Site Assessment, Load Analysis, and System Design

Site Assessment, Load Analysis, and System Design

This capstone lab brings together every skill you have built in the program. You will assess a real or mock site, calculate a system size, and produce the design documents a licensed contractor would submit for a permit. Work carefully — errors here cost money and can create safety hazards on an energized roof.

Site Assessment

Before you size a single module, you must understand the physical site. Arrive with your compass, inclinometer, tape measure, and shading tool.

Roof or ground-mount survey. Measure the available area and record roof pitch (or proposed tilt angle for a ground mount) and the compass azimuth of each candidate array location. True south (180° azimuth) is the reference for maximum annual production in the continental United States. Deviations east or west reduce yield; quantify this for your written justification.

Shading analysis. Use a Solar Pathfinder, Solmetric SunEye, or an equivalent app-based tool (such as SunSurveyor or the shade analysis feature built into Aurora Solar) to map obstructions at each candidate location. The tool projects the sun's annual path onto your measurement. Record the percentage of annual solar access (or equivalent metric) at each of your three locations. Select the location with the highest unshaded solar access and document your choice with the tool output and a written explanation of how shading would affect annual energy production. Even partial shading on one cell in a series string can reduce output of the entire string — this is why location selection matters.

Safety during site assessment. Follow all fall-protection requirements whenever you are on a roof: wear a hard hat, non-slip footwear, and a properly anchored personal fall arrest system if you are at or above six feet. On a cooperating building, follow the site owner's safety plan. Never walk on a roof surface you have not confirmed can bear your weight. Keep tools tethered to prevent dropped-object hazards.

Load Analysis and System Sizing

System size is driven by what the customer actually uses, not by how much roof space is available.

Step 1 — Annual consumption. Read the utility bill provided by your instructor. Sum twelve months of kWh usage to get annual consumption. If only one month is shown, use the annualization factor on your worksheet.

Step 2 — DC system size using peak sun hours. The standard method divides the daily kWh target by the peak sun hours (PSH) available at the site, then applies a system derate factor to account for real-world losses (wiring, inverter efficiency, soiling, temperature, and mismatch). A commonly used overall derate factor is around 0.80, though you should calculate or use the value provided on your worksheet.

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Daily kWh target = Annual kWh ÷ 365

Required DC kW = Daily kWh target ÷ (PSH × derate factor)

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Step 3 — Module and inverter selection. Choose modules from the manufacturer specification sheets provided. Calculate the number of modules needed: divide the required DC wattage by the module's STC-rated power (Watts). Round up to a whole number. Select a string inverter or microinverter that accepts the resulting DC input range.

String Sizing and NEC 690 Principles

String sizing keeps the system within the voltage and current limits set by both the NEC and the equipment manufacturer.

Voltage. NEC Article 690.7 requires that the maximum system voltage be calculated using the module's open-circuit voltage (Voc) corrected for the lowest expected ambient temperature at the site using the temperature coefficient provided in the module spec sheet. The corrected Voc per module multiplied by the number of modules in series must not exceed the inverter's maximum input voltage and must not exceed the system's maximum voltage rating (commonly 600 V for residential, 1000 V or 1500 V for commercial systems).

Current. NEC Article 690.8 principles require that conductor and overcurrent device ampacity be based on the module's short-circuit current (Isc) multiplied by 1.25 (to establish the continuous-duty baseline), and then further multiplied by 1.25 for overcurrent protection sizing where required, unless the equipment is rated for continuous operation under these conditions.

Safety during electrical design. A design error that results in a string voltage above inverter or NEC limits creates an arc-flash and fire hazard. Check every voltage calculation twice before submitting. During any physical work with energized conductors, apply lockout/tagout (LOTO) procedures and wear appropriate arc-rated PPE.

Single-Line Diagram and Bill of Materials

A single-line diagram (SLD) is the electrical schematic that AHJs (Authorities Having Jurisdiction) and inspectors review for permit approval. Every component and disconnect must appear and be labeled.

Required elements for a residential grid-tied SLD:

• PV array (show number of strings and modules per string)
• DC disconnect (rapid shutdown and array disconnect)
• Inverter
• AC disconnect
• Main service panel (show point of interconnection)
• Utility meter
• Grounding electrode conductor and equipment grounding conductors
• All conductor sizes and conduit types on each segment
• System voltage and current ratings at each segment

Draft by hand first on graph paper, then reproduce in your design software. The hand draft forces you to think through every connection before the software automates anything.

Your bill of materials (BOM) must list every component in the SLD with manufacturer, model number, quantity, and the rated specification that governs its selection (e.g., inverter max input voltage, conductor ampacity).

Completing Your Written Narrative

A short written narrative (one to two pages) accompanies the SLD and BOM. Explain why you chose each major piece of equipment, how your string sizing keeps the system within NEC 690.7 and 690.8 limits, and why you selected the final array location over the alternatives. This narrative is your professional justification — write it as if you are presenting to a building department.