Time Definition in PVsyst

Basic time, DST

In PVsyst, the basic reference time is the Legal time.

For any geographic location, the legal time is defined with respect to the GMT (Greenwich Mean Time), by the Time zone.

PVsyst doesn't make use of the DST (Daily Saving Time). The reference time is always the basic time (usually winter time) all over the year. If weather data or other hourly data involving DST have to be imported in PVsyst, the time reference should be corrected accordingly for Summer data at the import.

Time stamp

The denomination of a given time interval in PVsyst is always defined as the beginning of this interval. This is valid for hourly, daily or monthly values.

For weather data, the time stamp is the time interval over which the irradiance measurement (or any other value) is averaged. For example, the time stamp 11:00 corresponds to measurements averaged between 11:00 and 12:00.

Solar geometry

When using irradiance models, the solar geometry should be calculated for an "average" of the time interval. PVsyst uses the middle of this time interval (in the previous example 11:30).

However in the morning the time interval is not complete. The solar geometry will uses the middle between the sunrise hour, and the end of the interval. Idem for the evening.

Time shift

The time stamp is not defined in this way for all external weather data. Some of them use the end of the interval, some are measurement between 11:30 and 12:30, or any shift within the hour.

Therefore when importing weather data, PVsyst has to adapt its internal time interpretation, in order to always calculate the Solar Geometry in the middle of this interval.

This is done by defining a variable named "Time Shift", which should be manually adjusted for all imported hourly data files. This "time shift" should be defined already at the importing time, as the importing process may use irradiance models (retro-transposition, diffuse calculation) which use the Solar Geometry.

PVsyst provides several tools for identifying and correcting the time shift. See details here and here.

Generic year 1990

All meteorological data which are not related to real meteorological measurements of a given year are considered as a generic year.

This is the case for monthly data in the database, which sometimes proceed from averages over several years (for example Meteonorm).

This also holds for any synthetic hourly data or Typical Meteorological Years (TMY) or Design Reference Years (DRY).

By convention in PVsyst, the generic years are labeled 1990.

This information of a "generic year" is used in some outputs of PVsyst, for suppressing the year reference.

NB: you cannot modify the reference label 1990 into another year, this would physically not make sense.