Tutorial

Solar System Design Tutorial – Step by Step Guide

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.

1

Understanding Energy Needs

Establish baseline load requirements

Example Project

5 kW Residential Grid-Tied System

Location Phoenix, AZ
Annual Usage 7,200 kWh
Array Type Roof-mounted
Module 400W, 38.5V Vmp
Inverter 5kW String Inverter
Roof Tilt 20° South-facing

Before designing any solar system, you must understand the energy requirements. Review historical electricity usage data to capture seasonal variations in consumption.

Example: This home uses 600 kWh/month average × 12 months = 7,200 kWh annually. Peak usage in summer reaches 900 kWh/month due to air conditioning.

Key Considerations:

  • Identify monthly usage patterns and seasonal peaks
  • Account for future load increases (EV charging, pool, home additions)
  • Determine desired energy offset (80%, 100%, or 120% of usage)
  • Review utility rate structure (tiered, time-of-use, net metering policies)

Load Analysis Best Practices

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For grid-tied systems, review 12 months of utility bills. For off-grid systems, perform detailed load calculations for each appliance. Include startup surge currents for motors and compressors. Consider future expansion and plan for 20-30% overhead in system capacity.

2

Site Assessment & Solar Access

Evaluate shading, orientation, and available space

A proper site assessment determines system viability and optimal placement. Evaluate roof condition, orientation, tilt angle, and shading from trees, buildings, or obstructions.

Site Evaluation Checklist:

  • Roof condition: At least 15 years of remaining life
  • Orientation: South-facing ideal (90-110% of max production), East/West acceptable (75-85%)
  • Tilt angle: Optimal = latitude ±15°, flat roofs = 10° minimum
  • Shading analysis: Document obstructions throughout the year
  • Structural capacity: 3-4 lbs/sq ft for typical installations
Row Spacing Calculation (Ground Mount)
Row Spacing = Module Height × sin(Tilt Angle) / tan(Solar Altitude)
Where Solar Altitude = 90° – Latitude – 23.5° (winter solstice)

Example: This Phoenix installation has a south-facing roof with 20° tilt and minimal shading. Roof is 5 years old (20+ years remaining). Shading analysis shows 95%+ solar access year-round.

Use Shading & Spacing Calculator

Advanced Shading Analysis

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Use tools like Solmetric SunEye or drone-based software to create detailed shade reports. Document seasonal sun paths and calculate Total Solar Resource Fraction (TSRF). Systems with TSRF < 75% may not be economically viable without mitigation strategies like microinverters or optimizers.

3

Energy Production Estimation

Calculate expected system output using solar resource data

Use NREL’s PVWatts database to determine solar irradiance at your location. Account for system losses including soiling, temperature, mismatch, wiring, and inverter efficiency.

Annual Production Estimate
Annual kWh = System Size (kW) × Peak Sun Hours × 365 × System Derate Factor

Typical System Derate = 0.84 (accounts for 16% total losses)

Example Calculation:
5 kW × 5.5 PSH (Phoenix) × 365 days × 0.84 = 8,418 kWh/year
This exceeds the 7,200 kWh annual usage requirement (117% offset)

System Loss Factors:

  • Soiling: 2-5% (dust, pollen, bird droppings)
  • Temperature: 3-5% (modules lose ~0.4%/°C above 25°C)
  • Mismatch: 2% (module parameter variations)
  • Wiring: 2% (DC and AC conductor losses)
  • Inverter efficiency: 2-4% (typically 96-98% efficient)
  • Degradation: 0.5-0.7% per year (for financial models)
Use Production Estimate Calculator
4

String Configuration & Sizing

Design series/parallel array layout within inverter voltage windows

String sizing ensures your array operates within the inverter’s MPPT voltage range at all temperatures. Calculate both cold temperature maximum voltage (Voc) and hot temperature minimum voltage (Vmp).

