PVWatts and SAM paths

The PVWatts path selects the methods of PVWatts version 8. The SAM path selects the methods of SAM's detailed photovoltaic model. Both take the same request as every other run. The answer carries the full loss diagram.

The PVWatts path

The PVWatts path uses these methods.

• NREL SPA for the position of the sun
• Perez 1990 with the settings of NREL's SAM Simulation Core (SSC)
• Deline's linear self-shading for the rows
• the De Soto cover for the incidence-angle modifier
• the CEC six-parameter model of a representative module
• the NOCT cell temperature
• King's air-mass spectral modifier
• the Sandia inverter model, clipped at the AC nameplate

The steps that PVWatts does not model are switched off. These are the low-light threshold, the horizon, the night consumption and every AC component except the medium-voltage transformer and the unavailability.

The SAM path

The SAM path takes the PVWatts path's method where SAM shares it. Where SAM differs, it takes SAM's own method.

• the module's CEC parameters, or the representative module when the module has none
• the NOCT cell temperature with the module's rated NOCT
• Deline's non-linear self-shading
• the Sandia inverter model with its voltage terms
• the operating point on the module's current-voltage curve inside the MPPT window, with the AC output clipped
• the night consumption
• the rear mismatch calculated from the rear cell rows
• the transmission loss

Methods at each step

The table gives the methods for a fixed monofacial array, and the methods that change with the request.

Step	PVWatts path	SAM path
Irradiance
Solar position	NREL SPA (SSC application)	NREL SPA (SSC application)
Sun timing	Interval midpoint	Interval midpoint
Dark intervals	Zero	Zero
Transposition	Perez 1990	Perez 1990
Perez settings	NREL SSC settings	NREL SSC settings
Circumsolar treatment	As diffuse	As diffuse
Low-light threshold	No threshold	No threshold
Far shading (horizon)	Do not model	Do not model
Near shading (row to row)	Deline linear self-shading	Deline non-linear self-shading
Row count	Square field from the DC nameplate	Rows of the racking section
Sky-diffuse masking	View factor	View factor
Backtracking width	GCR	GCR
Backtracking angle	Planar backtracking, PVsyst application	Planar backtracking, PVsyst application
Irradiance-optimised tracking	Off	Off
Incidence-angle modifier, beam	De Soto cover	De Soto cover
Incidence-angle modifier, diffuse and albedo	Effective angle	Effective angle
Soiling	Monthly table	Monthly table
Rear side and front ground reflection	MonofacialWith a bifacial module: bifacialvf 1° sectors (Marion 2017)	MonofacialWith a bifacial module: bifacialvf 1° sectors (Marion 2017)
Rear-side geometry	PVWatts 8 fixed layout	Racking and bifacial sections
Energy
Cell temperature	NOCT (Duffie and Beckman)	NOCT, module's rated NOCT
Module model	CEC six-parameter, representative module	the request's choiceWith the module's CEC parameters: CEC six-parameter, this module
One-diode STC point	Maximum at the nominal power	Maximum at the nominal power
Spectral correction	Air-mass modifier (King 2004)	Air-mass modifier (King 2004)
Where the correction acts	Before the cell temperature	Before the cell temperature
Townsend snow	the request's choice	Do not model
Electrical shading effect	Linear	Linear
Rear mismatch	No rear mismatch	Deline 2020, from the rear cell rows
DC ohmic wiring	Flat fraction	Flat fraction
MPPT inputs	Strings share the inverter	Strings share the inverter
Inverter efficiency	Sandia, no voltage terms	Sandia, with voltage terms
Inverter operating limits	Clip at the AC nameplate	the request's choiceWith the module's CEC parameters: MPPT window on the module curve, AC clip
Night consumption	Off	On
AC ohmic wiring	k·P² on the inverter output	Flat fraction
AC resistive reference	Inverter output	Cascade
AC power reference	P_nom inverter	P_nom inverter
Medium-voltage transformer	Iron and copper losses	Iron and copper losses
System-level AC losses	On the sum of the inverters	On the sum of the inverters
MV line	Do not model	Do not model
HV transformer	Do not model	Do not model
HV line	Do not model	Do not model
Transmission and other losses	Do not model	Flat fractions
Auxiliaries	Do not model	Do not model
Grid power limitation	Do not model	Do not model
Unavailability	Flat fraction and periods	Flat fraction and periods
Power factor	Inside the inverter and AC rows	Inside the inverter and AC rows

Running a path over the API

POST /v1/tree/irradiance/describe and POST /v1/tree/energy/describe take a run request and return each path's changes to it as presets[].patch. Apply the irradiance patch, then the energy patch, and send the result to POST /v1/run. In the workbench, the Model tab's PVWatts path and SAM path buttons apply the same changes.

The simple run

POST /v1/run/simple takes a location, a DC size in kW, a module type, the racking, the tilt and azimuth, a DC/AC ratio, an inverter efficiency and a total loss in percent. Its defaults follow PVWatts version 8. The module runs on the PVWatts DC model and the inverter at a flat efficiency of 96 %. Every other step uses the pvperf path. The default total loss is 11.42 %. It is PVWatts's default of 14.08 % without its 3 % for shading. pvperf computes the row-to-row shading from the racking. The simple run guide gives every field, its default and the default's source.

curl -X POST https://pvperf.com/v1/run/simple \
  -H "Authorization: Bearer pvp_live_…" \
  -H "Content-Type: application/json" \
  -d '{"latitude": 39.8, "longitude": -89.85, "system_capacity_kw": 8, "racking": "single_axis"}'

Further pages

• PV simulation API
• PVsyst files and the PVsyst 8 path
• Reference, with every method of the table and its equations