True Peak Load Occurs on Unexpected Day — How to Identify Peak Date from Solar Gains and Geometry? [closed]

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Hi all,

I’m working on ASHRAE 140 Case 600 using EnergyPlus with a 15-minute timestep, and I’ve encountered an interesting situation that I’d really appreciate insight on. (I had asked a similar question earlier but didn’t receive any responses, so I’m rephrasing and asking again here in hopes of reaching more eyes.)

The actual peak cooling load occurs around January 22, not on the hottest design day in July (e.g., July 21). After analysis, I found this is largely due to solar geometry: the low-angle winter sun causes significant solar gains through the south-facing window, even though the outdoor temperatures are lower.

This observation highlights that:

The true peak cooling (or heating) load isn’t necessarily aligned with the hottest/coldest days.

The loads reported by Sizing:Zone/System (design day-based) don’t reflect the actual thermal demand during the year.

The actual peak load is more geometry- and solar-driven — affected by window orientation, occupancy, internal gains, and time-of-day exposure.

What I’m trying to figure out:

How to identify the exact date and time when the actual peak load occurs?

Which EnergyPlus output variables should I request to monitor:

Zone thermal loads (especially in Ideal Loads Air System mode)

Solar gains through windows or surfaces?

Is there an efficient way (e.g., ReadVarsESO, CSV, Python/Eppy) to extract this info and correlate it with surface/window gains?

Is this solar-angle-based peak shift common in practice, and how do professionals typically reconcile it with design-day sizing?

Any tips or experience from those who’ve encountered similar solar-driven peak conditions would be really helpful. Thanks in advance!