Conventions

Origin of Azimuth

Solar azimuth is an important element in solar positioning. PVGIS follows the
North-Up = 0° convention for azimuth angular measurements.

In overview :

- North-Based System (N=0): Default in PVGIS.
- East-Based System (E=0): Common in some literature and solar
  positioning models.
- South-Based System (S=0): Alternative system in specific
  applications.

Below is a visual representation of various azimuth angle conventions and
conversions, i.e. from a North-based system to either East- or South-based
systems :

             ┌─────────────┐  ┌────────────┐  ┌────────────┐
             │     N=0     │  │     N      │  │      N     │
             │      ▲      │  │     ▲      │  │      ▲     │
     Origin  │   W ◄┼► E   │  │  W ◄┼► E=0 │  │   W ◄┼► E  │
             │      ▼      │  │     ▼      │  │      ▼     │
             │      S      │  │     S      │  │     S=0    │
             └─────────────┘  └────────────┘  └────────────┘
             ┌─────────────┐  ┌────────────┐  ┌────────────┐
             │             │  │            │  │            │
             │             │  │            │  │            │
Input South  │     180     │  │     90     │  │     0      │
    (IS)     │             │  │            │  │            │
             │             │  │            │  │            │
             └─────────────┘  └────────────┘  └────────────┘
             ┌─────────────┐  ┌────────────┐  ┌────────────┐
             │             │  │            │  │            │
   Internal  │             │  │            │  │            │
             │      =      │  │  IS - 90   │  │  IS - 180  │
  Conversion │             │  │            │  │            │
             │             │  │            │  │            │
             └─────────────┘  └────────────┘  └────────────┘

Units and Conversions Draft Text

Most angular calculations in PVGIS are performed in
radians for consistency and precision. However, there is
an important exception : the atmospheric refraction adjustments
use degrees.

The relevant API function in PVGIS is
calculate_refracted_solar_altitude_series() which requires input in degrees and
outputs adjusted values in degrees.

The refraction correction formula ensures accurate modeling of the sun's
observed position, especially near the horizon, where the refraction effect is
strongest.


UTC & Local Time Zones Draft Text

By default, all calculations in PVGIS are performed in UTC.
This ensures consistency across different geographic regions. Nonetheless,
local time support is under review. The goal is to:
    - translate local timestamps to UTC
    - perform calculations
    - and convert them back to local time for final output

Date & Time

PVGIS respects input data timestamps and will then
filter the data based on user-defined parameters such as
`start_time`, `end_time`, and `periods`.

If no data is provided, PVGIS generates a DatetimeIndex using the
Pandas library and based on at least two or at most three out of the following
parameters :

- `start_time`
- `end_time`
- `frequency` : the default frequency is set to hourly
  (h)
- or/and `periods`

┌─────────────┐  ┌────────────┐  ┌────────────┐
│  Input      │  │  Generated │  │  Filtered  │
│ Timestamps  │  │ Timestamps │  │ Timestamps │
└─────────────┘  └────────────┘  └────────────┘

┌─────────────┐  ┌────────────┐
│ Start Time  │  │  End Time  │
├─────────────┴──┴────────────┤
│     Frequency or Periods    │
└─────────────────────────────┘


Assumptions for Input Data Draft Text

Solar irradiance input data must have a consistent temporal resolution
and extent. PVGIS assumes all provided data adhere to these requirements
for accurate modeling.


Solar Incidence Angle Draft Text

The default definition of the solar incidence angle in PVGIS
follows the "sun-vector to surface-normal" convention, specifically as modelled
by Iqbal.

Positioning Algorithm

PVGIS uses the NOAA solar geometry equations for solar
positioning. The system has undergone extensive testing, including hundreds of
thousands of automated tests to verify its accuracy. Comparisons with data from
the United States Naval Observatory (USNO) further validate the
correctness of the calculated solar angles.


Photovoltaic Efficiency and Spectral Data Draft Text

PVGIS integrates photovoltaic efficiency coefficients derived from
research in the ESTI Lab, part of Unit C2 (JRC, European
Commission) alongside the PVGIS team. These coefficients are critical for
modeling the performance of PV modules under various conditions.

Concepts

- Spectral Factor : The ratio between the nominal reference
  spectrum (used for rating PV panels) and the real solar irradiance spectrum
  at the Earth's surface.

- Efficiency Curves : Based on module types and are used to adjust
  performance predictions across different environmental conditions.