NEC 690.7 (US): Maximum PV system voltage must account for lowest expected ambient temperature. Use correction factors from manufacturer datasheets or industry standard -0.33%/°C for Voc. International standards may vary – check local electrical codes.
String Voltage Calculations
Max Voltage (cold) = Voc × # Modules × Temp Correction Factor
Min Voltage (hot) = Vmp × # Modules × Temp Correction Factor

Must satisfy: Inverter Min MPPT < Vmp_array < Inverter Max MPPT
And: Voc_array < Inverter Absolute Max Voltage

Example System:
Module: 400W, Voc = 48.5V, Vmp = 38.5V, Temp Coeff Voc = -0.28%/°C
Inverter: 150-500V MPPT range, 600V max input
Phoenix low temp: -3°C (28°C below STC)

13 modules in series:
• Voc at -3°C = 48.5V × 13 × 1.078 = 680V ⚠️ EXCEEDS 600V MAX
12 modules in series:
• Voc at -3°C = 48.5V × 12 × 1.078 = 627V ⚠️ STILL TOO HIGH
11 modules in series:
• Voc at -3°C = 48.5V × 11 × 1.078 = 575V ✓ Within limits
• Vmp at 65°C = 38.5V × 11 × 0.888 = 376V ✓ Within MPPT range

✓ Final config: 11 modules/string × 1 string = 4.4 kW (close to 5 kW target)

DC-to-AC Sizing Ratio:

Industry standard is 1.15-1.35× oversizing (DC watts ÷ AC watts). This compensates for inverter clipping during peak production hours while maximizing energy harvest during lower irradiance.

Use String Sizing Calculator

String Inverter vs. Microinverters

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String Inverters: Lower cost, centralized maintenance, higher efficiency (96-98%). Best for unshaded installations with uniform array orientation.

Microinverters: Module-level monitoring, better performance in shading, easier expansion, longer warranties (25 years). Higher upfront cost but excellent for complex roofs.

5

Equipment Selection & Sizing

Choose appropriate inverters, combiners, and protection devices

Select equipment that matches your system voltage, current, and power requirements while providing adequate safety margins and meeting code requirements.

Inverter Selection Criteria:

  • Power rating: Match or slightly exceed DC array size (consider 1.25× DC-to-AC ratio)
  • Voltage compatibility: Verify MPPT range accommodates string voltages at all temps
  • Current rating: Must handle maximum string current (Isc × 1.25)
  • Efficiency: Look for CEC weighted efficiency > 96%
  • Warranty & support: 10-25 year coverage, local service availability
Combiner Box Requirements
  • NEMA 3R or better rating
  • String-level fusing or breakers
  • SPD (surge protection device)
  • Ground fault protection (if required)
Disconnect Requirements
  • AC and DC disconnects required
  • Rated for maximum fault current
  • Within sight of inverter (or lockable)
  • Labeled per NEC 690.56
NEC 690.8 (US): Circuit sizing and overcurrent protection must be rated at 156% of Isc (1.25 × 1.25 safety factor) to account for irradiance variations and temperature effects. Check local electrical codes for regional requirements.

Example Equipment List:
• (11) 400W modules = 4.4 kW DC
• (1) 5 kW string inverter with 600V max input
• (1) DC combiner with 15A breaker per string
• (1) DC disconnect 30A/600V
• (1) AC disconnect 30A/240V
• SPD devices on DC and AC sides

6

Electrical Design & Wire Sizing

Calculate conductor sizes, voltage drop, and conduit requirements

Proper wire sizing ensures code compliance, minimizes voltage drop, and provides safe operation. Consider ampacity derating for temperature, conduit fill, and bundling.

NEC 690.8(B) (US): PV circuit conductors must be sized at 156% of maximum current (Isc × 1.25 × 1.25). This accounts for edge-of-cloud enhancement and ensures conductor longevity. Always verify with local electrical codes.
Conductor Ampacity Requirement
Required Ampacity = Isc × 1.56
After Derating = Required Ampacity ÷ (Temp Factor × Conduit Fill Factor)

Example: 11.5A Isc × 1.56 = 17.94A
With 0.88 temp derate: 17.94 ÷ 0.88 = 20.4A → Use 10 AWG (30A @ 90°C)

Voltage Drop Guidelines:

  • DC circuits: Maximum 2% voltage drop (1% source to combiner, 1% combiner to inverter)
  • AC circuits: Maximum 3% to main panel
  • Lower voltage drop = better efficiency, especially on long runs
  • Use voltage drop calculator to verify conductor selection
Voltage Drop Calculation (Single-Phase DC)
VD% = (2 × K × I × L) ÷ (CM × V) × 100

