Knowledgebase: Weather
Why doesn't the weather data in the output file match the weather data file?
Posted by - NA -, Last modified by Linda Lawrie on 26 May 2010 02:48 PM
Q. When I run my simulation, the weather data reported in the output file doesn't match the values in the weather file. Why is that?

A. If you are reporting values hourly, the values should match. If you are reporting values sub-hourly (i.e., detailed or timestep), then the weather data are interpolated between the hourly values in the weather file.

More explanation: The difference comes from interpolating hourly weather data for subhourly timesteps in EnergyPlus. In an hourly weather file, the temperatures and other state-point readings are the value at the time the reading was taken. For example, in the USA_IL_Chicago-OHare_TMY2.epw file, the outdoor dry bulb value for July 2, hour 1, is 19.4C. This is the temperature at 1:00 am.

If you set Timestep = 1, then EnergyPlus will report 19.4C for 07/02 01:00 and will use that value for the entire one hour timestep.

If Timestep = 4, then 19.4C is used only for the time step which ends at 01:00. The other timesteps use linearly interpolated values between the hourly weather file values. When you report at the "hourly" frequency in EnergyPlus, you see the average temperature over the hour. If you report at
the "timestep" frequency, you will see the values from the weather data file appear at the last timestep of each hour.
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