It usually means you have a panel cabinet or bus that is listed for up to 200 amps, with a 100-amp main breaker installed. The service is the 100-amp main (and the conductors and meter socket that feed it), unless an electrician and the utility have already upgraded those parts. Swapping the main breaker for a 200-amp breaker is not a service upgrade by itself and can be dangerous if the conductors or meter are still 100-amp equipment.
This mismatch showed up in the r/askanelectrician 60-amp charger thread: a 100-amp main in the upper right was the service limit, even if the can looked “big.” Angi, in its 2026 panel-upgrade article, quoted a master electrician making the same point in the other direction — a new 200-amp-rated panel does not help if the exterior wire and meter box are still 100- or 150-amp equipment.
For load calculations, compare against the service rating you actually have, which is generally the main-breaker or service size, not the marketing size of the box. The calculator asks for that rating explicitly. See also the meter-base page.
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.