Service Size Calculator — Rating & Headroom

Divide the calculated load in VA by 240 to get amps, then pick the smallest standard service at or above that number. For most single-family homes the honest answer is 200 A — not because the calculation demands it, but because the cost gap over 125 A or 150 A is small and the headroom covers future EV charging and electrification. Run the Residential Load Calculator first to get the VA figure.

For a gas-heated house with an electric range, dryer, and water heater, a 100 A service is usually adequate on paper — those homes typically calculate in the 80–100 A range. It becomes marginal the moment you add EV charging plus a heat pump, and inadequate for a fully electrified home. The deciding factor is the calculation, not the age of the panel.

They are completely different problems with different fixes. Full means no physical breaker spaces left — solvable with tandem breakers where the panel is listed for them, or with a subpanel, for a few hundred dollars. Overloaded means the calculated load exceeds what the service can carry — that needs a service upgrade, which is an order of magnitude more expensive. A panel can be completely full with plenty of capacity, or have empty spaces and no capacity at all.

It is a planning convention rather than a Code requirement for services. NEC 210.20(A) requires continuous loads to be at 125% — equivalently 80% — but a service is sized from a calculated load that already includes demand factors, so it is not double-discounted. Staying under 80% utilisation is nonetheless good practice: it leaves room for future load and keeps the main breaker off its thermal limit. This calculator flags anything above 80%.

Often yes. A 48 A charger is 11,520 VA of nameplate but adds only about 4,600 VA to a 220.82 calculated load thanks to the 40% remainder factor. Whether it fits depends entirely on what else is on the service. If it does not fit, an energy management system under NEC Article 750 that sheds or limits charging during peak demand is almost always cheaper than a service upgrade — and it is an explicitly permitted compliance path.

Not necessarily, and conflating them costs customers money. Replacing a full or obsolete panel while keeping the same service rating is a panel swap — no utility involvement, no new service conductors, no meter work. Increasing the service rating means new service-entrance conductors, usually a new meter socket, utility coordination, and often a mast or lateral upgrade. The second job can cost three or four times the first.

Market data for 2026 puts a residential panel or service upgrade at roughly $1,300 to $5,000 installed, with most 100 A to 200 A upgrades landing near $3,000. The spread is driven by whether the meter socket and service conductors have to change, mast versus underground lateral, panel location, permit and inspection fees, and how much circuit re-termination is involved. Price your own job with the Electrical Estimate Calculator.