Structural and Electrical Load Assessment for Rooftop Solar Installations

Installing rooftop solar panels requires a dual-layered analysis—structural and electrical load assessment. While the structural load assessment ensures the building can physically support the solar array and withstand environmental forces, the electrical load assessment guarantees safe and efficient integration of the PV system with the building's existing power system and utility grid.

This article covers detailed engineering methods, load types, formulas, examples, and frequently asked questions for a complete technical understanding of the subject.

1. Introduction

The integration of photovoltaic (PV) systems onto rooftops should never begin without assessing the load-bearing capacity of the roof and understanding the implications of connecting a new electrical generation source. Failing to assess structural and electrical loads correctly can lead to:

Roof failures
Fire hazards
Inverter faults
Code violations
Insurance non-compliance

2. Structural Load Assessment

2.1 Types of Structural Loads

Dead Load (DL):
Weight of permanent fixtures like panels, mounting structure, and conduits.
Live Load (LL):
Temporary loads from workers, tools, or snow accumulation.
Wind Load (WL):
Uplift and lateral forces due to wind, based on local building codes.
Seismic Load (SL):
In seismic zones, panels and mounting systems contribute to the building's seismic mass.

2.2 Load Calculation Framework

Total Structural Load (TSL):

TSL = DL + LL + WL + SL

Dead Load (DL):

DL = W_{panel} + W_{mount} + W_{cable}

🔹 Typical Weights:

PV Panel: 12–15 kg/m²
Mounting Structure: 10–20 kg/m²
Total Dead Load: 20–35 kg/m²

2.3 Wind Load Calculation (ASCE 7-16 Method)

P = q_z \cdot G \cdot C_p - q_i \cdot (GC_{pi})

Where:

$P$ = Design wind pressure (psf or N/m²)
$q_z$ = Velocity pressure at height z
$G$ = Gust effect factor
$C_p$ = External pressure coefficient
$q_i$ = Internal pressure
$GC_{pi}$ = Internal pressure coefficient

Simplified Equation (for low-rise buildings):

P = 0.613 \cdot V^2 \cdot C_e \cdot C_d \cdot K_d

Where:

$V$ = Basic wind speed (m/s)
$C_e, C_d$ = Terrain and height factors
$K_d$ = Directionality factor

🔸 In Phoenix, Arizona (wind speed = 105 mph ≈ 47 m/s):

P \approx 0.613 \cdot (47)^2 = 1353.9 \text{ N/m}^2

Apply this value to structural load combinations using ASD or LRFD methods.

2.4 Structural Safety Factors

Use load combinations as per IBC or ASCE 7
Include appropriate factor of safety (FoS):
- Dead Load: FoS = 1.5
- Wind Load: FoS = 2.0

2.5 Roof Suitability Checklist

Roof material (concrete slab, metal sheet, clay tile)
Roof slope and tilt
Accessibility
Age and maintenance history
Drainage system

3. Electrical Load Assessment

Electrical load assessment ensures compatibility and safe operation between the PV system, building load, and the grid.

3.1 System Rating and Load Profile

Step 1: Determine Average Load

\text{Average Daily Load} = \sum (\text{Power}_{i} \times \text{Time}_{i})

🔹 Example:

HVAC = 2.5 kW × 6 hrs = 15 kWh
Refrigerator = 0.2 kW × 24 hrs = 4.8 kWh
Lighting = 0.3 kW × 6 hrs = 1.8 kWh
Total = 21.6 kWh/day

3.2 System Sizing

Assuming 5.5 peak sun hours:

\text{Required PV Capacity} = \frac{21.6}{5.5} = 3.93 \approx 4 \text{ kW}

Derate by 10–15% for inefficiencies:

\text{Adjusted Size} = 4.5 \text{ kW}

3.3 Maximum Voltage and Current

Use module datasheets to calculate string parameters:

V_{oc, array} = N_{modules} \cdot V_{oc}

I_{sc, array} = I_{sc} \cdot N_{strings}

Adjust for temperature extremes per NEC 690.7.

3.4 Conductor and Protection Sizing

DC Cable Size:

Use voltage drop formula:

V_{drop} = I \cdot L \cdot R

\%V_{drop} = \left( \frac{V_{drop}}{V_{system}} \right) \times 100

Limit: < 2% for DC; < 3% for AC

3.5 Breakers and Fuses (NEC 690.9)

Overcurrent Protection Device (OCPD) rating:

I_{OCPD} = 1.25 \times I_{sc} \times 1.25 = 1.56 \times I_{sc}

3.6 Inverter Integration

Must match AC voltage (e.g., 240V split-phase for residential)
Size ≤ 120% of panel rating (per NEC 705.12(D)(2))
Anti-islanding protection required

3.7 Utility Load Coordination

Perform a load flow study and grid injection analysis using tools like:

PVSyst
ETAP
Helioscope

Submit to utility for interconnection approval.

4. Case Study – Phoenix, AZ Home

Roof type: Concrete slab
Available area: 32 m²
Chosen module: 400 W, 1.94 m²
No. of panels: 16 (6.4 kW)
Inverter: 6 kW (240V, UL 1741 listed)
Wind load: 1353.9 N/m²
Roof capacity: Verified for 50 kg/m² dead load
Cable size: 6 mm² DC, 10 mm² AC
Performance Ratio: ~82%
Annual output ≈ 10,000 kWh/year

FAQs

Q1. What is the minimum roof strength required for solar?

Generally, ≥ 25 kg/m² is needed, but 40–50 kg/m² is safer for added wind loads and system weight.

Q2. Can I install solar on an old or cracked roof?

It’s not recommended. Re-roof or reinforce the structure before solar installation to ensure longevity and safety.

Q3. Is electrical load assessment required for net metering?

Yes. Utilities require system ratings and expected export/load analysis before permitting.

Q4. Do wind loads increase with elevation?

Yes. Wind pressure increases with height and building exposure category (A, B, C, D per ASCE 7).

Q5. What codes apply to rooftop PV load assessments?

ASCE 7-16 (Wind/seismic loads)
IBC (Structural safety)
NEC Article 690/705 (Electrical design)
UL 1741/1699B (Inverter safety)

Conclusion

Structural and electrical load assessments are essential for the safe and compliant deployment of rooftop solar PV systems. Proper evaluation ensures longevity, performance, and peace of mind. Whether you're an engineer, installer, or homeowner, understanding these principles leads to more reliable and efficient solar installations.

Prasun Barua