HOW TO DETERMINE SOLAR INSOLATION  (W-Hr/m2)

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USING GRAPHICAL RECTANGULAR, TRAPAZOIDAL, PARABOLIC

AND INTEGRATION METHODS

A REFRESHER

ABSTRACT

At times there may be the need to determine the total quantity of solar energy (watts per square meter per day, W-day/m2), that is available during a day’s worth of sunshine (solar insolation), based on instantaneous solar irradiance (watts per square meter, W/m2) measurements.  This information is then used to determine how many photovoltaic panels are required for a solar energy project.

Four ‘area under the curve’ methods are discussed for determining insolation from measured irradiance data:

  • Graphical sum of rectangles;
  • Graphical sum of trapezoidals;
  • Graphical sum of parabolic bounded areas (Simpson’s Rule); and
  • Integration (Calculus) of trendline equations.

Each method can provide progressively more accurate results.

This paper summarizes the expectation that the Integration (Calculus) of trendline equation is the most accurate (provided the trendline correlation factor is close to unity (1)) and the quickest of the methods to use.  The Integration formula is presented in a user friendly ‘cook book’ procedure.

Irradiance PaperDownload