2.1: Measurement of the solar constant
The flux of solar radiation energy that arrives at the outermost layers of the atmosphere is called the total solar irradiance (TSI).
It varies slightly in an annual cycle (by about 3%) because the earth revolves around the sun on an elliptic orbit.
The figure below shows the daily means measured by satellites. These values are normalised to the sun’s distance of 1 astronomical unit (AU),
which is close to the mean distance between Earth and sun. Despite this normalisation, the data show very slight variations (by about 0.1%)
due to the 11-year solar cycle.
The long-term average of the TSI is called the solar constant S. Its reference-value is 1366.1 W/m2 [2],
which is near the average of the measurements shown in the figure below. S has changed by less than 0.2 W/m2 (0.015%) over
the past 1000 years, otherwise the observed climate variations would have been stronger [3].
In photovoltaics, the solar constant is used because it quantifies the amount of radiation energy that arrives from the sun at the outermost layers
of the atmosphere. Usually, both the seasonal variations due to the Earth's orbit and the 11-year solar cycle can be neglected in PV.
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Figure: Each red dot plots the daily mean of the total solar irradiance (TSI), measured at vertical
incidence above the Earth’s atmosphere by satellites [4][5], and normalised to the sun’s distance of 1 astronomical unit.
The dashed line plots the long-term average of the TSI, called the solar constant S [2]. Data from [6].
