Contents

Solar Performance Insight provides both a Dashboard and an API.

Dashboard

The dashboard provides access to Solar Performance Insight capabilities through a browser. After logging in, the Dashboard guides you through the following steps:

  1. Define or select the system
  2. Choose a workflow:

After the workflow completes, you can plot and download results.

Systems

A PV system is defined by four sets of data:

  1. The performance model defines the sequence of models to be used to calculate output of the system. Options are pvsyst-like, pvwatts and SAM.
  2. System metadata, including:
    • name
    • latitude and longitude. Use decimal degress (e.g., 35.5) without a direction character (e.g. N, E) and use negative longitude for locations west of the prime meridian.
    • elevation above mean sea level in meters
  3. Inverters. A system can have one or more inverters. The options for inverter parameters depend on the performance model.
  4. Arrays. An array is a collection of modules with the same fixed or tracking orientation. One or more arrays can be associated with each inverter. Array parameters depend on the performance model.

System performance models

Solar Performance Insight uses functions from pvlib to calculate system output consistent with the selected performance model: pvsyst-like, pvwatts and SAM.

Pvsyst-like model

Calculate system output consistent with the Pvsyst software package. The ‘pvsyst-like’ model includes only those steps in a Pvsyst which are fully and publicly documented. Some differences between Solar Performance Insight and Pvsyst output are to be expected. Users must provide parameters for the module model. Because the Pvsyst inverter model is not fully and publicly documented, Solar Performance Insight substitutes the Sandia Inverter Model that is used in SAM, giving users the option to select inverter parameters from a database.

PVWatts

Calculate system output consistent with the PVWatts application. With the exception of the module temperature model, the ‘pvwatts’ model calculates system output using the same methods as does the PVWatts website. For the module temperature model, Solar Performance Insight uses the Sandia temperature model in place of a more complex calculation used by PVWatts.

SAM

Calculate system output consistent with the PV performance model in the System Advisor Model (SAM). The ‘SAM’ model calculates system output using many, but not all, of the steps implemented in the SAM application. Users have access to databases of module and inverter model parameters that support the models implemented in SAM.

Inverters

Each inverter is assigned a unique name and (optionally) a make and model.

Inverter Parameters are the values needed for the function that calculates output AC power from input DC power and DC voltage. For the ‘pvsyst-like’ and ‘SAM’ performance models, the Sandia Inverter Model is used and users have the option to select inverter parameters from a database. For the ‘pvwatts’ performance model, Solar Performance Insight offers the PVWatts inverter model parameters as default values.

For the ‘pvwatts’ performance model only, a set of Loss Parameters may be entered. Each parameter is a percentage of AC power lost due to a category of effects (e.g., soiling, snow, mismatch). Solar Performance Insight offers the PVWatts loss factors as default values.

Arrays

Each array is assigned a unique name.

Arrays can be Fixed, at constant Tilt and Azimuth, or on Single Axis trackers. Single axis trackers are defined by the tilt and azimuth of the tracker axis. All modules in an array are at the same orientation.

The modules in an array are considered to be arranged in series-connected strings, each string containing the same number of modules. The albedo of the ground surface can be selected from a list of representative values or manually entered.

Module parameters are required for the model that calculates module output. For the SAM performance model only, a database of module parameters is available.

Temperature model parameters are required for the model that calculates module temperature from weather data. The temperature model is specific to each performance model:

  • for ‘pvsyst-like’, the temperature model is the same as used in Pvsyst.
  • for ‘pvwatts’, the temperature model is the Sandia model, rather than the more complex calculation used in PVWatts.
  • for ‘SAM’, the temperature model is the default temperature model in SAM.

Calculate performance

This workflow runs the performance model for the system using the weather you upload. Weather includes irradiance and temperature data, and can be uploaded once for the full system, by inverter, or by array.

Upload Weather Data

Irradiance data must contain one of the following combinations:

  • global horizontal (GHI), direct normal (DNI) and diffuse horizontal (DHI) irradiance. If your data includes only GHI and DHI, you can estimate DNI using a model; see pvlib’s dni function for example.
  • global plane-of-array (POA), direct POA, and diffuse POA irradiance. If your data includes only global POA, consider using the option below instead.
  • global POA (not adjusted for reflections or spectral content) or effective irradiance (adjusted for reflections and spectral content). Typically, global POA is measured with a pyranometer mounted in the array plane, and effective irradiance is measured with a reference cell.

