doc: how to get dhi & bni

[dacoex]

pvlib.clearsky.ineichen produces

• GHI
• DHI
• BNI

It is not yet documented, how the non-clear-sky diffuse horizontal and direct horizontal components are derived.
Say,

I checked the tutorials. Either DHI is included in the TMY files or the DHI of clear-sky is used.

If my understanding is am correct, there is no procedure for deriving actual dhi, e.g. during overcast conditions, from a timerseries of GHI.
The same applies for bhi

Maybe a matrix wich function for converting which irradiance value from which start value using which function would be nice to have that overview.

[wholmgren]

pvlib implements and documents several models that calculate DNI from GHI data:

http://pvlib-python.readthedocs.org/en/latest/pvlib.html?highlight=disc#pvlib.irradiance.disc

http://pvlib-python.readthedocs.org/en/latest/pvlib.html?highlight=dirint#pvlib.irradiance.dirint

[dacoex]

Sure.
But I will still need to use the math for the DHI (GHI-dependant / non clear-sky)

[wholmgren]

You mean dhi = ghi - dni*cos(zenith)? Do you want a function for this? Do you want to make a new documentation example? I'm not sure what you're asking for.

[dacoex]

Yes, I will add this to my list. I will try to elaborate further on it.
I feel the description of the the irradiation module could need some overview at beginning and guidance.

[wholmgren]

Based on my understanding, we should do one or both of the following:

1. Rename this issue "Add overview to irradiance module documentation".
2. Add your specific use case as an example to be addressed in issues doc: describe modeling steps #60 and/or Port notebook tutorials to rst with doctests #120.

Can you clarify or provide guidance?

[wholmgren]

I think this was addressed by irradiance.dni and #692.