I am looking for information about what source of illuminance values are used by EnergyPlus daylighting calculations. So far I have found two documentation sources but they seem to be inconsistent:
(I) In the Auxiliary Programs document it specifically states that the 3 illuminance values are “not currently used in EnergyPlus calculations”.
2.9.1.17 Field: Global Horizontal Illuminance ... It is not currently used in EnergyPlus calculations.
2.9.1.18 Field: Direct Normal Illuminance ... It is not currently used in EnergyPlus calculations.
2.9.1.19 Field: Diffuse Horizontal Illuminance ... It is not currently used in EnergyPlus calculations.
This would seem conclusive except for…
(II) In the Input / Output Reference there is one section which touches on this but I’m not sure what it means:
Section: Missing Weather File Data
The weather description of data contains “missing” descriptors … EnergyPlus will fill in “missing” value with something “appropriate”. … the following are used:
Data item Supplied Value Units
…
Illuminance Values 0.0 lux (or CD/m2)
I don’t understand how it could assume zero values for illuminance; using any single fixed value does not seem possible. The idea that this data could be "missing" seems contradictory to the statements about weather data files above.
I can’t find any other documentation of illuminance values from weather data. There is a related question here but it is unanswered.
Trying to read between the lines, I assume that EnergyPlus (A) ignores illuminance data in the EPW weather file but (B) calculates illuminance at each time step based on general solar radiation (from the weather file). Is this correct? What is the calculation method if so?
Thanks
