PVWatts Solar Production Calculator
Estimate the annual energy production and capacity factor of a grid-tied photovoltaic (PV) system.
The total DC nameplate rating of your solar array in kilowatts (kW).
The average number of hours per day your location receives 1000 W/m² of solar irradiance. This value varies by location and time of year. (e.g., 4-6 hours)
Represents the overall efficiency losses from DC power at the modules to AC power at the inverter output. Includes inverter efficiency, wiring losses, temperature effects, etc. (Typical: 70-85%)
Accounts for other losses not covered by the derate factor, such as shading, soiling (dirt/dust on panels), and snow cover. (Typical: 0-5%)
Estimated Production Results:
"; resultsHTML += "Estimated Annual AC Energy Production: " + annualProduction.toFixed(2) + " kWh"; resultsHTML += "Estimated Capacity Factor: " + capacityFactor.toFixed(2) + "%"; resultDiv.innerHTML = resultsHTML; }Understanding the PVWatts Solar Production Calculator
The PVWatts calculator is a widely used tool developed by the National Renewable Energy Laboratory (NREL) to estimate the electricity production of grid-connected photovoltaic (PV) systems. While the full NREL PVWatts tool uses detailed hourly weather data for specific locations, this simplified calculator provides a quick estimate based on key system parameters and average solar insolation for your area.
How Solar Production is Estimated
At its core, solar energy production is a function of your system's size, the amount of sunlight it receives, and various efficiency losses. This calculator uses a straightforward approach to help you understand these factors.
Key Inputs Explained:
System Size (DC kW)
This refers to the total direct current (DC) power rating of all the solar panels in your array. For example, if you have 20 solar panels, each rated at 300 watts, your total DC system size would be 20 * 300W = 6000W, or 6 kW. This is the raw power your panels can produce under ideal test conditions.
Average Daily Peak Sun Hours
Peak sun hours are a measure of solar insolation, representing the equivalent number of hours per day when solar irradiance averages 1,000 watts per square meter (W/m²). This is a crucial factor as it directly reflects how much sunlight your location receives on average. For instance, 4.5 peak sun hours means that over the course of a day, your panels receive the same amount of energy as if they were exposed to full sun (1,000 W/m²) for 4.5 hours. You can find average peak sun hour data for your specific location from resources like NREL's solar maps or other meteorological data providers.
DC to AC Derate Factor (%)
The DC to AC Derate Factor is a critical efficiency multiplier that accounts for various losses as DC power from your solar panels is converted into usable alternating current (AC) electricity for your home or the grid. These losses include:
- Inverter Efficiency: The efficiency of the inverter converting DC to AC.
- Wiring Losses: Energy lost as heat in the electrical wiring.
- Temperature Losses: Solar panels become less efficient as their temperature increases.
- Mismatch Losses: Slight differences in performance between individual panels.
- Soiling (partial): Some initial consideration for dirt/dust.
A typical derate factor ranges from 70% to 85%, depending on the quality of components, system design, and climate.
Additional System Losses (%)
While the DC to AC Derate Factor covers many common losses, additional factors can further reduce your system's output. These "additional losses" often include:
- Shading: Obstructions like trees, chimneys, or neighboring buildings blocking sunlight.
- Soiling (remaining): More significant accumulation of dirt, dust, pollen, or bird droppings.
- Snow Cover: Panels covered by snow, especially in winter months.
- System Age: Degradation of panels over time (though often modeled separately).
These losses are typically estimated as a percentage reduction from the otherwise expected output.
Key Outputs Explained:
Estimated Annual AC Energy Production (kWh)
This is the primary output of the calculator, representing the total amount of alternating current (AC) electricity your solar system is estimated to produce over an entire year, measured in kilowatt-hours (kWh). This value is crucial for understanding your potential electricity bill savings and the overall economic viability of your solar investment.
Estimated Capacity Factor (%)
The capacity factor is a measure of how efficiently your solar power plant operates relative to its maximum possible output if it ran at full power 24/7 for the entire year. It's calculated as:
Capacity Factor = (Actual Annual AC Energy Production / (System Size DC * 8760 hours/year)) * 100
A higher capacity factor indicates a more productive system. For solar PV, typical capacity factors range from 10% to 25%, depending heavily on location, system design, and local weather patterns.
Limitations of This Simplified Calculator
This calculator provides a useful estimate but is a simplification of the full NREL PVWatts tool. It does not account for:
- Hourly or monthly variations in solar insolation.
- Specific module types (e.g., thin-film vs. crystalline silicon).
- Array type (e.g., fixed, 1-axis tracking, 2-axis tracking).
- Specific tilt and azimuth angles (these are implicitly factored into your chosen "Average Daily Peak Sun Hours" and "DC to AC Derate Factor").
- Detailed temperature modeling.
- Geographic-specific weather patterns beyond average peak sun hours.
For a more precise analysis, especially for system design and financial modeling, it is recommended to use the official NREL PVWatts Calculator or consult with a professional solar installer.