Rooftop Solar Feasibility for Urban Community Center in Brooklyn, NY

Scenario

A nonprofit community center in Brooklyn, NY seeks to install a 50 kWp rooftop solar array on its flat, south-facing roof. Constraints include structural load limits (max 25 kg/m²), existing HVAC units causing partial shading, and limited budget for advanced monitoring or cleaning services. The roof has no obstructions to the east or west but experiences moderate morning shading from an adjacent 3-story building.

Given Data

• Latitude: 40.6782° N
• Longitude: -73.9352° W
• Tilt Angle: 15° (low tilt to minimize wind load and maximize roof coverage within structural limits)
• Azimuth Angle: 180° (true south)
• Global Horizontal Irradiance (GHI): 1,320 kWh/m²/year (measured TMY3 data for ZIP 11201)
• Performance Ratio: 0.78 (conservative due to aging roof surface and lack of active soiling mitigation)
• Ground Coverage Ratio: 0.45 (limited by HVAC setbacks and fire code setbacks)
• Shading Loss: 7% (validated via SketchUp + PVWatts shade analysis)
• Soiling Loss: 5% (urban environment with infrequent rain and high particulate matter)
• Mismatch Loss: 2%
• Wiring Loss: 2.5%
• Inverter Efficiency Loss: 4.5%
• Transformer Efficiency Loss: 0% (no step-up transformer; direct grid interconnection at 120/240 V)

Calculation

The tool computes annual energy yield (kWh/kWp) using a physics-informed empirical model:

1. Effective Plane-of-Array (POA) Irradiance Estimate: Adjusts GHI for tilt/azimuth using Perez transposition model (embedded). At 15° tilt, 180° azimuth, and 40.7° latitude, POA irradiance ≈ GHI × 1.08 = 1,320 × 1.08 = 1,425.6 kWh/m²/year.
2. System Loss Factor: Multiply all fractional losses:
   • Shading: 1 − 0.07 = 0.93
   • Soiling: 1 − 0.05 = 0.95
   • Mismatch: 1 − 0.02 = 0.98
   • Wiring: 1 − 0.025 = 0.975
   • Inverter: 1 − 0.045 = 0.955
   • Transformer: 1.00
     → Combined loss factor = 0.93 × 0.95 × 0.98 × 0.975 × 0.955 ≈ 0.802
3. Apply Performance Ratio & GCR scaling:
   Final yield = POA × PR × GCR × system loss factor
   = 1,425.6 × 0.78 × 0.45 × 0.802 ≈ 403.2 kWh/kWp

Result and Decision

The estimated yield of 403 kWh/kWp fell below the project’s internal threshold of 420 kWh/kWp for financial viability (based on NYSERDA incentive payback targets). As a result, the engineering team recommended increasing tilt to 22° (still within structural limits) and adding microinverters to mitigate shading impact — re-running the tool yielded 438 kWh/kWp. The revised design was approved, enabling PPA financing.

Lesson

Low tilt angles on urban rooftops often underperform due to compounded shading and soiling — always validate with site-specific transposition and loss modeling before finalizing mechanical layout.