Solar System Design Tutorial
Learn how to design a complete solar PV system from start to finish. This step-by-step guide walks you through site assessment, system sizing, equipment selection, electrical design, and financial analysis with practical examples and code references.
Understanding Energy Needs
Establish baseline load requirements
Before designing any solar system, you must understand the energy requirements. Review historical electricity usage data to capture seasonal variations in consumption.
Example: This commercial facility uses 6,250 kWh/month average × 12 months = 75,000 kWh annually. Peak usage in summer reaches 9,500 kWh/month due to HVAC loads.
Key Considerations:
- Identify monthly usage patterns and seasonal peaks
- Account for future load increases (facility expansion, equipment upgrades)
- Determine desired energy offset (80%, 100%, or 120% of usage)
- Review utility rate structure (demand charges, time-of-use, net metering)
Site Assessment & Layout Planning
Evaluate land availability and solar access
Ground-mounted systems require careful site assessment including land area, soil conditions, grading requirements, and shading analysis from nearby structures or vegetation.
Site Evaluation Checklist:
- Available land: Approximately 5-7 acres per MW for fixed-tilt systems
- Soil conditions: Assess for foundation requirements (driven piles vs. concrete ballast)
- Terrain: Flat or gently sloping preferred; minimize grading costs
- Shading analysis: Document obstructions from trees, buildings throughout the year
- Access: Vehicle access for construction and maintenance equipment
- Setbacks: Property line, road, and utility easement requirements
Where Solar Altitude = 90° – Latitude – 23.5° (winter solstice)
Example: For Phoenix (latitude 33.4°), with 25° tilt modules:
Solar Altitude = 90° – 33.4° – 23.5° = 33.1°
Row Spacing = 2.28m × sin(25°) / tan(33.1°) = 1.48m minimum
Ground Coverage Ratio (GCR) ≈ 0.40 for this configuration
Energy Production Estimation
Calculate expected system output
Use NREL’s PVWatts database to determine solar irradiance at your location. Ground-mounted systems typically achieve higher production than roof-mounted due to optimal tilt and reduced temperature effects.
Typical System Derate = 0.86 (ground-mount have fewer losses than roof)
Example Calculation:
50 kW × 5.7 PSH (Phoenix, optimized tilt) × 365 days × 0.86 = 89,537 kWh/year
This exceeds the 75,000 kWh requirement (119% offset)
String Configuration & Sizing
Design series/parallel array layout
String sizing ensures your array operates within the inverter’s MPPT voltage range at all temperatures.
Example System:
Module: 550W, Voc = 49.5V, Vmp = 41.2V
Inverter: 200-850V MPPT range, 1000V max
Phoenix low temp: -3°C
18 modules in series:
• Voc at -3°C = 49.5V × 18 × 1.078 = 960V ✓ Within limits
• Vmp at 65°C = 41.2V × 18 × 0.888 = 658V ✓ Within MPPT
5 strings × 18 modules = 90 modules × 550W = 49.5 kW
Electrical Design & Wire Sizing
Calculate conductor sizes and voltage drop
Ground-mounted systems often have longer wire runs from array to inverter. Proper wire sizing minimizes voltage drop and ensures code compliance.
AC circuits: Maximum 3% to main panel
Example Wire Sizing:
DC Circuits (13.8A Isc, 740V string voltage):
• Required: 13.8A × 1.56 = 21.5A
• After temp derate: 21.5 ÷ 0.88 = 24.4A
• Selected: 10 AWG USE-2 (30A rated)
• 150 ft run voltage drop: 1.4% ✓
Financial Analysis & ROI
Calculate payback period and lifetime savings
Commercial ground-mounted systems often have attractive economics due to economies of scale and favorable utility rate structures.
Example Financial Analysis:
System Cost: 49.5 kW × $2.25/W = $111,375
Tax Credit (30%): -$33,413
Depreciation Benefit: -$22,000
Net Cost: $55,962
Annual Production: 89,500 kWh
Commercial Rate: $0.12/kWh
Annual Savings: $10,740
Payback: 5.2 years
25-Year Savings: $285,000+