How to Choose the Right Generator Size for Your Home

How to Choose the Right Generator Size for Your Home

The right generator size for your home is determined by calculating the total wattage of the appliances and systems you want to power during an outage, accounting for both running watts and starting watts. Most Ontario homes require a standby generator between 14kW and 26kW, though smaller and larger homes fall outside this range.

Choosing the wrong generator size is the most common and most costly mistake homeowners make when investing in standby power. An undersized generator trips under load when you need it most. An oversized generator wastes thousands of dollars upfront and shortens its own lifespan through chronic underloading. Getting the size right is the foundation of a reliable, long-lasting backup power system.

This guide walks you through the sizing process step by step. For a definitive answer based on your home's actual electrical system, schedule a professional generator sizing consultation with our team.

Understanding Watts and Kilowatts

Before you can size a generator, you need to understand the units of electrical power measurement.

Watts (W)

A watt is the basic unit of electrical power. Every electrical appliance in your home has a wattage rating that indicates how much power it consumes during operation. A 60W light bulb uses 60 watts. A 1,500W space heater uses 1,500 watts.

Kilowatts (kW)

A kilowatt equals 1,000 watts. Generator capacity is rated in kilowatts. A 22kW generator can supply 22,000 watts of power continuously.

The Key Formula

To convert your total appliance wattage to the generator size needed:

Total watts ÷ 1,000 = kilowatts required

If your total load calculation shows 18,500 watts, you need at minimum an 18.5kW generator. In practice, you should select a generator with 20–25% more capacity than your calculated load to avoid running at 100% capacity continuously.

Starting Watts vs. Running Watts

This distinction is critical to accurate generator sizing and is the factor most commonly overlooked by homeowners doing their own calculations.

Running Watts (Rated Watts)

Running watts is the continuous power an appliance consumes during normal operation. A refrigerator uses approximately 150–400 running watts depending on size and efficiency. This is the steady-state power draw after the appliance has started and is operating normally.

Starting Watts (Surge Watts)

Starting watts is the brief spike of additional power an appliance draws when its electric motor first starts. This surge typically lasts only a fraction of a second to a few seconds, but the generator must be able to supply it or the appliance will not start — and the generator may trip its overload protection.

Motor-driven appliances have starting watts significantly higher than their running watts:

• Central air conditioner: 3,500–7,000 running watts / 8,000–15,000 starting watts
• Sump pump (1/2 HP): 800 running watts / 2,000 starting watts
• Well pump (1 HP): 1,500 running watts / 4,500 starting watts
• Refrigerator: 150–400 running watts / 1,200–2,000 starting watts
• Furnace blower: 500–800 running watts / 1,500–2,500 starting watts

Your generator must handle the scenario where multiple motor-driven appliances start simultaneously — for example, the air conditioner compressor kicks on while the sump pump activates during a storm.

Why Starting Watts Matter in Ontario

Ontario's climate creates scenarios where multiple high-draw appliances operate simultaneously:

• Summer storms: Air conditioning, sump pumps, and dehumidifiers all running and potentially restarting during the same outage
• Winter outages: Furnace blower, well pump, and space heaters running concurrently
• Spring thaw: Sump pumps cycling frequently while heating and cooling systems also operate

A generator sized only for running watts will fail in these real-world scenarios.

Step-by-Step Generator Sizing Process

Follow these steps to estimate the generator size your home needs. This provides a solid starting point, though a professional load assessment provides a more precise calculation.

Step 1: Decide What You Want to Power

First, determine whether you want whole-home backup (every circuit in your home) or essential-circuit backup (only critical systems). This decision is the single biggest factor in generator size and cost.

Essential circuits typically include:

• Furnace / boiler / heat pump
• Refrigerator and freezer
• Sump pump
• Well pump (if applicable)
• Kitchen lighting and a few outlets
• One bathroom
• Garage door opener
• Security system
• Internet/router

Whole-home backup adds:

• Central air conditioning
• Electric range/oven
• Clothes dryer
• Dishwasher
• All lighting and outlets throughout the home
• Electric vehicle charger
• Pool/hot tub equipment
• Workshop tools
• Entertainment systems

Step 2: List Your Appliances and Their Wattages

Create a list of every appliance you want to power and record both its running watts and starting watts. You can find wattage ratings on the appliance's nameplate, in the owner's manual, or on the manufacturer's website.

