In the United States most of your appliances and lights will use 120 volts. However, things like dryers and electric stoves use 240 volts.
The line into your house from the street will have 240 volts which can be broken down to circuits of 120 volts in your home's circuit box.
How the Capacity Is Measured
Electricians usually measure the capacity needed in your house using a parameter called volt-amps. This is the apparent power, the rate of flow of energy past a certain point in a circuit.
By simply dividing by the number of volts (240 in most American homes) you can find the number of amps needed.
Depends What You Are Using
The power you need depends on how many electric devices you are using in your home. As an example, if your house has a garbage disposal, garage-door opener, electric water heater, dishwasher and electric stove, you will need approximately 18,000 volt-amps, or circuits to handle 75 amps.