Sizing an existing electrical panel?

How do you properly determine the capacity of an existing breaker panel ? The panel is a 240/120 VAC unit with a 60 Amp main disconnect.
Is the proper way to calulate the panel capacity by performing 240VAC X 120A (total main breaker size)=28800 Watts. Or 120 VAC X 60A = 7200 Watts mutiplied by two.

Thanks

 

[jbartos]

Suggestion:
240V x 60A = 14400Watts or
120V X 60A x 2 = 14400Watts
is the answer for the standard interpretation of 240/120V panel, i.e. single phase three wire power distribution system.