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Understanding EPW leap year header

asked 2024-06-12 11:18:44 -0500

Jérôme's avatar

updated 2024-06-12 15:02:54 -0500

I'm developing a tool that exports weather data as epw. I'm wondering about the leap year header.

I'm exporting "true data", so for leap years, I always have Feb 29th data.

When exporting full years, the logic is simple, the leap year header should be "Yes" on leap years.

What if I'm exporting January data, or May-September data?

It boils down to what the flags is used for. IIUC, it means that if "Yes", E+ will expect Feb 29th data on leap years, otherwise not. In this case, and since I'm exporting true data, it shouldn't hurt to always set it to yes, right?

This seems to be confirmed by the reading of this PR so it should be true for E+ >= 9.1.0.

I'd be happy to get confirmation.

Assuming my understanding of E+ behaviour is correct, I'm still interested in what the "EPW standard" defines for this field, as other softwares consuming EPW might be more strict about it. I don't find the docs so explicit about it.


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answered 2024-06-13 03:52:05 -0500

PmP's avatar

To my knowledge, as I had to research a bit about how E+ treats gap years for some simulations, your understanding is correct.

First, here is a really old post that talks about it, just to add information to the subject:

You asked, "it shouldn't hurt to always set it to yes, right?" The answer is yes, as you have to go out of your way to simulate a gap year: The .idf NEEDS to have the runperiod on a gap year, the gap year data has to be in the .epw and the flag set to yes. If you simulate the runperiod on the .idf as 2019 for example, the gap year won't be simulated nor affect the simulation.

"What if I'm exporting January data, or May-September data?" then, as long as the .idf does not try to simulate days that are not in the .EPW, the gap year flag won't affect anything.

Good luck making your tool and I hope you can share it here once its done!,


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Thansk for confirming. I'd be happy to share the tool. It is currently private but I'll advocate in my company for releasing it under a free license.

Jérôme's avatar Jérôme  ( 2024-06-14 08:35:12 -0500 )edit

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Asked: 2024-06-12 11:18:44 -0500

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Last updated: Jun 14