This guide covers the two critical first steps in solar system design: determining how many panels are required to meet energy goals and ensuring the installation site is structurally sound.
Designing a solar array is not just about fitting as many panels as possible on a roof. It starts with a target energy output (kWh) and working backward to determine the hardware requirements.
Before selecting hardware, review the site's electricity usage history.
Look at the last 12 months: Total the kilowatt-hours (kWh) found on utility bills.
Adjust for future usage: If an EV charger, heat pump, or pool will be added, increase the target kWh accordingly (typically by 15–25%).
To convert energy needs (kWh) into system power (kW), you must account for local sunlight conditions, known as "Peak Sun Hours."
Note: The Derate Factor (usually ~0.75 to 0.82) accounts for real-world inefficiencies like wiring loss, inverter efficiency, shading, and soil/dust on panels.
Once you have the required System Size (kW), divide by the wattage of the specific panel model you intend to use.
Example: A 6 kW system using 400W panels requires: (6×1000)/400=15 panels.
Solar panels add significant weight and change the aerodynamics of a structure. Every design must verify that the building can support these new forces.
Adding solar arrays introduces new loads that the roof structure must support.
Dead Load: This is the static weight of the PV system itself (panels, racking, clamps, and ballast). Most residential systems add 2.5 – 4 lbs per sq. ft.
Live Load: Temporary weights the roof must support, such as snow, rain, or maintenance workers.
Engineering Review: Older roofs or those with non-standard framing (e.g., 2x4 rafters) may require structural reinforcement or "sistering" of rafters to handle the additional dead load.
Wind does not just push against panels; it tries to lift them off the roof (uplift). Designs must comply with ASCE 7-10/7-16 standards regarding wind speeds.
Wind Zones: Roofs are divided into three zones based on wind pressure:
Zone 1 (Interior): Lowest pressure. Standard attachment spacing.
Zone 2 (Perimeter): Higher pressure. often requires closer attachment spacing.
Zone 3 (Corners): Highest pressure/turbulence. Panels should ideally be avoided here, or they require the highest density of attachments.
Exposure Categories: You must also categorize the surrounding terrain (e.g., Exposure B for urban/suburban, Exposure C for open terrain/coastlines), as this affects wind velocity calculations.
[ ] Obtained 12 months of utility usage data.
[ ] Applied appropriate production ratio/derate factor for the region.
[ ] Verified panel wattage availability.
[ ] Checked rafter size and spacing (e.g., 2x6 @ 24" O.C.).
[ ] Identified wind exposure category and edge zones.