Design and Planning Fundamentals

🎯 Learning Objectives

✓ Calculate daily energy consumption from appliance-level load data
✓ Design PV array size (kWp) using insolation data and system losses
✓ Analyze inverter sizing relative to array DC output and AC load profile
✓ Apply NEC Article 690.7(A) to determine maximum DC source circuit voltage
✓ Explain the impact of tilt, azimuth, and shading on annual energy yield

📖 Why This Matters

A perfectly installed solar system will underperform—or fail—without sound design and planning. Real-world projects fail not due to faulty panels, but because of undersized wiring, mismatched inverters, or unaccounted shading that cuts yield by 30%+. In Module 3, you’ll learn how to translate client needs and site realities into a robust, code-compliant, bankable design—the foundation of every successful PV project.

📘 Core Principles

Solar PV design begins with energy demand quantification: aggregating loads by wattage, duty cycle, and duration to compute daily kWh. Next, site assessment evaluates solar resource (using TMY3 or NSRDB data), orientation, tilt, and shading via tools like Solar Pathfinder or PVWatts. System architecture follows—selecting string configuration, inverter type (string vs. micro), battery integration (if applicable), and balance-of-system components. Finally, electrical design applies engineering margins: derating factors for temperature, soiling, wiring losses (typically 10–14%), and NEC safety requirements—including rapid shutdown compliance (690.12) and conductor ampacity rules (310.15). Each decision cascades into performance, safety, and ROI.

📐 PV Array Sizing (DC Nameplate)

This formula determines the minimum DC nameplate capacity required to meet the AC load after accounting for all system losses. It ensures the design meets energy targets under real-world conditions—not ideal lab ratings.

Required DC Array Size (kWp)

P_{DC} = \frac{E_{AC} / \eta_{inv}}{1 - L_{total}} \div PSH

Calculates minimum DC nameplate capacity needed to meet AC energy demand after accounting for inverter efficiency and total system losses.

Variables:

Symbol	Name	Unit	Description
P_{DC}	Required DC array size	kWp	DC nameplate power rating of PV modules needed
E_{AC}	Daily AC energy consumption	kWh	Total usable AC energy required per day
\eta_{inv}	Inverter efficiency	decimal	AC output / DC input ratio under nominal operating conditions
L_{total}	Total system loss factor	decimal	Sum of all derating factors (soiling, wiring, temp, etc.)
PSH	Peak Sun Hours	h/day	Equivalent full-sun hours based on location-specific irradiance data

Typical Ranges:

Residential US grid-tied: 3–12 kWp

Commercial rooftop: 50–500 kWp

💡 Worked Example

Problem: A residential client consumes 18 kWh/day (AC). Historical PSH at site = 4.8 kWh/m²/day. Inverter efficiency = 96%. Total system losses (soiling, wiring, temp, etc.) = 14%. What minimum DC array size (kWp) is required?

1. Step 1: Calculate required DC energy output: AC load / inverter efficiency = 18 kWh / 0.96 = 18.75 kWh (DC)

2. Step 2: Account for total losses: divide by (1 − loss fraction) → 18.75 kWh / (1 − 0.14) = 18.75 / 0.86 ≈ 21.79 kWh (DC before losses)

3. Step 3: Divide by peak sun hours: 21.79 kWh / 4.8 h = 4.54 kWp

Answer: The result is 4.54 kWp, which falls within the typical residential range of 3–12 kWp.

🏗️ Real-World Application

In Austin, TX, a 2022 multifamily retrofit used PVWatts v7 with NSRDB TMY data showing 5.1 PSH. Load audit revealed 32 kWh/day per unit. Design team applied 12% total losses, selected a 97.5% efficient Fronius GEN24 Plus inverter, and sized a 7.2 kWp array (24 × 300W monocrystalline modules). Shading analysis identified 8% annual loss from adjacent roof parapet; this was mitigated via module-level power electronics (MLPE) and tilt optimization. Final modeled yield: 10,120 kWh/yr—within 1.2% of measured first-year production.