How to Size a Standby Generator for a House

Buy a standby generator too small and half the home stays dark during an outage. Buy one too large and you overspend on a unit that idles most of its life. Sizing is the single most consequential decision in a Toronto or GTA generator project. This guide walks through the two approaches to sizing — essentials-only versus whole-home — and shows what a proper load calc looks like.

How to size a standby generator for a house

Standby generator sizing starts with deciding what you want to run during an outage. Essentials-only sizing (furnace, fridge, some lights, sump pump) typically needs 7–12 kW. Whole-home sizing including AC, dryer, and water heater typically needs 18–24 kW for a mid-sized GTA home. A licensed contractor runs a formal load calc using the Canadian Electrical Code demand factors to size exactly. Add 20% headroom for future electrification (EV, heat pump).

Step 1: decide your coverage level

• Essentials only: furnace/boiler, fridge/freezer, sump pump, some lighting, phone charging.
• Comfort: above plus microwave, TV, water heater, most receptacles.
• Full comfort: above plus central AC and clothes dryer (one at a time).
• Whole home: everything runs simultaneously, no restrictions.

Most Ontario homeowners land at Comfort or Full Comfort. True whole-home is rare and expensive.

Step 2: list your major loads

Common Ontario residential loads with typical wattages:

• Furnace fan: 500–800W (natural gas heat).
• Electric furnace/heat pump backup: 5,000–10,000W.
• Central AC (2–3 ton): 3,000–5,500W running, 8,000–12,000W starting.
• Fridge: 150–600W running, 800–1,500W starting.
• Chest freezer: 100–400W.
• Sump pump: 750–1,500W starting.
• Well pump: 1,000–2,500W starting.
• Electric water heater: 3,000–4,500W.
• Electric dryer: 5,000–6,000W.
• Electric range: 6,000–12,000W.
• Microwave: 1,000–1,800W.
• Lighting circuits: 300–1,000W total.
• General receptacles (TV, computer, etc.): 500–2,000W total.

Step 3: apply CEC demand factors

Not all loads run simultaneously. The Canadian Electrical Code allows demand factors to size generators realistically. A licensed contractor applies these correctly, but the rough principle:

• Sum continuous loads (furnace fan, fridge, freezer) at 100%.
• Add starting current of the largest motor (usually AC or well pump).
• Add other intermittent loads at 40–80% depending on category.

Rough sample for comfort-level coverage:

• Continuous: 500 (furnace) + 400 (fridge) + 200 (freezer) + 200 (sump) + 800 (lights/receptacles) = 2,100W.
• Peak startup (AC compressor): 12,000W momentary.
• Intermittent: 1,500 (microwave, TV, etc.) at ~50% factor = 750W.
• Total continuous: ~3,000W. Peak: ~15,000W. Generator size: 15–18 kW.

Step 4: add headroom for the future

• Plan for EV charging on generator? Add 6–8 kW.
• Plan for heat pump conversion? Add 3–8 kW depending on system.
• Plan for induction range? Add 4–8 kW.
• Kids growing up (more devices)? Add 20%.

Generator lifespan is 20–25 years. Sizing for today only usually means undersized in 10.

Standard generator sizes and typical use

• 7–10 kW: essentials only, small homes, condos.
• 11–14 kW: comfort coverage, small to mid homes, no AC on generator.
• 15–17 kW: comfort coverage with AC on lower stage, most 2,000 sq ft homes.
• 18–22 kW: full comfort including AC and one large appliance, 2,500–3,500 sq ft.
• 24–26 kW: whole-home coverage or homes with heat pump/EV.

Load management as an alternative to upsizing

Automatic transfer switches with load management can shed non-essential circuits when the generator approaches capacity. This lets a smaller generator handle a larger home by never running everything simultaneously.

• Sheds dryer or AC when other heavy loads engage.
• Prioritizes essential circuits automatically.
• Adds $500–$1,500 CAD to transfer switch cost.
• Can save $2,000–$4,000 on a smaller generator vs the next size up.

Common sizing mistakes

• Sizing by home square footage alone. Two homes of the same size can have vastly different loads.
• Ignoring motor start current. Sizing for AC running instead of AC starting means the AC never starts.
• Ignoring future electrification. EV and heat pump plans need generator headroom.
• Trusting online calculators only. Rough estimates only. Use a licensed contractor for the actual sizing.
• Undersizing to save $2,000. Regretted daily during winter outages.

Expert tip from our ESA-licensed electricians

The best sizing question we ask GTA homeowners is: how do you want to feel during a 3-day January outage? Cold, dark, and rationing? Or normal-ish life? The answer sets your generator size. In our field data, homeowners who bought 20–22 kW for a mid-sized home never regret it. The homeowners who bought 12 kW for the same home are usually calling back within two years asking to upgrade. Buy right the first time.

Frequently Asked Questions

What size generator for a 2,000 sq ft home?

Typically 15–18 kW for comfort coverage including central AC.

Do I need to include AC in the sizing?

Depends on whether you want AC to run during an outage. Summer outages without AC are miserable.

Can load management downsize my generator?

Yes. Automatic transfer switches with load management shed non-essentials automatically.

How do I know my exact loads?

Licensed contractor does a CEC load calc reading nameplates and applying demand factors.

Is bigger always better?

Not endlessly. Very oversized generators run inefficiently at low loads and cost more.

Related reading from ezsmart.ca

• Best Whole-House Generator
• Portable vs Standby Generator
• How to Install a Transfer Switch for a Portable Generator
• How to Calculate Home Electrical Load Capacity
• What to Do in a Power Outage

Contact us

Need a proper generator sizing for a Toronto or GTA home? Call ezsmart at 416-838-9006 or use our contact page. An ESA-licensed electrician will run a load calc and recommend the right kW size.