[Radiance-general] unit of irradiance using epw as input

[Greg Ward]

Hi David,

The EPW weather file should contain total solar irradiance values in watts/sq.meter.  Radiance also reports values in watts/sq.meter, but only over the visible spectrum.  Therefore, we normally include an "efficacy" calculation that converts the EPW's radiometric values into these semi-photometric values.  (It's a bit confusing, but suffice it to say that Radiance's units have a relation to photometric units that employs a predefined efficacy/conversion value of 179 lumens/watt.)

If you want Radiance to actually give you total solar irradiance, then you should employ the -O1 option of gendaylit and gendaymtx.  This should forgo the usual correction factors.

Cheers,
-Greg

> From: David Davidson <this.is.david.davidson at gmail.com>
> Subject: [Radiance-general] unit of irradiance using epw as input
> Date: April 26, 2015 12:22:55 AM PDT
> 
> Hi, may I ask if the unit of irradiance is Wh/m2 if I use the "epw weather file + gendaylit/gencumulativesky + rtrace" method to get the irradiance of a given point for a particular hour in a year or annual total irradiance? 
> 
> The three radiation variables in epw file are using the unit of Wh/m2, so I assume the output of rtrace simulation using epw file as input should have the same unit. Is it correct?
> 
> Thanks!
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