Energy Production Calculators in OpenSolar

OpenSolar provides two methods for calculating system energy output:

1. PVWatts (Legacy – Default)

2. System Advisor Model (SAM) with OpenSolar 3D

PVWatts has been the default calculator since OpenSolar’s launch in 2019.
SAM was introduced in July 2020 alongside OpenSolar 3D to provide enhanced modelling capabilities, particularly around shading and component-level losses.

Switching Between Calculators

You can select your preferred energy production calculator at either:

• The Control (Template) Level, or

• The Project (Design) Level

Switching at the Control Level (Template Default)

To change the default calculator for your organization:

1. Navigate to Control

2. Go to Design & Hardware

3. Select Setbacks & Design Settings

4. Edit your design template

5. Expand Simulation Settings

6. Change the Energy Production Calculator to your desired option

This will apply the selected calculator as the default for all projects using that template.

Switching at the Project (Design) Level

To change the calculator for a specific project:

1. Navigate to Design

2. Open the Summary tab

3. Expand Advanced Settings

4. Use the Energy Production Calculator dropdown to select your preferred option

This allows flexibility to test or compare modelling approaches on a per-project basis.

PVWatts (Legacy Default)

OpenSolar currently uses PVWatts Version 6, with hourly resolution.

PVWatts calculates system generation output using established modelling assumptions and typical meteorological year (TMY) weather data.

For detailed methodology documentation from NREL, refer to their official PVWatts documentation here.

Weather Data

PVWatts uses hourly TMY data (8,760 data points annually), including:

• Direct Normal Irradiance (DNI) [W/m²]

• Diffuse Horizontal Irradiance (DHI) [W/m²]

• Ambient Dry Bulb Temperature [°C]

• Wind Speed at 10m height [m/s]

Albedo assumptions:

• TMY2 files use a fixed albedo of 0.2 for all hours

• TMY3 files use hourly albedo values provided in the dataset

No adjustments are made for:

• Leap years

• Daylight saving time

Sun Position Calculation

PVWatts calculates solar position hourly, including:

• Solar azimuth angle

• Solar zenith angle

These values are used to determine how much irradiance falls onto the PV array.

Plane-of-Array (POA) Irradiance

POA irradiance is calculated hourly and accounts for:

• Solar azimuth and altitude

• Module tilt

• Module azimuth

This ensures accurate modelling of incident radiation on the PV module surface throughout the year.

System Losses and Derating

PVWatts applies default system derating assumptions. These are fixed loss values based on system configuration.

Important notes:

• Microinverter systems

  • Module mismatch loss reduces to 0%

  • Total system losses reduce to approximately 12.32%

• DC Optimiser systems

  • Module mismatch loss reduces to 0%

  • DC optimiser efficiency can be entered manually

  • See the Inverters and Stringing section in Studio for setup instructions

• Inverter efficiency losses
  Based on CEC/Euro efficiency ratings for each inverter model

• Shading losses
  Must be entered manually in Studio (see related article)

New: OpenSolar 3D + System Advisor Model (SAM)

OpenSolar 3D + System Advisor Model (SAM)

OpenSolar integrated the System Advisor Model (SAM) for energy production modelling.

SAM is developed by the National Renewable Energy Laboratory (NREL) and is a comprehensive techno-economic model used widely in the renewable energy industry.

For detailed methodology, refer to official SAM documentation here.

Key Inputs Used in SAM on OpenSolar

SAM modelling includes:

• Irradiance and weather data

• Plane-of-Array (incident) irradiance

• Effective POA irradiance

• DC array output modelling

• System AC output modelling

Like PVWatts, SAM uses TMY weather data to calculate beam and diffuse irradiance throughout the year.

However, SAM’s major advantage in OpenSolar is its integration with OpenSolar 3D shading analysis.

Shading & Sun Access with OpenSolar 3D

OpenSolar 3D allows you to create detailed 3D site environments to model shading dynamically.

Using OpenSolar 3D:

• Total Sun Access* is calculated for each array

• Shading is applied hourly across the year

• SAM uses these shading values to calculate system output more accurately

Sun Access* is defined as:

The ratio of available solar irradiance after shading compared to solar irradiance before shading.

Loss Modelling in SAM

Losses can be viewed in:

Design → Summary → System Losses

SAM accounts for component-specific and configuration-specific losses.
 

Note: 

• If a string inverter is used with no optimisers then the losses will be: 
  • Module mismatch = 2%
  • Diodes and connections = 0.5%
  • DC wiring = 2%
• If an optimiser is used with a string inverter then the module mismatch will go to 0%, the DC wiring will go to 1% and the optimiser's efficiency will be used
• If a micro-inverter is used the module mismatch will go to 0% and the DC wiring will go to 0.1%. 
• For shaded string inverter (no optimiser) systems, SAM will incorporate a validated data set to look up what the output will be based on the shading values calculated from OpenSolar 3D. 
• For shaded micro and optimiser systems, the shading values will have a direct relation with the output. 

Side-by-Side Comparison

*Sun Access: A ratio of the available solar irradiance after shading compared to the solar irradiance before shading.