Re: [EnergyPlus_Support] discrepancy of weather data in epw file and csv report file

["Michael J. Witte" <mjwitte@xxxxxxxx>]

The standard EnergyPlus weather files (epw) contain hourly data.  For state 
variables such as temperature and humidity, the values in the epw file are the 
instantaneous measurements at 1:00 am, 2:00 am, and so on.  Because EnergyPlus 
uses zone time steps of less than one hour, these values are linearly 
interpolated for each time step.  Weather data values reported from EnergyPlus 
at the Timestep frequency will match the weather file one timestep per hour on 
the hour.  Values reported at the Hourly frequency represent the average value 
over the hour, so they will not match the instantaneous value in the weather 
file.  The solar radiation values in the weather file are integrated total 
values for the hour, so the interpolation must be done differently.  The total 
radiation in [Wh/m2] is used as an average rate for the hour in [W/m2], and 
this average rate is assumed to be the value at the midpoint of the hour.  
Again, linear interpolation is used to determine the value for each timestep.  
The reported solar values in the output will match the value in the weather 
file on the half hour.

Mike


On 4 Jun 2004, at 16:11, Rahul Chillar wrote:

> hi
> No , it does not matter . The program does calculations in time steps ,
> temperatures at those time steps are are calculated by division of an interval
> of temperature from the weather file and when the various temperatures are added
> back to give final temperature the report variable, they will not be same due to
> addition of truncation errors. It does not affect the results at all.
> 
> Thanks
> Rahul Chillar
> Grad Student
> Building Systems
> Univ Of Illinois UC
> 
> liao_lucia <liao@xxxxxxxxxxx> wrote:
> I notice that the same weather data (such as the Outdoor Dry Bulb 
> and Outdoor Relative Humidity) are slightly different in the epw 
> weather file and csv report file.
> 
> Does it matter ?
> 
> Lucia
> 
> 
> 
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx



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