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

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Hi all,

I’m working on ASHRAE 140 Case 600 using EnergyPlus with a 15-minute timestep, and I’ve noticed something interesting:

The true peak cooling load does not occur on the hottest design day (e.g., July 21). Instead, it occurs on a completely different day — around January 22, which is counterintuitive.

After analyzing the model, I found that this shift is mainly due to solar geometry:

In January, the sun angle favours the south-facing window, leading to higher solar gains in the zone, even though it’s colder outside.

As a result, the zone experiences higher internal heat buildup than it does during the “hottest” design day in summer.

This has made me realize that the true peak cooling (or heating) load is not necessarily driven by ambient temperature, but by the combined effect of building geometry, solar exposure, window placement, and internal gains.

My questions:

How can I determine the exact date and time when the actual peak load occurs, based on these dynamic solar effects?

Which EnergyPlus output variables would help me trace this — especially for solar gains through windows and zone thermal loads?

Is there a way to correlate solar gains and internal loads to identify the cause of the peak, and whether it shifts due to geometry?

Any suggested workflow or tools (e.g., ReadVarsESO, CSV analysis, Python/Eppy) to extract this information precisely?

I also want to know — is this behaviour expected to vary with geometry, and how do experienced modelers track these conditions?

Ultimately, I want to understand the drivers of actual peak loads rather than relying on default design day sizing. Any advice would be greatly appreciated!