Add together the wattage capacity of all general lighting branch circuits.
Add in the wattage rating of all plug-in outlet circuits.
Add in the wattage rating of all permanent appliances (ranges, dryers, water heaters, etc.)
Subtract 10,000.
Multiply this number by .40
Add 10,000.
Look for the full wattage rating of permanent air conditioners, and the wattage rating heating appliances (furnace plus space heaters), then add in whichever is the larger of these two numbers. (You don't heat and cool at the same time, so don't need to add both numbers.)
Divide the total by 240.
the answer should give you the suggested amperage needed to power a home adequately.