Sighten‘s solar energy system production model incorporates key aspects of the industry’s most sophisticated models while maintaining a simple and clear user interface. This document summarizes Sighten’s solar production modeling capabilities, which include an internal production calculator (based on the PVSyst 5 parameter model) and an integration with the PVWatts v6 API.

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Sighten solar production model highlights

• Simple inputs: simple and non-assumptive user inputs required - module and inverter quantities/models, pitch, azimuth, soiling, and shading.

• Hourly production profile: hourly production calculations with granular weather data.

• NREL validated remote shading: Sighten's automated shading feature leverages Google's Project Sunroof to provide instant and accurate array-level shading estimates (within 2% of on-site measurements).

• Equipment-specific calculations: module and inverter-level electrical behavior modeled based on the actual equipment used.

• Production methodology: organizations can choose the production methodology used for solar systems designed by their users. A change to the selected production methodology will impact all new systems while existing systems will continue to use the production methodology selected when they were designed.

• Production override: Sighten also supports an annual production override, which scales up hourly solar production proportionate to the overridden annual production value.

• Financier derate and degradation: solar production can be modified with an annual degradation assumption set at an organization and financing product-specific level. For homeowner agreements, an annual degradation percent can also be set at a product-specific level for financier expected production and financier guaranteed production. A derate on on annual production can also be set on a financing product-specific level for financier guaranteed production.

Sighten compared to PVWatts 

Sighten offers several modeling improvements over PVWatts. As an example, Sighten's model takes into account shade recovery from optimizers and microinverters, unlike PVWatts. Key differences in methodology are listed below:

Sighten production calculation

Production output: production is simulated for each array on an hourly basis (8760 hour profile)

Data inputs: primary sources of input:

• TMY weather data (8760 hours) based on system location
• PV module electrical specifications per manufacturer spec sheet
  • This includes parameters calculated by Sighten to construct IV curve (DeSoto)
• Inverter electrical specifications per manufacturer spec sheets
• Array details (e.g. the inputs for tilt, azimuth, string size, soiling, shading, etc)

Calculation description: for each hour of a TMY dataset:

1. Determine sun position (solar azimuth and zenith) based on lat, lon, and datetime
2. Determine angle of incidence based on panel configuration (tilt, azimuth, elevation) and sun position
3. Determine absorbed direct and diffuse radiation based on TMY radiation data and angle of incidence
4. Calculate the operating temperature based on the electrical and thermal properties of the PV module and the absorbed radiation
5. Generate five parameters from operating temperature, absorbed radiation, and the properties of the PV module
   1. Modified ideality factor
   2. Light current
   3. Diode current
   4. Series resistance
   5. Shunt resistance
6. Construct I-V curve of solar cell based on IV curve parameters calculated as functions of solar cell temperature for both shaded and unshaded conditions (Desoto)
7. Determine optimal max output on the I-V curve with applied shading and DC losses
8. Calculate AC power generation (see next)

Power generation: determine the DC then the AC power generated from the array. Power generation calculations are based on the type of inverters - string inverters, micro inverters, or string inverters with optimizers

• Note on shading: the percentage shading implies the percentage of PV modules IN EACH STRING that receive "unshaded" radiation versus "shaded" radiation
• String inverters: if the array uses a string inverter:
  • The string voltage is calculated as a composite of the unshaded and shaded PV modules
  • Find array max power (DC) by calculating string voltage as a function of a universally-applied string current
  • Find module voltage on the I-V curve for unshaded module
  • Find module voltage on the I-V curve for shaded module
  • String voltage is the sum of voltages generated by unshaded and shaded modules in their respective proportion
  • Iterate until maximum of product of string voltage and array current is found
• DC to AC conversion: convert array power (DC) into array power (AC) based on inverter efficiency specifications (per spec sheet) and derate assumptions applied to all string inverters:
  • wiring_dc = 0.980
  • mismatch  = 0.980
  • diodes    = 0.995
  • soiling   = 0.950
  • wiring_ac = 0.990

• Microinverters: if the array uses microinverters
  • Find array max power (DC) by calculating unshaded and shaded module voltages as functions of a individually-managed current
    • Find module max power (DC) on the I-V curve for unshaded module
    • Find module max power (DC) on the I-V curve for shaded module
    • Convert module power (DC) into module power (AC) for unshaded and shaded modules based on inverter efficiency specifications (per spec sheet) and derate assumptions applied to all microinverters:
      • wiring_dc = 0.995
      • mismatch  = 1.000
      • diodes    = 0.995
      • soiling   = 0.950
      • wiring_ac = 0.990
    • Array power (AC) is the sum of power (AC) generated by unshaded and shaded modules in their respective proportion of the array

• Optimizers: if the array uses string inverters with optimizers
  • The calculation for optimizers is the same methodology as for microinverters (above), but uses the following derate factors:
    • wiring dc = 0.980
    • mismatch = 1.000
    • diodes = 0.995
    • soiling = 0.950
    • wiring ac = 0.990

For more information on the models and methodologies used for Sighten's calculations, please check out the PDF links below.

Sighten-PVWatts production calculation

PV Watts V6 API https://developer.nrel.gov/docs/solar/pvwatts/v6/

Retrieves monthly and hourly AC system output from an integration with PVWatts V6 API using the following inputs:

More detailed information about PVWatts is available at https://www.nrel.gov/docs/fy14osti/62641.pdf and https://pvwatts.nrel.gov/version_6.php

NREL Shading Report

Reference papers

• Simulating SolarEdge in PVWatts
• How Misuse of Solar Resource Datasets is Reducing Solar Industry Profits
• NREL Study - Power Level Electronics
• SolarEdge Methodology (Optimizers)

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