If a nameplate lists amps instead of watts, multiply by your voltage:

Amps × 240V = Watts (for 240V appliances like dryers, ranges, A/C)

Amps × 120V = Watts (for 120V appliances like refrigerators, lights, outlets)

Step 3: Use the Common Appliance Wattage Table

Use this table as a reference for typical wattage values when nameplate data is unavailable.

| Appliance | Running Watts | Starting Watts |

|---|---|---|

| Central air conditioner (3 ton) | 3,500 | 10,500 |

| Central air conditioner (5 ton) | 6,000 | 15,000 |

| Furnace blower (1/2 HP) | 600 | 1,800 |

| Heat pump | 4,700 | 14,100 |

| Electric water heater (40 gal) | 4,500 | 4,500 |

| Tankless water heater (electric) | 18,000–27,000 | 18,000–27,000 |

| Sump pump (1/3 HP) | 600 | 1,500 |

| Sump pump (1/2 HP) | 800 | 2,000 |

| Well pump (1/2 HP) | 1,000 | 3,000 |

| Well pump (1 HP) | 1,500 | 4,500 |

| Refrigerator | 150–400 | 1,200–2,000 |

| Freezer (upright) | 300–500 | 1,000–1,500 |

| Electric range/oven | 3,000–5,000 | 3,000–5,000 |

| Microwave (1,000W) | 1,000 | 1,000 |

| Dishwasher | 1,200–1,800 | 1,800–2,400 |

| Clothes dryer (electric) | 4,000–5,500 | 5,000–6,500 |

| Washing machine | 500 | 1,200 |

| EV charger (Level 2, 40A) | 7,700–9,600 | 7,700–9,600 |

| LED lighting (per bulb) | 8–15 | 8–15 |

| Garage door opener | 550 | 1,100 |

| Television (LED, 55") | 80–120 | 80–120 |

| Desktop computer | 200–500 | 200–500 |

| Router/modem | 15–30 | 15–30 |

| Security system | 100–200 | 100–200 |

| Pool pump (1.5 HP) | 1,800 | 5,400 |

| Hot tub | 3,000–6,000 | 6,000–12,000 |

Step 4: Calculate Your Total Load

Add up the running watts of all appliances you want to power simultaneously. Then identify the single largest starting watt value among your motor-driven appliances and add it to the running total. This accounts for the worst-case motor start scenario.

Formula:

Total running watts of all appliances + Largest single starting watt surge = Minimum generator capacity required

Example calculation for a typical 2,200 sq ft Ontario home (whole-home backup):

| Appliance | Running Watts |

|---|---|

| Central A/C (3 ton) | 3,500 |

| Furnace blower | 600 |

| Refrigerator | 300 |

| Freezer | 400 |

| Sump pump | 800 |

| Electric range | 4,000 |

| Microwave | 1,000 |

| Dishwasher | 1,400 |

| Washing machine | 500 |

| Lighting (20 LEDs) | 240 |

| Television | 100 |

| Computers (2) | 500 |

| Router/modem | 30 |

| Garage door opener | 550 |

| Security system | 150 |

| Total running watts | 14,070 |

| + Largest starting surge (A/C) | +10,500 |

| Peak demand | 24,570 |

In this example, the homeowner needs at minimum a 22kW generator to handle the peak demand when the air conditioner starts while other appliances are running. A 24kW or 26kW unit would provide a comfortable operating margin.

Step 5: Add a Safety Margin

Never size a generator to run at 100% capacity. A generator operating at full rated load continuously will overheat, consume excessive fuel, and experience accelerated wear. The ideal operating range is 50–75% of rated capacity during a typical outage, with the ability to handle brief peaks up to 100%.

Add 20–25% to your calculated running load to establish a comfortable operating margin. Using the example above:

14,070 running watts × 1.25 = 17,588 watts (17.6kW)

Combined with the starting surge requirement, a 22kW generator is the right fit — it handles the continuous load at approximately 64% capacity while having sufficient headroom for motor starting surges.

Generator Size Recommendations by Home Size

These are general guidelines based on our experience installing generators across Ontario. Your actual requirement depends on your specific appliances, heating system, and backup expectations.

Small Homes (Under 1,500 sq ft)

• Essential circuits only: 10kW–14kW
• Whole-home backup: 14kW–18kW
• Natural gas furnace, no central A/C on backup, standard appliances

Medium Homes (1,500–2,500 sq ft)

• Essential circuits only: 14kW–18kW
• Whole-home backup: 18kW–22kW
• Natural gas furnace, central A/C, standard kitchen appliances

Large Homes (2,500–4,000 sq ft)

• Essential circuits only: 18kW–22kW
• Whole-home backup: 22kW–30kW
• Multiple HVAC zones, large kitchens, home offices, workshops

Estate Homes (Over 4,000 sq ft)

• Whole-home backup: 30kW–48kW+
• Multiple HVAC systems, pools, EV chargers, guest houses, extensive outdoor lighting

Ontario-Specific Sizing Considerations

Ontario's climate and housing stock create unique sizing factors that homeowners in milder regions do not face.