Where:
K = 12.9 for copper (resistivity constant)
I = Current in amps
L = One-way distance in feet
CM = Circular mils of conductor
V = Circuit voltage

Example Wire Sizing:
DC Circuits (11.5A Isc, 425V Vmp):
• Required: 11.5A × 1.56 = 17.94A
• After derating (90°C, rooftop): 17.94 ÷ 0.88 = 20.4A
• Selected: 10 AWG USE-2 (30A rated)
• 50 ft run voltage drop: 0.8% ✓

AC Circuit (5kW @ 240V = 20.8A):
• Required: 20.8A × 1.25 = 26A
• Selected: 10 AWG THWN-2
• 75 ft run voltage drop: 2.1% ✓

Use Wire Sizing Calculator Use Voltage Drop Calculator Use Conduit Size Calculator

Conduit Fill Requirements

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NEC Chapter 9, Table 4 limits conductor fill to 40% for 3+ conductors. Always verify fill calculations using online tools or NEC tables. PV circuits typically require 2 conductors (+/−) per string, plus equipment grounding conductor.

7

Financial Analysis & ROI

Calculate payback period, NPV, and lifetime savings

A comprehensive financial analysis helps understand the investment value of solar. Include system cost, incentives, utility savings, and financing options.

Financial Inputs:

  • System cost: $2.50-3.50/watt installed (varies by region, size, complexity)
  • Incentives: Tax credits, rebates, SRECs, property/sales tax exemptions (varies by country/region)
  • Utility rate: Average $/kWh and escalation rate (typically 2-4%/year)
  • Net metering: Credit rate for exported energy (if available)
  • Financing: Cash, loan (interest rate, term), lease, or PPA options
Simple Payback Period
Payback (years) = Net System Cost ÷ Annual Savings

Net Cost = Total Cost − Tax Credits − Incentives
Annual Savings = Annual Production (kWh) × Utility Rate ($/kWh)

Example Financial Analysis (US-based):
System Cost: 4.4 kW × $3.00/W = $13,200
Federal Tax Credit (30%): −$3,960
Net Cost: $9,240

Annual Production: 7,600 kWh
Utility Rate: $0.13/kWh
Annual Savings: 7,600 × $0.13 = $988

Simple Payback: 9,240 ÷ 988 = 9.4 years
25-Year Savings (4% escalation): $34,500+

Note: Incentives and costs vary significantly by location

Advanced Metrics:

  • NPV (Net Present Value): Total lifetime value in today’s dollars
  • IRR (Internal Rate of Return): Effective annual return on investment
  • LCOE (Levelized Cost of Energy): Average cost per kWh over system lifetime
Use Financial ROI Calculator
8

Code Compliance & Permits

Final review before permit submission

Before submitting for permits, verify all local electrical code requirements are met. This checklist covers common compliance items based on NEC Article 690 (US), but always check your local jurisdiction’s specific requirements.

Common Code Requirements (NEC-based):

  • 690.7: Maximum system voltage calculated with temperature correction ✓
  • 690.8: Conductors sized at 156% of Isc (1.25 × 1.25) ✓
  • 690.12: Rapid shutdown system installed (within 1 ft of array boundary) ✓
  • 690.13: Photovoltaic system disconnecting means provided ✓
  • 690.31: Wiring methods appropriate for location (USE-2 for exposed) ✓
  • 690.35: Ungrounded PV systems require ground-fault protection ✓
  • 690.56: Labels and markings on all equipment and disconnects ✓
  • 705.12: Grid interconnection meets utility requirements ✓
NEC 690.12 Rapid Shutdown (US): PV systems on buildings must reduce conductors outside array boundary to 80V within 30 seconds of shutdown initiation. Most modern inverters include this functionality. Check if this applies in your region.

Required Labels & Documentation:

  • AC/DC disconnect labels with voltage and current ratings
  • “WARNING: ELECTRIC SHOCK HAZARD” labels on PV equipment
  • Directory label showing PV system location
  • Placard at service entrance showing PV interconnection
  • Single-line diagram showing system configuration
  • Roof plan with module layout and fire setbacks

Important Note: This tutorial references US National Electrical Code (NEC) standards. Always consult your local electrical codes, building regulations, and authority having jurisdiction (AHJ) for specific requirements in your region. Electrical standards vary significantly worldwide (IEC, AS/NZS, BS, etc.).