: the credit values above are the “All Other” column of Table R406.3. This column applies to single-family homes and duplexes. Group R-2 occupancies (multifamily) use a different column with different values for some options. For example, Option 3.7 credits 3.0 for Group R-2b versus 2.0 for single-family. Builders working on multifamily projects should confirm the R-2b column for their specific option before relying on these figures.
Frequently Asked Questions
Can a home claim both solar and heat pump equipment credits under the WSEC?
Yes. Solar falls under Category 6, Renewable Electric Energy. Heat pump equipment falls under Category 3, HVAC Equipment. Table R406.3 only restricts how many options can be claimed within the same category, not across categories. A home can claim one HVAC equipment option and the solar credit at the same time.
What’s the maximum credit a heat pump alone can provide?
For a single-family home, the ceiling is 2.0 credits, through Option 3.7. That option is a ductless split-system heat pump with no electric-resistance heat in the primary living areas. It must be rated at a minimum HSPF2 of 9 (HSPF 10), and sized to heat the entire dwelling unit.
How many credits can solar provide under the 2021 WSEC?
Up to 4.5 credits, under Option 6.1. The formula is 0.5 credit for every 600 kWh of on-site solar or wind generation, documented per housing unit each year. This is calculated using PVWatts.
Does a heat pump water heater count toward the same category as heat pump space heating?
No. Water heating credits sit in a separate category, Efficient Water Heating (Category 5). A heat pump water heater option doesn’t compete with the space-heating options in Category 3. It can be combined with either the solar path or a Category 3 heat pump credit.
Which option is simpler to document on a permit set?
Solar’s Option 6.1 generally requires one PVWatts run tied to the array size and roof plane. Heat pump options require the specific HSPF2 or COP rating for the exact equipment model. That can mean follow-up paperwork if the mechanical contractor substitutes a different unit after the permit is issued.