Cell temperature is the temperature of the cells inside the array’s modules. This quantity is rarely measured, but if it has been determined, you can upload it with your weather data.

Module temperature is the temperature on the outside module surface, and is often measured with an attached temperature sensor.

If your data doesn’t include module temperature, you can upload air temperature (and optionally, wind speed) and Solar Performance Insight will calculate cell temperature using a model.

Weather data must include a datetime index with a fixed time spacing in whole minutes. It is best if the datetime index is localized to the timezone. If the uploaded datetime index is not localized, Solar Performance Insight assumes that times are local to the selected timezone, and localizes the index for you.

After you choose the file containing weather data for upload, Solar Performance Insight guides you through matching the columns in the data to the weather variables expected for the calculation.

When matching is complete, the Upload Data button completes the data upload and queues the model calculation.

Calculation Results

Results are summarized by month. Summary results include:

  • Total Energy is total electrical energy.
  • Plane of Array Insolation is total broadband solar insolation summed over all arrays.
  • Effective Insolation is total solar insolation, adjusted for reflections and spectral content, summed over all arrays.
  • Average Daytime Cell Temperature is averaged over all arrays and times when the sun is above the earth’s horizon.

Detailed output can be downloaded in CSV or Arrow formats.

Solar Performance Insight can make plots of time series of uploaded data and/or model results - click the New Plot button.

Compare Performance

This workflow compares actual power (which you upload) to reference or modeled power. Reference power is a record of modeled output from the system, usually done as part of the system design. Modeled power is calculated by Solar Performance Insight from the weather data which you upload; see the Calculate Performance workflow for details.

Compare Actual to Reference Performance

The summary results for the Compare workflow are the ratio and difference between monthly actual energy and monthly reference energy that is adjusted for the differences between actual and reference weather. The comparison can be done with data at hourly or better resolution, or using monthly totals.

Comparing actual and reference with hourly data

The adjustment of reference power accounts for the differences in irradiance and temperature at each timestemp.

Case 1. When reference data includes DC power and weather, the adjusted reference AC power is calculated as

PAC, adj = min {PAC0, 0.985 PDC, adj}

PDC, adj = PDC, ref (POAactual / POAref) Ftem

Ftem = (1 - γ (Tcell, actual - 25)) / (1 - γ (Tcell, ref - 25))

PAC0 is the total AC capacity of the PV and γ is the temperature coefficient of power provided in the PV system metadata. The factor of 0.985 accounts for the DC to AC conversion efficiency.

Case 2. When reference data includes AC power and weather (DC power is not available), the adjusted reference AC power is calculated as

PAC, adj = min {PAC0, PAC, ref (POAactual / POAref) Ftem}

Ftem = (1 - γ (Tcell, actual - 25)) / (1 - γ (Tcell, ref - 25))

Case 3. When reference data includes only weather, Solar Performance Insight first runs the PV system performance model (provided with the system metadata) to estimate PDC, ref. Then, adjusted reference AC power is calculated as described above for Case 1.

Comparing actual and reference with monthly data

When reference data are at monthly resolution, the weather data must include plane-of-array (POA) insolation POAref, average daytime cell or module temperature Tavg, ref, and total AC energy EAC, ref. The adjusted reference AC energy for each month is calculated by

EAC, adj = EAC, ref (POAactual / POAref) - Ltem}

Ltem = (PAC0 / 1000) POAact γ (Tavg, actual - Tavg, ref)

The equation above derives from a simple energy performance model:

E = (PAC0 / 1000) POA (1 - γ (Tavg - 25))

Upload Actual and Reference Data

Actual data includes AC power and actual weather data for the entire system or per inverter. Options for reference data include:

  • weather data only, in which case Solar Performance Insight runs the performance model to calculate reference power.
  • weather and AC power data. In this case, the reference and actual weather data are used to adjust the reference AC power to the actual weather conditions.
  • weather, AC and DC power data. In this case, the reference and actual weather data and the reference DC power are used to adjust the reference AC power to the actual weather conditions.

See Upload Weather Data for data upload details.

API

You can also access the Solar Performance Insight capabilities through its RESTful API. Example python scripts are found here. For example, you can post json files containing system descriptions, define and post files containing weather data, request job execution and get results. Documentation for the API is located here.