Do not assume solar makes a measured-demand calculation easier, and do not use the 30-day recording exception as a workaround. Trade commentary on the existing-loads method (NEC 2023 220.87 / 2026 120.87, as commonly described) says the 30-day exception is not available where there is a renewable energy system or peak-load shaving. Whether a full year of utility data still qualifies, and whether that data is gross house demand or net of the array, is a fact about your meter, your utility, and the authority having jurisdiction. This page will not pick a method for a solar house.
The practical problem is concealment. A utility “peak kW” that is net of rooftop production can look low on a sunny afternoon while the panel is still carrying the air conditioner, the range, and, soon, a charger. If the array is offline — night, snow, an outage, a tripped inverter — the service has to carry the house without that offset. Peak-shaving batteries raise a similar issue: they can suppress the number on the bill without reducing the feeder’s job when the battery is empty or held in reserve.
What to bring to the electrician: 12 months of the most detailed interval data the utility will export, a one-line description of the solar and any storage, and the inverter or interconnection documents. If measured demand is not accepted, a calculated dwelling-unit method, a listed energy management system, or a service upgrade are the usual alternatives. The calculator still helps you sketch the arithmetic, with the limitation spelled out.
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.