How do you figure out how much power is needed for the whole house?

Power as in voltage, amps ? What project are you working on?

By doing a demand load calculation.

This doesn't mean you are going to add up all the circuit breakers, or all the loads--seeing that not all loads will be utilized simultaneously. So, there is a number of factors (demand factors) that has to be considered. Where do we start: The NEC-Article 220.

First: General lighting load
Second: AC over Heat or versus heat
Third: App's (Appliances)
Fourth: Dryer
Fifth: Cooking appliance or equipment
Sixth: Service or feeder size