Re: [EnergyPlus_Support] Difference between input and output weather data

["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 2 Feb 2005, at 22:25, wah wrote:

> 
> Hello.
> Would like to know why is it that the output weather data called from simulation
> differs slightly from the input values? Does the weather data (outdoor drybulb,
> relative humidity, dew point temperature,  direct/diffuse radiation, wind speed)
> change during the simulation process? @_@ Thank you and have a nice day.
> 
> PuiWah
> 
> 
> 
> 
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
> 
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
> 
> Attachments are not allowed -- please post any files to the appropriate folder
> in the Files area of the Support Web Site.
> 
> Yahoo! Groups Links
> 
> 
> 
> 
> 
> 
> 
> 
> 



========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx




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