Rooftop Solar Feasibility Study for Urban Commercial Building in Phoenix, AZ

Scenario

A 12-story mixed-use commercial building in downtown Phoenix, AZ, sought to install a 250 kW rooftop PV system. Constraints included: (1) structural load limits restricting panel tilt to ≤25°, (2) fixed azimuth due to roof orientation (true south = 0°), and (3) significant morning shading from an adjacent 8-story hotel until 10:30 a.m., necessitating conservative irradiance estimates.

Given Data

• GHI: 6.8 kWh/m²/day (Phoenix annual average, NREL NSRDB)
• DHI: 2.1 kWh/m²/day
• DNI: 4.9 kWh/m²/day
• Tilt angle: 22° (max allowed by roof structure & wind-load analysis)
• Azimuth angle: 0° (true south)

Calculation

The Plane-of-Array (POA) irradiance is computed using the standard decomposition model:

POA = DNI × cos(θ) + DHI × [(1 + cos(β))/2] + GHI × ρ × [(1 − cos(β))/2]

Where:

• θ = angle of incidence between DNI and panel normal (calculated via solar geometry; for Phoenix at solar noon on equinox, θ ≈ 12.5° → cos(θ) ≈ 0.976)
• β = tilt angle = 22° → cos(β) ≈ 0.927
• ρ = ground albedo = 0.2 (typical for urban concrete/roof gravel)

Substituting:

• DNI component = 4.9 × 0.976 = 4.78 kWh/m²/day
• DHI component = 2.1 × [(1 + 0.927)/2] = 2.1 × 0.9635 ≈ 2.02 kWh/m²/day
• Reflected component = 6.8 × 0.2 × [(1 − 0.927)/2] = 1.36 × 0.0365 ≈ 0.05 kWh/m²/day

POA = 4.78 + 2.02 + 0.05 = 6.85 kWh/m²/day

The Solar Irradiance Calculator (validated against PVWatts v8) returns 6.84 kWh/m²/day, confirming consistency within 0.2%.

Result and Decision

The calculated POA irradiance (6.84 kWh/m²/day) exceeded the project’s minimum threshold of 6.2 kWh/m²/day required for viable ROI under Arizona’s net metering rules. The engineering team approved the 250 kW system using monocrystalline bifacial modules mounted at 22° tilt, with enhanced soiling mitigation protocols due to high dust accumulation rates.

Lesson

Even modest tilt adjustments (e.g., 22° vs. flat 0°) in high-DNI desert climates yield >15% POA gain over horizontal surfaces — validating tilt optimization as a low-cost, high-impact design lever when structural constraints allow.