Sighten‘s solar energy system production model incorporates key aspects of the industry’s most sophisticated models while maintaining simplicity of user inputs. This document summarizes Sighten’s solar production modeling capabilities, which includes an internal production calculator (based on the PVsyst 5 parameter model) and an integration with the PVWatts v6 API.
Sighten 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
Equipment-specific calculations: module and inverter-level electrical behavior modeled based on the equipment used
Production methodology: organizations can choose their production methodology in Sighten’s tool at an organization-specific level. A change to the selected production methodology will impact all new systems. Existing systems will continue to use the production methodology selected when they were designed.
Production override: using either methodology, the tool also supports an annual production override. This scales the hourly production based on the overridden amount.
Financier derate and degradation: the system production can be modified with an annual degradation assumption 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 the yearly production can also be set on a product-specific level for financier guaranteed production.
Sighten vs PVWatts
Sighten offers several modeling improvements over PVWatts. For example, Sighten's model takes into account shade recovery from optimizers and microinverters, unlike PVWatts. Differences in methodology are listed below.
| Component | Sighten | PVWatts |
|---|---|---|
Weather data | NREL TMY Stations | NREL TMY Stations |
Shading | SAM2 shading model | User input |
Module production | DeSoto 5-parameter | User input |
Inverter efficiency | SAM CEC Efficiency | User input |
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 user inputs or defaults for tilt, azimuth, string size, soiling, shading, etc)
Field | Input | Source | Description |
|---|---|---|---|
latitude | x | Google Geocode API (or override) | Address Latitude |
longitude | x | Google Geocode API (or override) | Address Longitude |
elevation | x | Google Geocode API | Address Elevation |
GHI | TMY3 Hourly Data | Hourly Global Horizontal Irradiance | |
DHI | TMY3 Hourly Data | Hourly Diffuse Horizontal Irradiance | |
DNI | TMY3 Hourly Data | Hourly Direct Normal Irradiance | |
wind_speed | TMY3 Hourly Data | Hourly Wind Speed | |
t_ambient | TMY3 Hourly Data | Hourly Ambient Temperature | |
tilt | x | Google Sunroof or system design | Tilt of array (degrees) |
azimuth | x | Google Sunroof or system design | Azimuth of array (degrees) |
n_series | x | System Design | # of modules in series |
n_parallel | x | System Design | # of series in parallel |
solar_access_mo1-12 | x | Google Sunroof or system design | Monthly Solar Access |
cell_count | Sighten value for selected module | # of cells in the module | |
area | Sighten value for selected module | # surface area of module | |
t_noct | Sighten value for selected module | Nominal operation cell temperature | |
alpha_isc_pct | Sighten value for selected module | Temperature coefficient at short circuit current (%/C) | |
beta_voc_pct | Sighten value for selected module | Temperature coefficient at open circuit voltage (%/C) | |
bandgap | Sighten value for selected module | Band gap of the module | |
i_sc_stc | Sighten value for selected module | Short-circuit current at the SRC (A) | |
v_oc_stc | Sighten value for selected module | Open-circuit voltage at the SRC (V) | |
i_mp_stc | Sighten value for selected module | Current at maximum power at the SRC (A) | |
v_mp_stc | Sighten value for selected module | Voltage at maximum power at the SRC (V) | |
a_stc | Sighten value for selected module | Modified ideality factor at the SRC | |
i_l_stc | Sighten value for selected module | Light current at the SRC (A) | |
i_o_stc | Sighten value for selected module | Reverse saturation current at the SRC (A) | |
r_sh_stc | Sighten value for selected module | Shunt resistance at the SRC. | |
r_s_stc | Sighten value for selected module | Series resistance at the SRC | |
microinverter | Sighten value for selected inverter | Whether inverter is a microinverter | |
uses_optimizers | Sighten value for selected inverter | Whether inverter uses optimizers | |
v_dc_max | Sighten value for selected inverter | Absolute max voltage (V) | |
v_dc_mppt_lower | Sighten value for selected inverter | Lower bound of MPPT range (V) | |
v_dc_mppt_upper | Sighten value for selected inverter | Upper bound of MPPT range (V) | |
p_ac_nominal | Sighten value for selected inverter | Nominal output power (W) | |
efficiency_cec | Sighten value for selected inverter | CEC efficiency of the inverter | |
draw_night | Sighten value for selected inverter | Inverter power draw at night |
Calculation description: for each hour of a TMY dataset:
- Determine sun position (solar azimuth and zenith) based on lat, lon, and datetime
- Determine angle of incidence based on panel configuration (tilt, azimuth, elevation) and sun position
- Determine absorbed direct and diffuse radiation based on TMY radiation data and angle of incidence
- Calculate the operating temperature based on the electrical and thermal properties of the PV module and the absorbed radiation
- Generate five parameters from operating temperature, absorbed radiation, and the properties of the PV module
- Modified ideality factor
- Light current
- Diode current
- Series resistance
- Shunt resistance
- 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 optimal max output on the I-V curve with applied shading and DC losses
- 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
- Find array max power (DC) by calculating unshaded and shaded module voltages as functions of a individually-managed current
- 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
- The calculation for optimizers is the same methodology as for microinverters (above), but uses the following derate factors:
For more information on the models and methodologies used for Sighten's calculations, please check out the PDF links below.
All SolarEdge string inverters are considered to be equipped with optimizers in Sighten.
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:
Field | Input | Source | Description |
|---|---|---|---|
Capcity | x | System design | Module nameplate * number of modules |
Latitude | x | Google Geocode API (or override) | Address Latitude |
Longitude | x | Google Geocode API (or override) | Address Longitude |
Tilt | x | System design | Array Tilt |
Azimuth | x | System design | Array Azimuth |
Losses (Shading) | x | Google Sunroof or system design (or 5% default) | Shading percent based on solar access |
Losses (Soiling) | x | Optional input in system design (2% default) | Input or default soiling |
Loses (Other) | 2% Mismatch 2% Wiring 0.5% Connections 1.5% Light-induced Degradation 1% Nameplate Rating 3% Availability | All losses are applied multiplicatively | |
Array Type | Fixed - Roof Mounted | Constant (other options are Fixed - Open Rack, 1-Axis, 1-Axis Backtracking, 2-Axis) | |
Module Type | Sighten value for selected module | Thin film if cell type is thin film, Premium based on module efficiency and gamma pmp, Standard otherwise | |
Inverter Efficiency | Sighten value for selected inverter | Based on selected inverter’s CEC Efficiency | |
Climate Dataset | NSRDB Climate Data | Nearest NSRDB station to input address lat/lon |
More detailed information about PVWatts is available at https://www.nrel.gov/docs/fy14osti/62641.pdf and https://pvwatts.nrel.gov/version_6.php
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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