I use a very loose method here in northern Illinois, where we have the highest electrical rates in the country. Most people have gas clothes dryers, gas ranges, and gas forced air heating. If they have air conditioning, electric dryer and electric range, it gets a 200 amp service. If not 100 amp. However, most communities around here now require a 200 amp service. A call to the local building department may simplify your solution. Of course there is no way to calculate according to appliances not yet purchased.