Before a solar project can be evaluated financially, the project team needs an estimate of how much electricity the system may produce. The PVWatts Calculator, developed by the National Renewable Energy Laboratory, provides a practical way to create an initial production estimate using basic system and location information.
This guide explains how to use PVWatts, which assumptions matter and how to interpret the results.
What Is PVWatts?
PVWatts is a web-based calculator that estimates the energy production and basic value of grid-connected photovoltaic systems. It uses historical weather data, solar-resource information and user-provided system assumptions to estimate monthly and annual electricity generation.
PVWatts is particularly useful for early-stage analysis, education and high-level comparisons. It does not replace a detailed production study prepared for final engineering, financing or investment decisions.
Information to Gather Before You Begin
You can run a basic estimate with only a project location and system size. A more useful analysis should also consider:
- Expected direct-current system size
- Module type
- Array type
- System losses
- Array tilt
- Array azimuth
- Direct-current to alternating-current size ratio
- Inverter efficiency
If the project design is still preliminary, document which values are confirmed and which are assumptions.
How to Use the PVWatts Calculator
Step 1: Enter the Project Location
Enter the site’s address, ZIP code or geographic coordinates. PVWatts will identify nearby weather data and display the selected location on a map.
Review the weather-data source before continuing. A nearby station is helpful, but proximity alone does not guarantee that it perfectly represents the project’s conditions.
Step 2: Enter the System Size
Enter the system’s direct-current nameplate capacity in kilowatts. For example, a system rated at 500 kilowatts DC would be entered as 500 kW.
Be careful not to enter the alternating-current capacity unless the two values are intentionally the same.
Step 3: Select the Module Type
Select the module category that most closely represents the proposed equipment. The available options reflect different assumptions about module performance and efficiency.
For early-stage estimates, use the best available information and record the selected assumption for future comparison.
Step 4: Select the Array Type
Choose the configuration that best reflects the proposed mounting system, such as a fixed roof-mounted array, fixed ground-mounted array or tracking system.
The array type affects operating temperature, orientation and the amount of solar energy captured by the system.
Step 5: Review System Losses
PVWatts includes a default total system-loss assumption. These losses may represent soiling, shading, wiring, mismatch, availability, connections and other factors that reduce delivered energy.
Do not reduce the default simply to improve the result. Update the assumption only when project-specific information supports the change.
Step 6: Enter Tilt and Azimuth
Tilt describes the angle of the modules relative to horizontal. Azimuth describes the direction the modules face. In PVWatts, an azimuth of 180 degrees generally represents a south-facing array in the Northern Hemisphere.
For rooftop projects, tilt and azimuth may be constrained by the roof. Ground-mounted projects may offer greater design flexibility.
Step 7: Review Advanced Parameters
Advanced inputs include the DC-to-AC ratio, inverter efficiency and other modeling assumptions. If you do not have project-specific values, the defaults can support an initial estimate, but they should be documented as assumptions.
Step 8: Calculate the Results
Run the calculation and review the estimated monthly and annual electricity production. Save the inputs alongside the results so that another person can understand and reproduce the estimate.
How to Interpret the Results
Annual Energy Production
The annual production estimate is the total electricity the modeled system is expected to generate during a typical year, usually expressed in kilowatt-hours.
This figure can be used as an early input for revenue, savings and project-finance calculations.
Monthly Production
The monthly results show how generation changes throughout the year. Seasonal variation may be caused by solar-resource availability, weather patterns, module orientation and temperature.
Reviewing the monthly profile is especially important when the project’s value depends on when electricity is generated rather than only on annual production.
Capacity Factor
Capacity factor compares actual modeled production with the amount of energy the system would produce if it operated at its nameplate capacity during every hour of the year.
It is useful for high-level comparisons, but it should be interpreted in the context of the project’s location, design and DC-to-AC ratio.
Common PVWatts Mistakes
- Entering AC capacity instead of DC system size
- Selecting an array type that does not match the proposed design
- Using unrealistic tilt or azimuth assumptions
- Reducing system losses without supporting evidence
- Assuming the nearest weather station perfectly represents the site
- Treating an early-stage PVWatts estimate as a final bankable production study
- Failing to save the assumptions used to create the result
Build Useful Comparison Scenarios
PVWatts becomes more instructive when you compare multiple cases. Consider testing:
- Different roof orientations
- Fixed-tilt and tracking configurations
- Higher and lower system-loss assumptions
- Alternative DC-to-AC ratios
- Multiple project locations
- Different system sizes
Change one assumption at a time so you can identify what caused the change in production.
From Production to Project Economics
The production estimate is only one part of a project analysis. To evaluate economics, combine expected electricity generation with the project’s revenue or savings rate, operating costs, construction cost, financing and incentive assumptions.
You can explore that relationship with The Energy Academy’s interactive solar financial model.
Start Your Estimate
Open PVWatts, enter a project location and system size, and then test how different design assumptions affect the result.
Estimate Solar Production with PVWatts
For additional background, explore The Energy Academy’s solar engineering learning resources.