Weather Data Timestamp

The TMY data set starts from 1 to 24, but the weather data in SAM from 0 to 23. For example, at 8 AM on Jan 1 in Daggett (723815) GHI is 54 W/m2 from the TMY data set. However, the same value is recorded at 7 AM in SAM. Why do you shift the timestamp of the weather data in SAM?

Thank you in advance.

HJ Lee

Dear HJ Lee,

SAM assumes that the first row of data is for the time step that begins at midnight. For hourly data, that is equivalent to the time step that begins at 1 am. This is consistent with the time convention of files you download from the latest NSRDB update (PSM).

Best regards,
Paul.

Hi Paul,

I have the same issue as HJ and am confused by your answer. I have a .epw weather file for sydney (GMT+10) which starts at 1/1/1991 01:00 , with the first day's peak GHI at 12:00 noon. If I output the GHI from SAM, it starts at midnight and peaks at 11am.
I'm not clear what you mean when you say the timestep that begins at midnight is equivalent to teh timestep that begins at 1am for hourly data.
Please could you explain.

Thanks.

Mike

Hi Mike,
I just reread what I wrote and agree that it is confusing.
What I meant to say was that the time step that begins at midnight is equivalent to the time step that ends at 1 am. We can call that first time step either 12 am or 1 am (or, Hour 0 or Hour 1). The old NREL TMY files identified the first hour as Hour 1 (hour ending at 1 am, or beginning at 12 am), and the new NREL NSRDB files identify the first hour as Hour 0 (hour beginning at 12 am, or ending at 1 am).
SAM calls the first hour Hour 0 regardless of what it is called in the weather file. For the Sydney EPW file, the beam irradiance on January 1 in the Weather file peaks at 898 W/m2 in Hour 14, which is the hour ending at 14:00 and beginning at 13:00. SAM shows that peak value at 13:00, which is the hour beginning at 13:00 and ending at 14:00.
I hope that is somewhat clearer..
Best regards,

Paul.