Cautions for Interpreting the Results

PVWatts^® is designed to estimate electricity production and the cost of generating electricity for a photovoltaic system quickly using a few simple inputs. Use the guidelines below to help you understand factors that may affect the quality of your results.

If you need estimates based on more detailed characterization of the photovoltaic system and more detailed financial modeling, we recommend NLR's System Advisor Model (SAM). SAM's Detailed Photovoltaic Model models modules and inverters as separate components with options for shading, snow losses, and other factors that PVWatts^® does not account for. SAM\'s pro-forma cash flow models for different types of projects calculate financial metrics based on more detailed information about project costs, debt options, incentives, complex electricity rate structures, and building electricity consumption.

For cost of energy analysis that does not require detailed production estimates, we recommend NLR's Cost of Renewable Energy Spreadsheet Tool (CREST). It is a suite of Excel cash flow models different types of renewable energy systems designed to assess renewable energy project economics and incentives.

Weather Variability

The monthly and yearly energy production are modeled using the photovoltaic system parameters you selected and weather data that are typical or representative of long-term averages.

Because weather patterns vary from year to year, the values in the tables are better indicators of long-term performance than of performance for a specific month or year. Photovoltaic system performance is largely proportional to the amount of solar radiation received, which may vary from the long-term average by ±30% for monthly values and ±10% for annual values. How the solar radiation might vary for your location may be evaluated by examining the tables in the Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors.

For these variations and the uncertainties associated with the weather data and the model used to model the system's performance, future months and years may be encountered where the actual performance is less than or greater than the values shown in the table. The variations may be as much as 40% for individual months and up to 20% for individual years. Compared to long-term performance over many years, the values in the table are accurate to within 10% to 12%.

System design and operating conditions

The energy value estimates may be overly optimistic if you use the default system losses value and any of the following conditions are true:

• Nearby buildings, trees, objects, or other photovoltaic modules or supporting structures shade the modules.
• The modules are on one- and two-axis tracking devices that do not keep the modules at the optimum orientation with respect to the sun's position.
• Annual soiling or snow cover losses exceed 5%.
• The system performance has degraded from new. (A photovoltaic system's output may degrade by as much as 1% per year.)

If any of these conditions are true, use the Loss Calculator to generate a system loss value that accounts for these factors.

Module choice

The energy production values in the table are estimated using coefficients appropriate for crystalline silicon modules and some kinds of thin film modules.

Energy value, net-metering policy, and customer use habits

If net metering is not available and the photovoltaic system sends surplus electricity to the power grid, the electric service provider often buys this surplus electricity from the owner at a lower price than the retail price of electricity. Because PVWatts^® effectively assumes net metering, the energy value it reports is higher than it would be without net metering.

The energy value does not account for benefits of the the photovoltaic system such as greater energy independence, reduction in fossil fuel usage and air pollution. It also does not account for financial benefits such as the tax deduction for interest payments that residential system owners may claim by financing the project as a home mortgage.