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The math that decides 100A or 200A.
Every panel upgrade, service change, and new build starts with the same question: how big a service does this house actually need? This calculator runs the NEC Article 220 standard method line by line — lighting at 3 VA per square foot, demand factors from Table 220.42, the 8 kW range allowance, dryer minimums, fixed-appliance diversity, and the non-coincident heating/cooling rule — ending at total demand VA, service amps, and the minimum standard service size.
Electricians use it to justify a panel upgrade to the inspector — the demand-factored VA total is the number on the permit. Homeowners quoted a 200A upgrade use it to check whether the house actually calculates past 100A, or whether the EV charger is the only thing pushing it over.
The calculation assumes worst case with code-mandated diversity, not your actual usage. It exists to guarantee the service can carry the peak the code imagines — burners, dryer, AC, and water heater with realistic overlap. Demand factors like 35% on lighting and 8 kW on a 12 kW range are the code's model of how homes really behave.
Barely. A gas furnace contributes only its blower motor (often 400–800 VA), a gas water heater contributes nothing, and a gas range only its controls. Homes with gas heat, hot water, and cooking routinely calculate under 100A; all-electric homes are the ones pushing 150–200A.
Often it is the deciding load. A 48A Level 2 charger adds 11,520 VA at 100% as a continuous load — on a house calculating at 90A, that single addition pushes the service past 125A toward 150 or 200A. Add the charger's VA to "other fixed appliances" above to see it.
NEC 220.82 offers an alternative for dwellings (100% of the first 10 kVA, 40% of the rest) that frequently lands one service size smaller. The standard method above is always accepted; many electricians run both and submit the smaller. The local authority having jurisdiction makes the final call.