PV Design Software and Energy Production Modeling

Why Software Modeling Matters

Hand calculations give you a solid estimate, but a client signing a contract needs a more precise prediction. PV design software combines satellite weather data, roof geometry, module performance curves, inverter efficiency tables, and shading analysis into a single model. The result is an annual energy production estimate that is far more reliable than a pencil-and-paper figure—and it becomes the document you stand behind professionally.

NABCEP exam content and real job sites both expect you to use production modeling tools confidently. This lesson walks through the full workflow so that you can build a model, read the output, and explain it to a client or inspector.

Key Software Platforms

PVWatts (NREL): Free, browser-based, and maintained by the National Renewable Energy Laboratory. You enter location, system size (DC kW), module type, array tilt, azimuth, and system losses. PVWatts returns monthly and annual AC energy output in kWh. It is the most widely accepted tool for quick feasibility checks and utility interconnection applications.

Aurora Solar: A commercial platform used by many installation companies. Aurora ingests aerial imagery, lets you trace roof planes, automatically calculates pitch and azimuth, runs irradiance and shading simulations, and generates full proposal reports. Your instructor may substitute another licensed platform (e.g., Helioscope, OpenSolar); the workflow concepts are the same across tools.

The Modeling Workflow

1. Site Location and Weather Data

Enter the project address or GPS coordinates. The software pulls a Typical Meteorological Year (TMY) dataset—a statistically representative full year of hourly weather records for that location. TMY data drives all irradiance calculations, so location accuracy is critical. A one-degree latitude error can shift annual production by several percent.

2. Roof Plane Definition

Define each roof surface the array will occupy. Inputs include pitch (degrees or rise-over-run), compass azimuth (true south = 180°), and the usable area after setbacks. In advanced tools you trace the plane on aerial imagery; in PVWatts you enter tilt and azimuth directly.

3. Module and Inverter Selection

Choose equipment from the software's database using the same model numbers on your design drawings. The software applies the manufacturer's power-temperature coefficient and the inverter's efficiency curve. Mismatched equipment entries produce inaccurate results—always verify part numbers.

4. Shading Entry

PVWatts uses a single "shading loss" percentage (a conservative estimate). Advanced platforms run a full irradiance simulation hour by hour, casting shade from trees, chimneys, and parapets onto each module position. Accurate shading entry is one of the most important steps because even partial shade on one module can reduce string output significantly, depending on inverter topology.

5. System Loss Settings

PVWatts uses a default system loss value that accounts for soiling, wiring resistance, mismatch, availability, and other derate factors. Review each sub-category and adjust for site conditions (e.g., higher soiling loss in dusty climates). Never leave all defaults without checking that they fit your project.

6. Report Generation and Interpretation

Run the simulation and export the report. Key figures to check:

• Annual AC kWh – the production number you compare against the client's annual consumption.
• Monthly breakdown – confirms seasonal variation and helps set client expectations.
• Performance ratio – AC output divided by the theoretical DC output; a healthy residential system is generally in a reasonable range that reflects your site conditions and losses.
• Offset percentage – annual kWh produced divided by annual kWh consumed, expressed as a percent. Your design target is to hit the client's requested offset within ±10%.

Comparing Software Output to Your Hand Calculation

After modeling, calculate the percentage difference between the software result and your Lesson 3 hand estimate:

> Difference (%) = ((Software kWh − Hand-Calc kWh) / Hand-Calc kWh) × 100

A difference is expected and normal. Common reasons:

• The hand calculation uses peak sun hours and a single derate factor; software applies hour-by-hour irradiance and detailed loss components.
• Software models inverter clipping when the DC array slightly overloads the inverter at peak irradiance.
• Shading entered in software may not have been captured in the hand calculation.

Document the difference and write a clear, two-sentence explanation. This skill—reconciling methods—is tested on NABCEP exams and required in professional practice.

Safety and Professional Responsibility

Software use carries no direct electrical hazard, but it carries professional liability. An overestimated production figure can expose your employer to warranty claims or legal action. Follow these practices:

• Always use the actual project address, not a placeholder.
• Match equipment entries to the approved submittal schedule.
• Save and back up your model file before exporting the final report.
• Label every exported PDF with the project name, date, and software version.
• Never alter a report PDF after export to misrepresent results—this is unethical and potentially fraudulent.

When working at a physical site to collect data for modeling (measuring roof planes, assessing shading obstructions), apply standard fall-protection rules: use a hard hat, non-slip footwear, and follow your employer's rooftop safety policy. Do not access the roof alone.