The subpanel must be fed with the appropriate sized circuit breaker in order to have adequate protection for the panel and the wiing supplying it. To find the right sized breaker, simply divide the adjusted wattage by 240 volts and you'll arrive at the rated amperage needed for your subpanel. This, however, may not be a common circuit breaker size. In that case you'll need to choose the appriate sized breaker to fit the application.
Choose a double-pole circuit breaker equal to or larger than the amperage you calculated. This is the amperage that the circuit breaker needs to be to safely guard your feed. For instance, if the load calculation comes out to 48 amps, then a double-pole circuit breaker of 50 amps is required to protect it.