Using PVWatts for Preliminary Solar Production Estimates

Using PVWatts, a free tool from the National Renewable Energy Laboratory (NREL), is an essential first step for any master or journeyman electrician creating a preliminary solar proposal. This powerful calculator allows you to generate a reliable initial estimate for a system’s energy production, which is crucial for early client conversations. By inputting a location, system size, and basic equipment parameters, PVWatts processes decades of solar irradiance data to forecast monthly and annual output. Understanding how to adjust inputs like system losses and the DC to AC size ratio helps create a more realistic picture of how a solar PV system will perform. While it doesn’t replace a full electrical design or a detailed voltage drop calculation, it provides a foundational analysis that builds client confidence and sets the stage for a comprehensive, code-compliant final proposal.

Why Preliminary Estimates Matter for Master and Journeyman Electricians

In today’s competitive solar market, clients expect fast, transparent, and data-driven answers. As a professional electrician, your ability to provide a quick yet reasonably accurate production estimate can be the difference between winning a job and losing a lead. This initial analysis, often based on a tool like PVWatts, serves as a critical conversation starter. It helps manage client expectations about the potential output of a solar PV system for their home before investing significant time in a full-scale engineering design. The growth in the solar industry continues to present new avenues for licensed professionals, and mastering these preliminary tools is a key skill. For those looking to expand their expertise, exploring solar career opportunities for electricians can be a significant professional step.

This preliminary estimate is a stepping stone toward a binding proposal. While PVWatts provides the energy-production side of the equation, a final quote requires a thorough understanding of the current NEC code book, local regulations, and the complexities of system integration. Sophisticated clients understand that an initial estimate is not the final word. The goal is to build trust by demonstrating expertise from the very first interaction, using reliable data to frame the project’s potential.

Getting Started with the PVWatts Calculator: A Step-by-Step Guide

Using PVWatts is a straightforward process that provides immense value. Follow these steps to generate your first preliminary solar energy estimate. This process forms the foundation of modern electrician training for renewables.

1. Navigate to the PVWatts Website: Open your browser and go to the official NREL PVWatts Calculator website.
2. Enter Project Location: Input the site address. The tool uses this to pull from its extensive database of location-specific solar irradiance data, which is the primary factor determining a system’s potential output.
3. Define Basic System Information: Here you will enter the core specifications of the proposed system.
   • DC System Size (kW): This is the total rated power of your solar panels. For a simple residential system, this might be 5-10 kW.
   • Module Type: Choose between Standard, Premium (higher solar panel efficiency), and Thin Film.
   • Array Type: Select from Fixed (roof or ground mount), 1-Axis Tracking, or 2-Axis Tracking.
4. Input System Losses: This is a critical step for a realistic estimate. PVWatts defaults to a 14% system losses value. On the PVWatts site you can edit detailed loss categories (soiling, shading, wiring, etc.) to see how they affect the total loss; however, inverter clipping is not an editable loss category in the system losses input — clipping is produced by the inverter model when the DC-to-AC ratio causes the predicted AC output to exceed the inverter nameplate rating. While not a substitute for a full conductor sizing analysis, adjusting these loss inputs provides a solid initial adjustment.
5. Adjust Advanced Parameters (Optional but Recommended): For a more refined estimate, you can modify the tilt angle, azimuth angle, and the DC to AC size ratio. Proper inverter sizing is key, and modeling a ratio between 1.1 and 1.3 is common practice to maximize energy harvest.
6. Calculate and Review Results: Click the “Calculate” button to see the results. The output will show monthly and annual AC energy production in kilowatt-hours (kWh). The results page also provides valuable metrics like the plane-of-array irradiance, which is the amount of solar radiation striking the panels at their specific tilt and azimuth.

Understanding Key Inputs and Their Impact on System Performance

The accuracy of your PVWatts estimate depends on the quality of your inputs. For a journeyman electrician or master electrician, understanding these variables demonstrates a higher level of expertise.

Solar Irradiance Data and Location

This is the most critical input. PVWatts uses decades of weather and solar radiation data for thousands of locations, giving it a strong statistical foundation. An identical system will produce vastly different amounts of energy in Phoenix, Arizona, versus Seattle, Washington, purely due to available sunlight.

