Tankless water heaters get sold on endless hot water and closet-sized footprints — and the brochure rarely mentions that the electric versions are among the most demanding appliances you can put in a house. Whether you need electrical work depends entirely on which type you're buying.
Gas tankless: a non-event electrically
A gas tankless unit (Rinnai, Navien, Rheem gas models) heats water with a gas burner; electricity only runs the ignition, fan, and controls. It needs a standard 120V receptacle — a dedicated circuit is good practice and cheap to add during installation ($250–$500 if one isn't nearby). If your home has gas service, this is why the overwhelming majority of Ontario tankless installs are gas: the electrical scope is trivial.
Electric tankless: read this before you buy
Heating water instantly with electricity takes enormous instantaneous power. A whole-home electric tankless unit rated for Ontario's cold groundwater runs 24–36 kW — that's 100–150 amps at 240V, fed by two to four separate double-pole breakers. For perspective, that one appliance can demand more capacity than everything else in your house combined. On a 100-amp service it's simply not happening; even on 200 amps, the load calculation often fails once the EV charger, range, dryer, and AC are counted.
If you're set on electric tankless, budget honestly: the wiring (multiple 8 or 6 AWG 240V runs) costs $1,500–$3,000, a panel upgrade adds $3,200–$4,500 if needed, and the ESA permit applies. Point-of-use electric tankless units — a small heater under a far-away bathroom sink — are the exception: at 15–30 amps they're a normal dedicated-circuit job.
The alternative that usually wins
For homeowners going all-electric, a heat-pump (hybrid) water heater delivers most of the benefit with a fraction of the demand: one modest dedicated circuit, 60–70% less energy than a resistance tank, and substantial rebates in current federal and provincial programs. We install the circuit, and the energy audit math almost always favours it over electric tankless in Ontario's climate.
Whatever direction you're leaning, the first step is free: we'll run your panel's load calculation during an estimate visit and tell you exactly what your service can support — before you've bought equipment that doesn't fit. 416-837-4038.