What size generator do I need to run a whole house in the GTA?

It depends far more on your air conditioner than on your floor area. Tick a 3-ton central air conditioner and the calculator has to cover 5,000 running watts and an 11,500 watt start on that appliance alone (Norwall PowerSystems), which on its own pushes a typical GTA house to the 22 to 26 kW end of the air-cooled range. Leave the air conditioner off the generator, which many GTA installs do, and the same house often lands at 10 to 14 kW. That single decision is worth thousands of dollars on the quote.

Why does the calculator add only one starting surge instead of all of them?

Because that is how both published sizing guides do it. Honda's power management worked example totals the running watts of five appliances at 1,400 W, then lists the starting figures separately and takes the largest, 1,600 W, not their 5,300 W sum. Norwall's guide says the same in words: add up total running watts, then pick the appliance with the largest starting watts and add that. Motors draw their surge for a fraction of a second, and sizing for every motor starting in the same instant would put every household two tiers above what any installer quotes.

How many watts does a refrigerator use?

The two published tables disagree by a factor of four, and both are right about different fridges. Honda rates an Energy Star refrigerator at 132 to 192 watts running and 1,200 watts to start. Norwall rates a general refrigerator at 600 watts running and 2,800 to start. An older fridge behaves like Norwall's figure and a recent one like Honda's, so this site publishes both ends rather than splitting them. Size to the high end unless you know your appliance's nameplate.

Is there a Canadian code that says what size generator I need?

No. CSA C282 covers emergency power in institutional buildings, not a house, and the Canadian Electrical Code governs how a generator is wired rather than how big it is. Residential sizing is done by the manufacturers' published method, which is what this calculator uses. What Ontario does regulate is the installation: the electrical side needs an ESA permit and the gas side a TSSA permit, and a licensed contractor has to pull them.

What is the 25 percent margin for?

Norwall's sizing guide adds 25 percent to the running total to prevent overloads or running at maximum load for minimum capacity. Honda's example does not add a margin. Rather than average two published methods into a figure neither one states, this calculator prints both: the bare running-plus-surge floor, and the same figure with Norwall's stated 25 percent on top. The recommended size is the one with the margin, because a generator held at its ceiling for a three-day outage is the one that fails.

Should I size for the generator or for my electrical panel?

For the load, not the panel. Norwall's guide makes this point directly: the main breaker in your panel is sized to protect your wiring and does not represent how much power you use, and a home with a 200 A main breaker probably needs no more than 80 to 90 amps at 240 volts. Sizing off the panel rating is how people end up quoted a generator twice the size they need. Your installer should still do a load calculation on site before anything is ordered.

What Size Generator Do I Need? Calculator 2026

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Generator size you need

10 kW

The smallest air-cooled standby size that covers 5,925 W, which is your running total plus the single largest starting surge plus Norwall's stated 25 percent margin. On the low end of the published wattage tables the same list needs 3,465 W.

Running watts, everything at once: 1,732–2,500 W
Largest single starting surge: Refrigerator: 1,300–2,800 W
Bare minimum: running plus that one surge (Honda's method): 3,032–5,300 W
With the 25 percent margin (Norwall): 3,465–5,925 W

The surge is the largest ONE appliance starting, not every motor added together, which is how both published guides size a generator. Adding a second fridge raises the running total and leaves the surge where it is. Ranges are wide because the two source tables disagree: a fridge is 132 W running to Honda and 600 W to Norwall, and neither figure was averaged away. Size to the high end.

How does this calculator work out the size?

Two numbers matter for every appliance and they are not the same number. Running watts are what it draws once it has settled. Starting watts are the spike a motor pulls in the first fraction of a second, and Norwall PowerSystems puts it plainly: anything with a motor requires up to three times the amount of power to start as it does to run. A 1/2 hp sump pump runs on 1,050 watts and needs up to 3,200 to start. Miss that and you buy a generator that trips the moment the pump calls for water.

The surge is counted once, for the hungriest appliance only. That is not a simplification this site invented: Honda's own power-management example totals five appliances at 1,400 running watts, lists their starting figures separately, and takes the largest of them at 1,600 rather than the 5,300 they sum to. Its conclusion for that house is a 3,000 watt generator. Adding every surge together would size the same house at roughly double, and nobody sells or installs it that way.

There is no Canadian standard to point at here, and it would be easy to imply one. CSA C282 is about emergency power in institutional buildings and the Canadian Electrical Code is about how the machine is wired, not how large it is. Residential sizing is the manufacturers' method, so this page names whose method it is on every line.

