How big a generator do I need?

Size the generator around what must keep running, not the square footage of the house

The right generator size starts with the loads you need during an outage, not a rule like “one size for a detached home.” Two houses with the same floor area can need very different generator capacity if one has a sump pump, electric range, central air, freezer, medical equipment, and a finished basement, while the other only needs heat, a fridge, and lights.

For most Toronto and GTA homes, the first decision is whether you want essential-load backup or whole-home backup. Essential-load systems keep selected circuits alive. Whole-home systems try to carry the entire service, often with load management so the generator is not asked to run every large appliance at once.

Essential loads usually include:

Large electric loads change the answer quickly. Electric range, dryer, hot tub, EV charger, heat pump backup heat, central air, and electric water heating can push a generator far beyond an essential-load design. In many homes, those loads are either locked out during generator operation or controlled by load-shedding equipment.

If you are still deciding between portable and standby, start with the practical comparison in standby vs portable generator options, then have the final size confirmed from the actual panel and equipment nameplates.

A load calculation shows the real generator size and what can be safely backed up

A generator should be sized from a load calculation, the equipment starting requirements, and the transfer switch design. Guessing from online charts is where people end up with generators that stall, trip, or cannot start a sump pump and fridge at the same time.

During a site assessment, we look at the panel schedule and the actual equipment, not just the breaker sizes. A 15A lighting breaker does not mean 15 amps are always in use. A motor load may run at a modest level but need a higher inrush current for a moment at startup. That difference matters for furnace blowers, well pumps, sump pumps, fridges, freezers, and air conditioning.

The calculation usually separates loads into three groups:

• Must run automatically: furnace or boiler, sump pump, fridge, freezer, internet equipment, medical equipment, selected lights.
• Nice to have if capacity allows: microwave, garage door opener, some kitchen receptacles, laundry receptacle, additional lighting.
• Usually managed or excluded: electric range, dryer, EV charger, hot tub, sauna, central air, pool equipment, and large electric heat.

A standby generator for a typical GTA home is commonly in the 10–26 kW range, but the correct number depends on what is connected and what is blocked from running. The installed cost for a typical standby system is often quoted in the $12,000–$20,000 range, with size, transfer equipment, gas work, pad location, trenching, and permits all affecting the written quote.

This is also where selected-circuit backup can save money and complexity. If your goal is heat, sump protection, cold food, internet, and safe lighting, a smaller system with a properly chosen transfer switch may be more sensible than trying to power every appliance in the house.

The transfer switch decides how generator power enters the house

The generator size and transfer switch have to be designed together. A transfer switch is not an accessory; it is the safety device that separates your house from the utility grid and prevents backfeed into Toronto Hydro, Alectra, Hydro One, Elexicon, Oshawa Power, or another local utility system.

There are two common paths:

• Automatic standby generator: the generator starts after utility power fails, and an automatic transfer switch moves the selected loads or service onto generator power.
• Portable generator with inlet and manual transfer: you start the generator manually, connect it to a proper inlet, and move selected circuits with a manual transfer switch or approved transfer equipment.

What is not acceptable is a cord from a generator into a dryer receptacle, range receptacle, or homemade adapter. That can energize the utility side, damage equipment, and create a shock hazard. If you are considering a portable unit, read why you cannot plug a generator into a house outlet before buying cables or adapters.

The transfer switch design answers practical questions:

• Which circuits are backed up?
• Is the neutral switched or solid, based on the equipment design?
• Are generator breakers and conductors sized correctly?
• Does the system need load shedding for air conditioning, electric heat, or other large loads?
• Is the panel labelled so a homeowner can operate it safely during an outage?
• Is the generator protected from being overloaded when multiple motors start?

For a portable-generator setup, a manual transfer switch with an inlet box is the lower-cost route, often in the $1,500–$2,500 range when installed as a proper alternative to a standby system. For more on the equipment choice, see generator transfer switch cost and options.

In Ontario, the installation needs ESA notification and inspection where required

A permanently connected generator system in Ontario is electrical work that needs proper notification, installation, and inspection where required. The generator itself may sit outside, but the electrical work involves feeders, overcurrent protection, grounding and bonding, transfer equipment, panel connections, labelling, and sometimes service coordination.

For a standby installation, the electrical inspection is not just a formality. The ESA inspector may review items such as:

• transfer switch rating and installation method;
• conductor sizing and protection;
• grounding and bonding connections;
• working space around panels and transfer equipment;
• labelling of backed-up circuits and operating instructions;
• weatherproof equipment and outdoor wiring methods;
• separation between generator power and utility power.

Generator placement also has to be planned before the electrical quote is final. The unit needs a safe outdoor location with manufacturer-required clearances, carbon monoxide safety in mind, service access, and a practical route for wiring. Gas connection and generator pad requirements are part of the overall project, even though the electrical contractor’s scope is the wiring, transfer equipment, notification, and commissioning on the electrical side.

If your project is in a condo, townhouse complex, laneway property, or tight side yard, placement can be the limiting factor. Condo rules, municipal requirements, noise considerations, and manufacturer clearances may affect what is possible. If an exact clearance or approval rule matters, confirm it with the manufacturer, municipality, condo corporation, and ESA before ordering the unit.

For the permit side, see whether a generator needs an electrical permit in Ontario. For installed cost planning, the broader breakdown is covered in home standby generator cost in Ontario.

The safest next step is a site visit before you buy the generator

The best time to size the generator is before the equipment is purchased. Once a generator is sitting in the garage, the options narrow: the unit may be too small for the desired loads, too large for the available gas capacity, awkward to place, or incompatible with the transfer equipment you actually need.

A proper generator assessment should leave you with a written plan that identifies:

• the circuits that will be backed up;
• whether the design is selected-load or whole-home;
• expected generator size range;
• transfer switch type and location;
• any load-shedding requirements;
• the panel work needed;
• ESA notification and inspection requirements;
• placement concerns before the unit is ordered.

City Power Electrical Services is a Licensed Electrical Contractor, ECRA/ESA #7015314, based in North York and serving Toronto and the GTA; we provide free written estimates for generator wiring, transfer switch design, ESA notification where required, and final cleanup. If you are planning backup power, start with our generator installation service so the size, switch, and permit path are designed together instead of guessed after purchase.