How is Output Calculated in OpenSolar?

You have two options to calculate system output in OpenSolar

The first option is PVWatts. This is the default calculator, and has been used since OpenSolar's inception in 2019. 

The second option is the System Advisor Model (SAM). This is a new calculator that OpenSolar has introduced in July 2020.

Please read below to learn more about both of these calculators and see a summary here.

If you would like to learn how to switch your calculator between PVWatts and SAM, please see this article.

Legacy: PV Watts

Currently, system generation output is calculated using PVWatts Version 6, with hourly resolution.

A brief summary of some of the inputs we use are shown here but for more information on PVWatts methodology, see here.

Weather Data

• PVWatts utilises hourly typical meteorological year (TMY) data (8760 data points) of:
  • Direct normal irradiance (DNI) and diffuse horizontal irradiance (DHI) [W/m2]
  • Ambient dry bulb temperature [˙C]
  • Wind speed at 10m above the ground [m/s]
  • Albedo = 0.2 for all hours of the year for TMY2 data files and uses hourly value provided in TMY3 files
• No adjustments are made for leap years or daylight saving times.

Sun Position

• PVWatts calculates the sun position every hour, finding the azimuth and solar zenith angles which are then later used to determine the insolation falling on the PV array.

Plane-of-ARRAY (POA) Irradiance

• The POA irradiance is calculated hourly throughout the year, taking in the effects of the sun’s azimuth and altitude angle, as well as the module tilt and azimuth to accurately calculate the insolation on the PV module throughout each day.

System Losses and Derating

A summary of the default PVWatts derating can be seen in the following table:

 Note:

• If a microinverter is used then the mismatch loss given above will be reduced to 0%, giving a total of 12.32%. 
• If a DC optimiser is used, then the mismatch loss will go to 0% and you will be able to enter in the DC optimiser efficiency. You can find out how to add DC optimisers in Studio here at the bottom of the Inverters and Stringing section. 
• Losses due to inverter efficiency are based on the CEC/Euro efficiency ratings of each inverter model.
• Shading losses can also be configured manually within Studio (see how in this article)

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

With the release of OpenSolar 3D on OpenSolar, we have integrated SAM for system output calculations. SAM, created by the National Renewable Energy Laboratory (NREL), is a techno-economic software model that is designed specifically for the renewable energy industry.

Below are some key inputs we use on OpenSolar, but if you'd like to learn more about SAM methodology, please see here.

• PV system performance
• Irradiance and weather data
• POA (incident) irradiance
• Effective POA irradiance
• Model DC Output
• Array DC output
• System AC Output

Similarly to PVWatts, SAM takes into account the data from the weather file to calculate the irradiance levels (both beam and diffuse) throughout the year. However, a key difference is that you can now use the new OpenSolar 3D tool alongside this data to accurately model the shading of certain systems throughout the year in order to create systems which will perform more optimally. 

OpenSolar 3D allows you to create 3D environments used for calculating total Sun Access* which then uses SAM to calculate the system output, taking into account the type of inverter used and whether optimisers are used or not. See notes below losses for more details. 

When designing with SAM, the following losses are accounted for (these can be seen in the design zone, by clicking on the summary tab and then system losses. (Note the figures below are taken as an example from a dummy project.) 

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. 

Summary

If you would like to learn how to switch your calculator between PVWatts and SAM, please see this article.

Note: You can manually scale the output of your systems. Find out how to do so here. 

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