What moves the answer most in a GTA house?

Central air conditioning, and it is not close. A 3-ton unit brings 5,000 running watts and an 11,500 watt start, which is larger than the entire essentials load of most houses put together. Deciding whether the air conditioner goes on the generator is usually the difference between a 14 kW quote and a 26 kW one. Plenty of GTA installs leave it off deliberately and accept a warm July outage.

After that it is whatever is electric rather than gas. An electric dryer is 5,400 to 5,750 running watts and an electric water heater 4,500; their gas equivalents draw almost nothing, because only the drum motor and the ignition are electrical. A house on gas heat, gas water and gas drying has a far smaller backup problem than an all-electric one at the same square footage. The sump pump is the other one worth arguing about: it is small, and it is the appliance whose failure costs the most. Public Safety Canada puts the average cost to repair a flooded basement at over $40,000, and lists sump pump failure among the flood types insurance is bought against.

Once you have a size, the cost calculator prices it against published GTA quotes, and the standby generator cost guide breaks a quote into its line items. The full appliance wattage chart has every figure this calculator uses, with its source on each row.

Frequently asked questions

What size generator do I need to run a whole house in the GTA?: It depends far more on your air conditioner than on your floor area. Tick a 3-ton central air conditioner and the calculator has to cover 5,000 running watts and an 11,500 watt start on that appliance alone (Norwall PowerSystems), which on its own pushes a typical GTA house to the 22 to 26 kW end of the air-cooled range. Leave the air conditioner off the generator, which many GTA installs do, and the same house often lands at 10 to 14 kW. That single decision is worth thousands of dollars on the quote.
Why does the calculator add only one starting surge instead of all of them?: Because that is how both published sizing guides do it. Honda's power management worked example totals the running watts of five appliances at 1,400 W, then lists the starting figures separately and takes the largest, 1,600 W, not their 5,300 W sum. Norwall's guide says the same in words: add up total running watts, then pick the appliance with the largest starting watts and add that. Motors draw their surge for a fraction of a second, and sizing for every motor starting in the same instant would put every household two tiers above what any installer quotes.
How many watts does a refrigerator use?: The two published tables disagree by a factor of four, and both are right about different fridges. Honda rates an Energy Star refrigerator at 132 to 192 watts running and 1,200 watts to start. Norwall rates a general refrigerator at 600 watts running and 2,800 to start. An older fridge behaves like Norwall's figure and a recent one like Honda's, so this site publishes both ends rather than splitting them. Size to the high end unless you know your appliance's nameplate.
Is there a Canadian code that says what size generator I need?: No. CSA C282 covers emergency power in institutional buildings, not a house, and the Canadian Electrical Code governs how a generator is wired rather than how big it is. Residential sizing is done by the manufacturers' published method, which is what this calculator uses. What Ontario does regulate is the installation: the electrical side needs an ESA permit and the gas side a TSSA permit, and a licensed contractor has to pull them.
What is the 25 percent margin for?: Norwall's sizing guide adds 25 percent to the running total to prevent overloads or running at maximum load for minimum capacity. Honda's example does not add a margin. Rather than average two published methods into a figure neither one states, this calculator prints both: the bare running-plus-surge floor, and the same figure with Norwall's stated 25 percent on top. The recommended size is the one with the margin, because a generator held at its ceiling for a three-day outage is the one that fails.
Should I size for the generator or for my electrical panel?: For the load, not the panel. Norwall's guide makes this point directly: the main breaker in your panel is sized to protect your wiring and does not represent how much power you use, and a home with a 200 A main breaker probably needs no more than 80 to 90 amps at 240 volts. Sizing off the panel rating is how people end up quoted a generator twice the size they need. Your installer should still do a load calculation on site before anything is ordered.

Where these wattage figures come from

Two published tables, named on every row of the chart, and where they disagree Home Generator Index keeps both ends rather than picking one. Their running-watt columns agree exactly on the furnace fan across all five horsepower sizes, which is the cross-check that makes the rest usable; their starting-watt columns diverge widely, which is why every result here is a range.

This is an estimate to take to a quote, not a substitute for one. Norwall's own guide says a sizing guide cannot anticipate your power-use habits and that you should work with an installer who will do a load calculation on site. Browse standby generator installers on Home Generator Index

What Size Generator Do I Need? Work It Out From Your Appliances