Skip navigation to main content.
NREL - National Renewable Energy Laboratory
About NRELEnergy AnalysisScience and TechnologyTechnology TransferTechnology DeploymentEnergy Systems Integration

PVWatts Legacy Calculators (Version 1 & 2) will no longer be supported after June 2014.

Please use the most current version PVWatts®.

How to Interpret Results in Legacy Calculators

Weather Variability

The monthly and yearly energy production estimates are modeled using the selected photovoltaic (PV) system parameters and weather data that are typical or representative of long-term averages. For reference or comparison with local information, the solar radiation values modeled for the PV array are included in the performance results.

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 particular month or year. PV performance is largely proportional to the solar radiation received, which may vary from the long-term average by 30% monthly and 10% yearly. Solar radiation variance for a specific location can 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 model, future months and years may have actual PV performance that differs from 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, however, the values in the table are accurate to within 10%–12%.

If the default overall DC-to-AC derate factor is used, the energy values in the table will overestimate actual energy production if:

  • Nearby buildings, objects, or other PV modules and array structures shade the PV modules
  • Tracking mechanisms for one- and two-axis tracking systems do not keep the PV arrays at optimum orientation
  • Soiling or snow cover-related losses exceed 5%
  • System performance has degraded from new.

System design and operating conditions

If any of these situations exists, an overall DC-to-AC derate factor that accounts for system-specific component derate factors should be used. For more information, see How to Change Parameters in Legacy Calculators.

Module Choice

The PV system size is derived from the nameplate DC power rating. The energy production values in the table are estimated using coefficients relevant to crystalline silicon PV systems, assuming common silicon module designs. Adjusting these coefficients for specific silicon products and/or for thin-film products may result in results varying by as much as ~10%. If the user's goal is to differentiate performance of specific products, a module-specific calculation must be used.

Net metering policy and/or customer use habits

The cost savings are determined as the product of the number of kilowatt-hours and the cost of electricity per kilowatt-hour. These savings occur if the owner uses all of the electricity produced by the PV system or if the owner has a net-metering agreement with the utility. With net-metering, the utility bills the owner for the net electricity consumed. When electricity flows from the utility to the owner, the meter spins forward. When electricity flows from the PV system to the utility, the meter spins backward.

If net metering is not available and the PV system sends surplus electricity to the utility grid, the utility generally buys the electricity from the owner at a lower price than the owner pays the utility for electricity. In this case, the cost savings shown in the table should be reduced.

Besides cost savings, other benefits of PV systems include greater energy independence and a reduction in fossil fuel use and air pollution. For commercial customers, additional cost savings may come from reducing demand charges.