System Losses and the Derating Factor

No system operates at 100% efficiency. The derating factor accounts for real-world production losses. Key components include:

• Soiling: Dust, pollen, and grime on panels.
• Shading: Trees or adjacent structures blocking the sun.
• Wiring and Connections: Minor energy loss in conductors, which a detailed voltage drop calculation would quantify more precisely.
• Inverter Efficiency: The loss incurred when converting DC power from the panels to usable AC power.
• Temperature: Solar panel efficiency decreases as temperature increases.

Azimuth Angle and Tilt Angle: Optimizing Panel Orientation

These two inputs define how the solar array is oriented toward the sun. The azimuth angle is the compass direction (180° is due south in the northern hemisphere), while the tilt angle is the slope of the panels relative to the ground. Small adjustments here can significantly impact the annual energy output, making it a key consideration during site assessment.

DC to AC Size Ratio and Inverter Sizing

The DC to AC size ratio is the total DC power of the solar panels divided by the AC power rating of the inverter. A ratio greater than 1.0 (e.g., 1.25) means the solar array is oversized relative to the inverter. This is a common and effective strategy. It allows the inverter to operate at its peak efficiency for more hours of the day and capture more energy during morning and evening hours, at the cost of minor “clipping” (lost energy) during the absolute peak sun hours on a few days of the year. Getting this ratio right is a key part of proper inverter sizing.

From PVWatts Estimate to a Full Proposal: Bridging the Gap with the NEC

Using PVWatts provides a powerful energy model, but it is not an electrical design tool. A professional, install-ready proposal must be grounded in the National Electrical Code, particularly NEC Article 690, which governs solar photovoltaic systems. After the preliminary estimate, a qualified electrician must perform a detailed design that accounts for safety, compliance, and performance.

Critical design elements that go beyond a PVWatts analysis include:

• Conductor Sizing: Calculating appropriate wire gauges for the DC and AC sides of the system based on ampacity, temperature, and conduit fill, as required by the NEC.
• Voltage Drop Calculation: Ensuring that energy loss across long wire runs remains within recommended limits. The NEC’s informational guidance suggests keeping voltage drop to about 3% for a feeder or branch circuit and 5% maximum combined; many PV designers aim for a total voltage drop around 2–3% (for example, 2% on the DC side and 1% on the AC side) for optimal performance.
• Rapid Shutdown: Designing a system that complies with NEC requirements for rapid shutdown, which allows firefighters and first responders to have reduced-voltage conditions for safer access to rooftop arrays during an emergency.
• Interconnection and Integration: Planning for the physical and legal connection to the utility grid, which involves an interconnection agreement. This may also include integration with a standby generator, requiring a proper transfer switch for generator use.
• Overcurrent Protection: Sizing fuses and breakers correctly to protect equipment and prevent hazards.

For highly detailed, bankable financial models, professionals often turn to more advanced software like the System Advisor Model (SAM), also developed by NREL. In fact, PVWatts is a simplified version of the more complex SAM.

To move from initial estimates to building complex, code-compliant systems, ongoing professional development is essential. Create accurate solar proposals. Learn system modeling in our courses. ExpertCE provides a range of online electrical courses designed to provide the electrician training needed to excel in this growing field.

Primary Sources

• NREL PVWatts Calculator: https://pvwatts.nrel.gov/
• PVWatts Version 5 Manual (detailing accuracy): https://www.nrel.gov/
• National Electrical Code (NEC): https://www.nfpa.org/nec

Frequently Asked Questions (FAQ)

How accurate is PVWatts for estimating a solar PV system’s output?

NREL’s PVWatts manual states that errors may be as high as ±10% for annual energy totals and ±30% for monthly totals for weather data representing long-term typical conditions; actual performance in a specific year may deviate further (up to ±20% annual and ±40% monthly in some cases). This level of accuracy is useful for preliminary analysis and initial client discussions, but site-specific modeling and measured data are required for bankable predictions.

Does using PVWatts replace the need for a full electrical design and NEC Article 690 compliance?

Absolutely not. PVWatts is a modeling tool for energy production, not an electrical design tool. A qualified master electrician or journeyman electrician must create a full design that addresses conductor sizing, overcurrent protection, rapid shutdown, and all other requirements of the NEC code book to ensure a safe and compliant installation.

Can PVWatts help with inverter sizing and determining the DC to AC size ratio?

Yes. While it doesn’t recommend a specific inverter, PVWatts is an excellent tool for modeling the effects of different DC to AC size ratios. By running multiple simulations with different ratios, you can see how oversizing the DC array impacts annual energy production and clipping losses, helping you determine an optimal inverter sizing strategy for a given project.