Open the door (not the dead front) and find the main breaker, usually at the top or in a side compartment, and read the ampere number printed on its handle or body: 100, 125, 150, 200 are common. Then find the manufacturer’s label inside the door or on the box, which lists the panel’s bus rating and permitted breaker configurations. Use the main-breaker or official service size in a load calculation, and do not assume a 200-amp label on the can overrides a 100-amp main.
If you have fuses rather than a main breaker, that is a different, older arrangement — see the fuse-box page and get an electrician involved before you add load. If there is no obvious main, stop; some older systems have a main in a separate enclosure at the meter.
Take a photo of the label and the main for the electrician. Then enter that rating in the panel calculator. If the numbers on the main and the label disagree, read the 100-amp-main / 200-amp-box page before you tell anyone you “already have 200 amps.”
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.