SolarFarmer Models

Modelling methodology, assumptions & key parameters explained

SolarFarmer Calculation Methodology

SolarFarmer’s simulation engine is built on a modular and physically-based modelling architecture that supports accurate energy yield prediction across a wide range of PV system designs. From raw solar irradiance inputs to AC power output and system-level loss assessments, each modelling stage reflects industry-validated physics and electrical engineering principles. This document provides a transparent, engineering-focused overview of how calculations are performed within SolarFarmer.

SolarFarmer is also frequently evaluated as a PVSyst alternative by engineering teams looking for high-fidelity modelling of complex terrain, bifacial systems, and multi-level AC architectures.

Irradiance Calculation

SolarFarmer calculates irradiance on the plane of array (POA) through a combination of geometric and atmospheric models, enabling accurate modelling of complex terrain and shading effects.

Key Modelling Steps:

Solar Position: Computed using the Solar Position Algorithm (SPA), as defined by Reda and Andreas (NREL, 2008).
Sky Model: Employs the Perez anisotropic model (1990) to resolve diffuse horizontal irradiance into circumsolar, horizon, and isotropic components.
Plane-of-Array Irradiance (POA): Computed as the sum of three components: direct irradiance projected onto the array tilt angle, diffuse irradiance adjusted using the Perez model, and ground-reflected irradiance based on surface albedo and array geometry.
Terrain Shading: High-resolution 3D ray tracing evaluates shading events, using configurable angular resolution and obstruction thresholds.
Inter-row Shading: Evaluated per time step using polygon intersection methods or horizon-based approaches for row-to-row analysis.

Ground Albedo and Bifacial Irradiance:

Parameter	Description
Ground Albedo	Configurable as spatially uniform or variable grid input
Rear Irradiance	Calculated using view factors and geometry between module rear and ground
Bifacial Gain	Weighted by bifaciality factor and irradiance on back-side plane

More detail: Irradiance Calculation

PV Conversion Modelling

SolarFarmer models the DC-side energy conversion using either detailed physical models or empirical performance tables depending on the module data available.

Core Inputs and Outputs:

Parameter	Unit	Description
POA Irradiance	W/m²	From irradiance model
apply_tracker_inactive	°C	Estimated using Sandia or IEC models based on wind speed and ambient temp
apply_tracker_inactive	-	Single-diode model or user-imported IV curves
DC Power Output	W	Post-degradation, post-shading electrical output

Shading and Mismatch Modelling:

Diode-level response: For detailed shading resolution on complex strings
Approximate method: For performance scaling in large-scale simulations

Degradation Modelling:

Supports linear degradation (e.g. 0.5%/year), stepwise adjustments, and custom user curves.

More detail: PV Conversion Modelling

Inverter Modelling

SolarFarmer applies inverter manufacturer data to simulate real-world performance with detailed electrical fidelity.

Electrical Modelling Table:

Parameter	Type	Source / Behaviour
Inverter Efficiency	Lookup Table	Based on input voltage and % loading (per datasheet)
MPPT Range	Fixed / Dynamic	Defined from datasheet, allows clipping losses outside tracking window
Reactive Power Capability	Curve or Limits	Optional inclusion for grid code compliance simulations
Self-Consumption	Fixed Value	Standby and night-time losses in W or %
Power Limiting Behaviour	Rule-Based	Includes user-defined temperature derating curves

More detail: Inverter Modelling

AC Collection System and Losses

The AC electrical model includes configurable topology and impedance detail from inverter terminals through to grid interconnection.

Electrical Network Representation:

Hierarchical collection tree (e.g., string → inverter → transformer → main panel)
Cable lengths and conductor sizes user-defined or auto-generated
Multi-voltage support (e.g., 400V, 22kV)

Loss Modelling Table:

Component	Modelled Losses	Description
Cables	Resistive (I²R)	Voltage drop and thermal loss
Transformers	No-load and load losses	Based on manufacturer data or IEEE formulas
Switchgear / Panels	Fixed percentage or absolute losses	Optional for balance-of-system accounting
Availability	Time-based (%) or schedule-based	Allows modelling of operational constraints or curtailment windows

More detail: AC Collection System

Summary Results and Interpretation

Aggregation and Temporal Resolution:

SolarFarmer supports flexible aggregation across components and time domains:

Aggregation Level	Time Granularity	Output Examples
Module	5-min, 1-hr	Temperature, current, voltage
String	1-hr	DC power, mismatch loss
Inverter	1-hr, 1-day	AC output, clipping loss
Site / Portfolio	Daily, Monthly	Energy yield, Performance Ratio

Performance Ratio (PR) Calculation:

PR is calculated both monthly and annually, normalized to irradiance on the POA:

Reference Irradiance: Calculated POA with soiling and shading included
Installed Power: Sum of nameplate capacities (DC)
AC Output: Net of inverter and AC losses

More detail: Performance Ratio

Output Files and Data Access

Time Series Output

Each simulation produces CSV files containing the following fields:

Parameter	Units	Description
Timestamp	UTC	Time of record, in ISO format
POA_Irradiance	W/m²	Plane-of-array irradiance
T_Module	°C	Module temperature
DC_Power_Module	W	Output of individual module
DC_Power_String	W	Summed DC output per string
AC_Power_Inverter	W	Net inverter output
Loss_Shading	%	Relative loss due to shading
Loss_Mismatch	%	Relative mismatch loss across modules/strings

More detail: Time Series Output

Cloud Platform Outputs

Simulations run on the SolarFarmer cloud platform export a structured results package:

File Name	Format	Content Summary
results_timeseries.csv	CSV	High-resolution energy and irradiance data
summary.json	JSON	Metadata and simulation configuration overview
diagnostics.log	Plain text	Simulation success and warning messages

More detail: Cloud Results

Theoretical Foundations and References

The SolarFarmer calculation engine incorporates best-practice models and methods used in the PV industry, including:

For extended mathematical formulations, boundary conditions, and validation references, see:

Try the Solcast API

Solcast takes on the many challenges of producing live and forecast solar data, so that you don’t have to. That means making the data easy to access, validate and integrate. We provide instant access to live and forecast data products via this web interface, which is free to try. These include direct estimates of global, direct and diffuse solar radiation, as well as PV power output.