Overview and comparison with PV Watts

Highlights

Simple and non-assumptive user inputs required: module and inverter quantities/models, pitch, azimuth, and shading
Hourly production calculations with granular weather data
Module and inverter-level electrical behavior modeled

Sighten‘s production model incorporates key aspects of the industry’s most sophisticated models while maintaining simplicity of user inputs.

Overview

User inputs	Irradiation	Module output	Array output
Location Equipment Pitch, azimuth, solar access	Weather data Sun position Solar access	Absorption Cell temps Module current- voltage curves	Aggregate modules Inverter clipping & efficiency

Sighten vs PVWatts

Component	Sighten	PVWatts
Weather data	NREL TMY Stations	NREL TMY Stations
Shading	SAM2 shading model	User input
Module production	DeSoto 5-parameter	User derate input
Inverter clipping	Sandia MPPT Tracking	None
Inverter efficiency	SAM CEC Efficiency	User input

Full description of Sighten's production calculation

production is simulated for each array on an hourly basis
primary sources of input:
- TMY weather data (8760 hours) based on system location
- PV module electrical specifications per manufacturer spec sheet
  - includes parameters we calculate to construct IV curve (DeSoto)
- inverter electrical specifications per manufacturer spec sheets
- array details (e.g. tilt, azimuth, string size, shading, etc)
for each hour of a TMY dataset:
- determine the solar radiation incident on the tilted surface
  - "unshaded" radiation includes beam radiation component
  - "shaded" radiation assumes no beam radiation
- determine solar cell temperature based on PV model specifications in both shaded and unshaded conditions
- 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)
- determine the AC power generated from the array:
  - NOTE: the percentage shading implies the percentage of PV modules IN EACH STRING that receive "unshaded" radiation versus "shaded" radiation
  - 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
  - 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
- 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

How is production calculated in Sighten?

Highlights

Overview

Sighten vs PVWatts

Full description of Sighten's production calculation