Only if the conditions for the exception are met, and that is not something to assume from a website. The existing-loads method commonly described for NEC 2023 220.87 / 2026 120.87 prefers 12 months of maximum demand. Trade commentary on the 30-day recording exception, used when a full year of data is missing, notes extra limits: it is not available where the service has a renewable energy system or peak-load shaving. A 30-day recording also has to be taken in a way that actually captures peak, typically with a demand meter, not a one-week guess in mild weather.
Why the 12-month path exists: heating and cooling peaks do not both happen in the same month. A January recording can miss the air-conditioner peak; a July recording can miss the heat. Thirty days is a fallback for houses that have no utility demand history, not a shortcut to a friendlier number.
If you have rooftop solar, a battery that peaks-shaves, or a utility portal that only shows net imported kW, stop and read the solar-and-measured-demand page before you treat a short recording as a permit method. The panel calculator can run the 125% arithmetic on whatever peak you enter; it cannot tell you whether that peak qualifies.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.