How does an electrician figure out what size service a home needs?

They do an NEC load calculation, which adds up expected loads and converts them to amps. The DOE's panel-upgrade guide walks through one method: count general lighting and outlets at 3 volt-amps per square foot, add small-appliance circuits at 1,500 VA each, list fixed appliances by nameplate rating, count the first 8,000 VA at 100% and the rest at 40%, then add the larger of heating or cooling. Dividing the total by 240 volts gives the needed amps. Electricians often aim to keep the load under 80% of service capacity. Smart panels or circuit-sharing devices can sometimes avoid an upgrade.

Sources

• Home Electrification and Electric Panel UpgradesU.S. Department of Energy, Building Science Education Solution Center