Commercial Rooftop Solar Installation in Phoenix, AZ

Case Study: Commercial Rooftop Solar Installation in Phoenix, AZ

Scenario

A 250-kWp rooftop PV system is being installed on a warehouse in Phoenix, Arizona. Local utility interconnection rules require the inverter AC output to be capped at 200 kVA to avoid mandatory bi-directional metering and advanced grid-support functions. High ambient temperatures (up to 45°C) and dust accumulation necessitate conservative derating. The selected inverter model has a nominal efficiency of 95% at 50% load and a rated power factor of 0.80 — verified from its UL 1741 SA-certified datasheet.

Given Data

• AC Output Power (p_ac): 200 kVA
• Inverter Efficiency (eff): 95% → 0.95
• Power Factor (pf): 0.80
• Derating Factor (derating): 0.80 (to accommodate thermal stress and long-term reliability in desert conditions)

Calculation

The tool computes required DC input power (p_dc) as:

p_dc = (p_ac × 1000 × pf) / (eff × derating)
     = (200 × 1000 × 0.80) / (0.95 × 0.80)
     = (160,000) / (0.76)
     ≈ 210,526 W = 210.53 kW

Note: The p_ac input is in kVA, but real power (kW) delivered is p_ac × pf. Since inverter efficiency applies to real power conversion, we first compute real AC output (200 kVA × 0.8 = 160 kW), then divide by (eff × derating) to back-calculate minimum required DC real power.

Result and Decision

The calculation yields 210.53 kW of required DC input capacity. To meet this while allowing for ±5% tolerance and future minor array degradation, engineers selected two identical 110-kW string inverters (total 220 kW DC rating), each with MPPT voltage range compatible with the 22-string, 21-module-per-string array (Voc ≈ 1020 V). This avoids oversizing a single large inverter that would suffer disproportionate clipping losses during morning/evening low-irradiance periods.

Lesson

Derating isn’t just about temperature — it’s a system-level reliability buffer. In hot climates, applying derating before efficiency correction (as this tool does) prevents under-sizing the DC source, which could otherwise force operation near inverter limits and accelerate thermal aging.