Tips and Tricks for Using EnergyPlus — EnergyPlus 9.4

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Output does not match EPW values[LINK]

Why do values in the EPW differ from the output reports of EnergyPlus?

This is expected. 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.