How to Size a Standby Generator for a House

How to Size a Standby Generator for a House

Sizing a standby generator means matching the generator’s kilowatt output to the loads you want to power during an outage. An undersized generator will trip on overload; an oversized one wastes money on equipment and fuel. The correct size depends on whether you want to run your entire home or only essential circuits, and on which appliances draw the most power.

Essential Circuits Only (Partial Home)

• A 10–14 kW generator typically covers essential circuits: furnace, refrigerator, sump pump, well pump (if applicable), a few lighting circuits, and phone charging
• Central air conditioning is usually excluded at this size — running the AC during an outage is optional for most households
• This is the most common choice for Ontario homes where outages are measured in hours or days, not weeks

Whole-Home Coverage

• A 20 kW generator covers a typical Ontario home including central air conditioning, electric range, and EV charging on a managed schedule
• Homes with electric heat (baseboard or heat pump as primary heat) may need 22–26 kW to handle the heating load
• Large homes with multiple HVAC zones, a pool pump, or hot tub typically require a load calculation before sizing

The Load Calculation

A licensed electrician performs a load calculation by adding the nameplate wattage of all loads intended to run simultaneously, then applying a demand factor. The largest motor load (typically the air conditioner or well pump) drives the sizing because motors draw 3–5 times their running current at startup. A generator must handle that starting surge without stalling.

EZSMART performs generator load calculations and installs Generac, Kohler, and Briggs & Stratton standby generators across Ontario. Contact us to assess your home’s load and recommend the correct generator size and automatic transfer switch for your service.

Amir Azimipour

Electrician Since 2008 Journeyman Electrician Designated Master Electrician at EZSMART Corp