A hundred amps was a generous service in 1985, when a house ran lights, a fridge, a television and an electric water heater. It’s a tight fit for a house with an induction cooktop, a heat pump, two cars charging overnight and a home office in the backyard.
The arithmetic that actually matters
Load calculation isn’t about adding up nameplate ratings — it’s about what runs at the same time. A charger and an electric water heater will never both be at full draw at 3 p.m., but the charger and the heat pump absolutely will be at 2 a.m. in February. We calculate against realistic coincident demand, then leave headroom for one more major appliance.
Signs you’re already at the limit
A full panel with no spare slots is the obvious one. Less obvious: main breaker trips that only happen in winter, lights that dip noticeably when something large starts, tandem breakers doubled up in a panel never designed for them, and any circuit that has been extended twice to serve rooms it was never sized for.
What an upgrade buys beyond capacity
Modern panels protect each circuit individually, so a fault in the outdoor lighting stops taking the freezer with it. You get surge protection for the whole house, a legend anyone can read, and — for most people the real benefit — the ability to say yes to the next project without having this conversation again.