Heating System Type

Your home's heating system has a major impact on generator sizing:

• Natural gas furnace: Requires only blower motor power (500–800W running). The furnace itself runs on gas, not electricity. This is the most generator-friendly heating option.
• Electric furnace: Draws 10,000–24,000 watts continuously. Electric furnaces require very large generators and are rarely backed up entirely; most homeowners switch to supplemental heating during outages or upgrade to gas/propane heat.
• Heat pump: Draws 4,000–7,000+ running watts with very high starting surges. Heat pumps with electric backup strips are especially demanding. Size your generator for the heat pump's starting requirement.
• Electric baseboard heating: Each zone draws 1,000–2,000 watts. Whole-home baseboard backup can require very large generators depending on the number of zones.

Sump Pump Requirements

Many Ontario homes rely on sump pumps to prevent basement flooding, especially during spring thaw and summer storms. Sump pumps are typically non-negotiable essential circuits for generator backup. If your home has multiple sump pumps, account for all of them running simultaneously during heavy rainfall.

Well Pump Considerations

Rural Ontario homes on well water lose all water supply during a power outage. Well pumps draw significant starting watts (3,000–6,000W for typical residential wells) and must be included in your generator sizing calculation. Deep wells with submersible pumps at 200+ feet draw even more.

Electric Vehicle Charging

An increasing number of Ontario homeowners have electric vehicles with Level 2 home chargers drawing 7,700–9,600 watts. If you want to maintain EV charging during outages, this load must be factored into your generator size. Some homeowners opt to exclude the EV charger from generator backup and charge at public stations during extended outages.

Air Conditioning in Summer Outages

Ontario's ice storm season gets the most attention, but summer heat waves cause outages too. Central air conditioning is the single largest electrical load in most homes and has very high starting surge requirements. If you want A/C during summer outages, plan for it in your generator sizing.

The Risks of Getting Size Wrong

Undersizing: When Your Generator Is Too Small

An undersized generator creates immediate problems during an outage:

• Overload shutdown: The generator trips its overload protection when too many appliances draw power simultaneously, leaving you in the dark
• Voltage and frequency instability: Running at or above rated capacity causes voltage drops and frequency fluctuations that can damage sensitive electronics
• Inability to start motor loads: Air conditioners, sump pumps, and well pumps may fail to start if the generator cannot supply their starting watts
• Accelerated wear: Sustained operation at maximum capacity dramatically shortens engine and alternator life
• Selective manual load management: You end up manually switching circuits on and off to stay within capacity — defeating the purpose of automatic standby power

Oversizing: When Your Generator Is Too Big

Oversizing is less immediately dangerous but still costly and harmful:

• Higher upfront cost: You pay thousands more for capacity you will never use
• Higher fuel consumption: Larger engines burn more fuel even at partial load
• Wet stacking: Generators running consistently below 30% of rated load experience carbon buildup in the exhaust system, which degrades engine performance and can cause costly repairs
• Larger physical footprint: Bigger generators take up more yard space, require larger concrete pads, and may be harder to place within setback requirements

The ideal generator runs at 50–75% of its rated capacity during a typical outage scenario. This balances efficiency, longevity, and headroom for peak demands.

Why Professional Load Assessment Matters

While the step-by-step process above gives you a reasonable estimate, a professional load assessment provides accuracy that a DIY calculation cannot match.

What a Professional Assessment Includes

During a generator sizing consultation, our technicians:

• Inspect your electrical panel and identify every circuit, its amperage, and its purpose
• Measure actual loads on critical circuits using clamp meters, capturing real-world consumption rather than nameplate maximums
• Account for diversity factors — in practice, not every appliance runs at full load simultaneously, and professional sizing methodologies account for this statistically
• Evaluate your heating and cooling systems including staging, auxiliary heat, and defrost cycles that affect peak demand
• Consider future electrical additions such as EV chargers, home office equipment, pool installations, or HVAC upgrades you are planning
• Apply National Electrical Code and manufacturer guidelines for proper generator selection

The Cost of Getting It Wrong

A professional sizing consultation costs a fraction of the price difference between generator sizes. Buying a 22kW generator when you actually needed 18kW wastes $3,000–$5,000. Buying an 18kW unit when you needed 22kW leaves you without reliable power during the outages that matter most. Neither outcome is acceptable when a professional assessment eliminates the guesswork entirely.

Get Your Home Professionally Sized

Choosing the right generator size is the most important decision in your standby power investment. An accurately sized generator delivers reliable, efficient, and long-lasting backup power. An incorrectly sized unit costs you money — either through overspending upfront or through unreliable performance when you need it most.

Ontario Generator Systems provides free generator sizing consultations as part of our site inspection process. Our technicians perform a complete load assessment, evaluate your property for optimal generator placement, and recommend the exact generator size and model for your needs.

[Schedule your free site inspection and sizing consultation today](/contact) — no obligation, no pressure. We will give you a clear, data-driven recommendation so you can invest with confidence. For more information about the installation process, visit our residential generator installation page or check our FAQ for quick answers to common questions.