They are related but not interchangeable. Service amperage is the rating of the service to the house: the service-entrance conductors, the meter socket, and the main overcurrent device that limits what the utility supply can deliver. The bus rating is a listing on the panel cabinet — the current the bus bars are designed to carry. Manufacturers often ship a cabinet whose bus is rated higher than the main breaker installed in it, so that the same box can be used in more than one service size. That is the 100-amp-main / 200-amp-box situation, and it does not give you a 200-amp service.
For a load calculation, compare the result with the service rating you actually have, which is generally the main-breaker or official service size. For physical work, the electrician also has to respect the bus rating, the main’s rating, and the panel’s labeled configurations (including whether tandems are allowed). Angi, in its 2026 panel-upgrade article, quoted a master electrician making the complementary point: a new 200-amp-rated panel does not help if the exterior wire and meter box are still 100- or 150-amp equipment.
Read the label and the main, as the “how to read the panel” page describes, and enter the service rating — not the marketing size of the can — in the calculator.
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.