I have some experience in installing residential electric but I never had to calculate the load for anyone’s home. I have a friend who wants me to run power out to his garage for a outlet and a light and he also wants a line ran up to the second floor for his home office for a computer and he has plans on adding another bath room to the back of his house. So that’s 2 lights and 5 outlets and maybe 1 more light. What I want to know is how to calculate whether he is going to need a 200A service or should he just keep his 100A service.

Unless he is planning AC and doing some arc welding the 100amp should be just fine.