Solar resource terminology can look simple until a designer compares two datasets and discovers that the values describe different physical quantities. The first discipline is to define the measurement or model variable before comparing numbers.
GHI
Global Horizontal Irradiance is the total irradiance received on a horizontal plane. It includes direct and diffuse components. GHI is widely used for broad resource comparison and as an input to many PV workflows.
DNI
Direct Normal Irradiance represents the direct beam component on a plane normal to the sun's rays. It is especially relevant to concentrating technologies and to resource decomposition/transposition models.
DHI
Diffuse Horizontal Irradiance is the diffuse sky component on a horizontal plane. It becomes important when converting horizontal resource data into the plane-of-array resource.
GTI
Global Tilted Irradiance describes the total irradiance incident on a selected tilted plane. Because fixed PV arrays are tilted and oriented, GTI is often closer to the geometry of the actual array than raw GHI.
PVOUT
PVOUT is an estimated PV electricity output metric generated by a model. It is not a physical irradiance measurement and should not be compared directly with GHI as if they were the same quantity.
Professional documentation
Record the site coordinates, dataset, time period, interpolation method, tilt/azimuth assumptions, temperature source, loss assumptions and software version. This makes the simulation reproducible and easier to review.
Useful sources
NASA POWER • Global Solar Atlas • ASHRAE climate data
Apply the concept
Use the corresponding GCC Solar Academy calculator after reading the article. For final engineering, verify the result against the actual equipment datasheet, installation method, governing standard and authority requirement.
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