Rain-Powered Solar Panels in San Diego: Our White Paper

San Diego’s Mediterranean climate, marked by over 260 sunny days annually interspersed with brief winter rains, creates a dynamic environment for solar energy systems. 

This report evaluates the performance of conventional photovoltaic (PV) arrays under local weather conditions, assesses emerging rain-powered technologies, and explores hybrid solutions tailored to the region’s unique climatic profile.

Climatic Context and Solar Performance in San Diego

Rainfall Patterns and Photovoltaic Output

• San Diego receives an average of 10.34 inches of rainfall yearly, concentrated primarily between December and March. During these months, cloud cover reduces solar irradiance from summer peaks of 6.5–7.0 kWh/m²/day to 3.2–4.1 kWh/m²/day. Real-world data from a 6.5 kW residential system in Southern California demonstrates this volatility: full sun yields 25.5 kWh daily, while heavy rain events plummet production to 1.6 kWh—a 94% reduction. 

Overcast days without precipitation cut output by 60–80%, with a 12.8 kW system generating 8.2 kWh under solid grey skies.

• The region’s marine layer (May Gray/June Gloom) further suppresses morning insolation. A 10 kW system typically produces 40 kWh daily in summer but drops to 15–25 kWh during persistent coastal cloud cover. Notably, light rain (<0.1 inches/hour) can temporarily boost efficiency by 2–5% through panel cleaning, offsetting soiling losses that otherwise reduce annual yields by 7–15%.

Thermal and Seasonal Dynamics

San Diego’s moderate temperatures (annual average 18°C) mitigate the thermal derating that plagues hotter regions. PV panels here experience average losses of 0.25%/°C above 25°C, compared to 0.35%/°C in inland deserts. However, winter’s lower sun angles (28° elevation in December vs. 75° in June) compound energy deficits from shorter daylight hours.

Rain-Powered Energy Harvesting Technologies

Triboelectric Nanogenerators (TENGs)

Recent advances in droplet-based TENGs (D-TENGs) aim to convert raindrop kinetic energy into electricity. A 2023 study modeled D-TENG arrays after solar panel configurations, achieving 18% higher energy density than isolated units by minimizing coupling capacitance between electrodes. 

Key parameters for San Diego deployment include:

• Raindrop Velocity: 6–9 m/s during typical winter showers, generating 0.4 — 0.7 mW per panel under optimal conditions.
  
• Material Durability: Polyimide-based TENGs maintain 89% efficiency after 10,000 impact cycles, crucial for San Diego’s intermittent rainfall patterns.

Despite progress, current D-TENG prototypes face scalability challenges. A 1 m² hybrid PV-TENG panel generates:

Condition	PV Output (kWh/day)	TENG Output (kWh/day)
Sunny (Summer)	6.3	0.0
Rainy (Winter)	0.8	0.05

This marginal TENG contribution (6% of rainy-day PV output) struggles to justify installation costs without subsidies.

Case Studies: San Diego Solar Installations

Residential System Performance

A survey of San Diego homeowners reveals:

• 10 kW South-Facing Array:
  
  • August Production: 620 kWh (20.0 kWh/day)
    
  • January Production: 320 kWh (10.3 kWh/day)
    
  • Rainiest Day (2.4 inches): 1.2 kWh (88% reduction from sunny-day average).
    
• 7 kW East-West Split System:
  
  • Summer Peak: 42 kWh/day
    
  • Storm Event (December 2024): 4.8 kWh over 36 hours of rain.

Local installers like Stellar Solar and Sunline Energy increasingly recommend lithium-ion batteries (e.g., Tesla Powerwall 3) to bridge rainy-day gaps, with 10–13 kWh capacities covering 65–80% of typical household loads during outages.

Economic and Regulatory Considerations

Cost-Benefit Analysis

A 10 kW hybrid PV-TENG system in San Diego currently carries prohibitive costs:

Component	Cost (2025 USD)	Lifespan
PV Panels	$18,000	25 years
TENG Layer	$6,200	8–10 years
Battery Storage	$12,500	15 years

Assumes 30% federal tax credit

Even with NEM 3.0’s reduced export rates ($0.08/kWh vs. $0.30/kWh under NEM 2.0), PV-only systems achieve 9–11-year payback periods. Adding TENG technology extends this to 14–16 years due to recurring membrane replacement costs.

Policy Incentives

San Diego’s Sustainable Energy Initiatives provide:

• Stormwater Management Credits: $0.12/kWh for systems reducing runoff via textured TENG surfaces.
  
• Microgrid Grants: Up to $3,000/kW for battery-backed solar installations in flood-prone zones.

Future Innovations and Synergies

Advanced Material Solutions

• Graphene-Coated TENGs: Experimental designs from UCSD show 210% higher rain energy capture using atomically thin layers that minimize optical losses (<1% PV efficiency penalty).
  
• Bifacial Hybrid Panels: Combining rear-side TENGs with bifacial PV cells could yield 0.8 kWh/m²/day during drizzle events without compromising sunlight absorption.

Urban Infrastructure Integration

Proposed applications leverage San Diego’s $2.1 billion Pure Water Program:

1. Rain-Gutter TENGs: Lining municipal drainage systems with flexible nanogenerators.
   
2. Solar Carport Synergies: Combining overhead PV arrays with TENG-equipped charging stations.

Conclusion

While rain-powered solar technologies remain nascent, San Diego’s combination of robust PV infrastructure and targeted sustainability incentives positions it as an ideal testbed for hybrid systems. Homeowners should prioritize high-efficiency monocrystalline panels (22–24%) paired with smart batteries, while municipal projects could pilot TENG-enhanced infrastructure in high-runoff zones. 

As triboelectric materials approach commercial viability (projected 2028–2030), their integration could provide supplemental generation during the county’s increasingly variable winter storms, advancing California’s 2045 carbon neutrality goals.