Seattle’s solar permitting is more integrated than most US cities — SDCI and Seattle City Light coordinate the permit and interconnection review, and the utility’s Approved Equipment List is a hard prerequisite. The mild, cloudy climate means lower production per kW than southern markets, but the combination of Washington’s sales tax exemption, federal ITC, and net metering provides reasonable economics.
Seattle Solar Snapshot
NEC Edition: 2023 (WA early adopter) | Utility: Seattle City Light (municipal) | Net Metering: RCW 80.60 (retail rate; April 30 excess forfeited) | State Incentive: Sales tax exemption (~9–10.5%) | SCL AEL: Must use approved equipment list
SurgePV’s cloud solar design software helps automate compliance documentation.
NEC 2023 in Seattle
Washington adopted NEC 2023 statewide. Seattle (SDCI) enforces NEC 2023 for all solar permits:
| NEC 2023 Section | Seattle Application |
|---|---|
| 690.12 PVHCS | Alternative RSD path available — confirm SCL/SDCI interpretation |
| 690.7 string voltage | Mild climate: T_min = -9°C |
| 690.8 conductor sizing | Mild derating (0.82 factor for SCL area) |
| 690.41 grounding | NEC 2023 reorganized language |
| Article 706 storage | Updated for solar-plus-storage systems |
Seattle Climate Engineering
Seattle’s mild climate makes conductor sizing less aggressive than desert markets:
| Parameter | Value |
|---|---|
| ASHRAE max air temp | 29°C |
| + 22°C rooftop adder | +22°C |
| Effective conductor temp | 51°C |
| THWN-2 (90°C) correction factor | 0.82 |
Seattle has the most favorable conductor derating of any major US city — 10 AWG at 0.82 factor = 40 × 0.82 = 32.8A available.
Seattle NEC 690.7 String Voltage
Seattle T_min = -9°C:
For Voc = 45.0V, β_Voc = -0.28%/°C:
Factor = 1 + (-9 - 25) × (-0.0028) = 1 + 0.0952 = 1.0952
Max modules (600V) = 600 / (45.0 × 1.0952) = 12.16 → 12 modules
Max modules (1000V) = 1000 / (45.0 × 1.0952) = 20.3 → 20 modulesSeattle’s cold minimum requires about the same maximum string count as LA — both limit to 12 residential modules, but for different reasons (Voc in Seattle; conductor derating in LA).
Seattle City Light Approved Equipment List
The SCL AEL is a mandatory prerequisite:
- SCL publishes the AEL at seattle.gov/city-light/solar
- List includes approved inverters, meters, and other interconnection equipment
- Updated periodically — always check the current list
- Equipment not on the AEL is not eligible for SCL interconnection, even if UL-listed
Why SCL has an AEL: SCL is a municipal utility with its own interconnection standards. The AEL ensures equipment meets SCL’s technical requirements for grid protection, power quality, and communication protocols (including SunSpec/DER communication requirements).
Seattle City Light Interconnection Process
| System Size | Process | Timeline |
|---|---|---|
| Under 10 kW residential | Standard | 15–25 business days |
| 10 kW–30 kW | Engineering review | 20–30 business days |
| Over 30 kW | Distribution study | 60–90 days |
Interconnection application requires:
- SCL solar interconnection application form
- One-line electrical diagram (NEC 2023)
- Site plan with module layout
- Inverter spec sheet (verify AEL status)
- SDCI permit number (or SolarAPP+ certificate)
SDCI and SCL Review in Parallel
Unlike most US markets where the permit and utility interconnection are separate sequential processes, Seattle’s SDCI and SCL coordinate their reviews. Submit both applications simultaneously — this parallel process can save weeks vs. sequential submission. The SDCI permit references the SCL interconnection application and vice versa.
Washington Net Metering for Seattle
| Parameter | Details (RCW 80.60) |
|---|---|
| Eligible size | Up to 100 kW residential; up to 1 MW commercial |
| Credit rate | Full retail rate |
| Monthly carryover | Yes |
| Annual true-up | April 30 — excess credits forfeited |
| Utilities covered | All utilities including SCL (municipal) |
April 30 credit forfeiture: Seattle’s net metering does not pay out excess annual credits — they are simply zeroed out. This is different from Oregon (market rate payout) or Minnesota (retail rate payout). System right-sizing is important: a system that exports heavily in summer (Seattle’s sunnier months) may see those credits forfeited if not self-consumed or stored.
SDCI Permit Process
SolarAPP+ Track
SDCI has accepted SolarAPP+ for qualifying residential systems:
- Submit through SolarAPP+-compatible software
- Automated approval
- Submit SolarAPP+ certificate to SDCI
- SDCI issues permit
- Install + inspections
- SCL net meter activation
Standard Track
- Submit through SDCI Permit portal (Seattle.gov/sdci)
- Upload electrical one-line, structural drawings, equipment specs
- SDCI and SCL coordinated review
- Plan review: 10–20 business days
- Install
- Schedule SDCI inspections
- SCL net meter activation
SDCI Permit Fees
| System | Approximate Fee |
|---|---|
| Residential (simple) | $150–350 |
| Commercial | Valuation-based |
Seattle Historic Districts
Seattle has several historic districts and landmark buildings:
Seattle Landmarks and Historic Districts
If the project is on a Seattle Landmark or in a historic district, the Seattle Landmarks Preservation Board must review the solar installation application before SDCI can issue a permit. This can add 4–8 weeks and may restrict panel placement to rear/non-visible roof surfaces. Identify potential historic designation early using the Seattle Landmarks database.
Washington Sales Tax Exemption
Seattle solar installations qualify for Washington’s sales tax exemption:
- Seattle combined sales tax rate: approximately 10.25–10.5%
- Exemption applies to solar equipment and installation labor
- Exemption taken by installer at time of sale (no customer application needed)
- Savings of ~10% on total system cost
Seattle Solar Permit Packages — SCL AEL Compliant
SurgePV generates NEC 2023-compliant permit packages for Seattle SDCI — with SCL Approved Equipment List verification, coordinated permit and interconnection documentation, and Washington net metering modeling.
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Frequently Asked Questions
Does Seattle use NEC 2023?
Yes — Washington adopted NEC 2023 as an early adopter state. SDCI enforces NEC 2023 for all Seattle solar permits. Key NEC 2023 feature: PVHCS as alternative rapid shutdown path — confirm interpretation with SDCI.
What is Seattle City Light’s Approved Equipment List?
SCL’s AEL is a mandatory list of approved inverters and equipment for SCL interconnection. Using an inverter not on the AEL prevents interconnection regardless of UL listing. Always verify AEL status for the specified inverter before finalizing equipment selection.
What happens to unused Seattle net metering credits?
April 30 annual reset: unused monthly credit carryforwards are forfeited (zeroed out). Unlike Oregon or Minnesota, Washington does not pay out annual excess credits. Right-size the system to minimize annual credit forfeiture in Seattle’s moderate production climate.
