Hi Gado

It is certainly the global irradiance on the plane of interest, which combines direct radiation plus diffuse radiation either from the sky or from the ground (this component may be high if there is a high albedo like for example that of snow or sand). 

I think Global irradiance on a fixed plane where DNI is usually used when working on concentrating solar power plants. 

The total incident solar radiation on a surface is the sum of three components: beam radiation, radiation reflected by the ground (surrounding area) and sky diffuse radiation. having these data, you can use the isotropic model with the Lui and Jordan formula. 

Global irradiance on a fixed plane

Many models were developed in this subject in the aim of representing the realistic total radiation reached by the panels and then the output power. The radiation, as mentioned above, is the sum of three components. You have to choose between models and with regard of the site climatic condutions, you can compute the total radiation reached by the panel. Besides isotropic model, the anisotropic model was elaborated by introducing corrections on the isotropic model. It is often used by scientis. Please take a look on my contributions for further information.

Global horizontal irradiance to be used for PV output calulations

Global Irradiance at a fixed horizontal plane is the best for calculations of the PV output

Global irradiance on a fixed plane

if you use software tools (such as PVsyst) to perform the analysis of a PV system.

you need 12 monthly daily  values for each variable:  12 global solar radiation on horizontal plane values and 12 ambient temperature values.  if available you can use also 12 diffuse irradiance values and 12 wind speed values.

Try to google it, You will find many papers and articles

regards.

direct solar radiation

Definitely Global Solar Radiation is better to be considered here. However the DNI has higher contribution to the estimated or predicted power output of the PV panel.

With best regards, 

You'll need to know the tilt & azimuth of the PV system.

Then, you'll need the DNI and Diffuse Horizontal radiation.

Then you convert DNI to incident radiation at the plane of array (the solar module surface) via DNI * cos(AOI) where AOI is the angle of incidence.

Then you can use a transposition model (I recommend Reindl 1990 for its elegance) to calculate the incident diffuse irradiance at plane of array.

Then you run these variables through a PV power model (along with a temperature estimate).

You can download these data for FREE via the Solcast API. Read more at:

https://articles.solcast.com.au/solar-data-api/free-access-to-solar-irradiance-data-for-non-commercial-research-use

And

http://solcast.com.au