Whole-Home Standby Generator Sizing Calculator
Calculate continuous running wattage, motor starting surge (LRA), fuel supply demand, and optimal generator kW capacity for your Ontario home. Model essential emergency circuits against complete whole-home comfort.
How Standby Generators Are Sized in Ontario
Unlike portable gasoline generators that require manual cord stringing in snowstorms, home standby generators permanently connect to your home’s natural gas line or propane tank. Within 10 seconds of a utility outage (ice storm, transformer failure, or severe wind), the generator automatically starts and the transfer switch restores power to your entire electrical panel.
1. Starting Surge (LRA) vs Running Watts
The most common sizing error is calculating only continuous running watts. Electric motors—especially air conditioner compressors, heat pumps, and well pumps—require 3 to 5 times their running wattage for 0.5 to 1 second when starting under pressure. A 3-ton standard AC that uses 3,500 Watts while running can pull 14,000 Watts at startup. If your generator cannot deliver this surge, the motor will stall or the generator breaker will trip.
2. Generac Guardian Capacity & Gas Demand Table
| Model | LP Rating (kW) | Natural Gas Rating (kW) | Natural Gas Flow (100% Load) | Recommended Household Sizing |
|---|---|---|---|---|
| Generac 14 kW | 14.0 kW | 14.0 kW | 206,000 BTU/hr (206 cu ft/hr) | Essentials + Sump + Modulating Inverter AC |
| Generac 18 kW | 18.0 kW | 17.0 kW | 249,000 BTU/hr (249 cu ft/hr) | Standard Home + 3-Ton AC + Essentials |
| Generac 22 kW (Best-Seller) | 22.0 kW | 19.5 kW | 281,000 BTU/hr (281 cu ft/hr) | Whole-Home + 4-Ton AC + Sump + Cooking |
| Generac 24 kW / 26 kW | 24.0 / 26.0 kW | 21.0 / 22.5 kW | 327,000 BTU/hr (327 cu ft/hr) | Estate Home + Dual AC Systems + Well Pump |
Frequently Asked Questions
Do I need to upgrade my Enbridge natural gas meter for a 22kW generator?
Most standard Ontario residential gas meters are rated at 250,000 to 275,000 BTU/hr. When a 22kW generator (drawing ~280,000 BTU/hr) runs alongside your furnace and water heater, the total demand can exceed 450,000 BTU/hr. Enbridge Gas will typically upgrade your meter regulator to a 400 CFH or 630 CFH high-capacity unit for a nominal fee upon application by a licensed gas technician.
What is an Automatic Transfer Switch (ATS) and do I need 200A?
An ATS is the electrical brain of the system. Installed between your main hydro meter and breaker panel, it detects power failure, commands the generator to start, and transfers electrical load in 10 seconds. In Ontario, homes with a 200A main service require a 200A service-rated ATS to power the whole house seamlessly without a separate subpanel.
What clearances are required for standby generator installation in Ontario?
Per CSA B149.1 and local Ontario fire/building codes, generators must be installed at least 1.5 meters (5 feet) from any window, door, or fresh air intake, at least 450 mm (18 inches) from home exterior walls, and have a clear 1 meter (3 feet) working clearance on all sides.