When using the LB Analysis Period component, I noticed that the generated time seems to be slightly offset from the actual time. After importing EPW data, I tried to use the LB Analysis Period to extract solar radiation for a specific moment (for example, May 15th at 12:00 PM ). However, it actually extracts the data for May 15th at 1:00 PM .
If the data is at minute-level resolution , and I want to extract all the minute values between 12:00 PM and 1:00 PM on May 15th , it ends up generating data between 12:00 PM and 1:00 PM on May 16th instead.
I’m really curious why this happens. Thanks in advance for your help!
You can trust that the Ladybug components work correctly to give you the correct values at finer timesteps. What you are confused about here is a (relatively poor) design decision that was made 3 decades ago when the EPW file schema was being developed, which we are all stuck with now.
You will see that the first hour of any TMY EPW is labeled 1AM (and not midnight as it should have been if it actually represents the start of the year). This means that the value reported in this slot is actually the average conditions between midnight and 1AM. So the ladybug libraries are correcting for this when they apply the analysis period, assuming that the period is actually representing the time range between the values that you specified and not some wacky interpretation of time like what the EPW file schema uses.
You can search the forum for “EPW 24:00” to see many other topics on this forum where people have been confused by this:
Also note that I say this was a poor design decision in full acknowledgement that the EPW schema developers could not have seen the repercussions of it 30 years later. I’m certainly guilty of similar types of poor design decisions in the file schemas that I have helped make. If only we could realize all of the ways that people want to use our file schema at the time that we’re first putting it together …