Generator Size Calculator Canada

Quick answer

Size a generator from the loads you actually need during an outage, not the square footage of your house. Add the running watts of the loads that may operate together, account for the largest likely motor-starting surge, leave planning headroom, and make sure the generator provides the correct voltage. A 120 V-only generator cannot run a 240 V well pump just because the wattage number looks big enough.

The calculator below works in both directions: “What size generator do I need?” and “What can my generator run?”. The presets are editable planning values. Replace them with the real nameplate or manufacturer numbers whenever you have them.

How to size a generator without lying to yourself

Generator sizing gets messy when every appliance is treated as one big pile of watts. A refrigerator compressor, sump pump, furnace blower, coffee maker, and router do not behave the same way. This calculator separates them into a few practical groups.

• Continuous loads stay on, such as networking gear, lighting, or other small electronics.
• Automatic cycling loads can start without asking you first, such as a refrigerator, freezer, sump pump, furnace blower, or well pump.
• Run-separately loads are things you can deliberately keep off until the automatic equipment is quiet, such as a microwave, kettle, coffee maker, or portable heater.
• Exact-data loads are deliberately blocked from generic automatic sizing when the equipment varies too much for a responsible preset.

For the normal sizing mode, the calculator adds the selected must-run loads and then adds the largest startup delta: the difference between one motor load's starting watts and running watts. That models the common case where one motor starts while the other selected loads are already running.

The optional coincident-start setting adds the two largest startup deltas. It is deliberately conservative for households where two automatic motor loads could start close together. Summing every appliance's peak surge would usually create a comically oversized number that nobody actually operates at for more than a blink.

Running watts vs starting watts

Running watts are the sustained electrical load after equipment is operating normally. Starting watts, sometimes called surge or peak watts, are the short burst some motors and compressors need while starting.

This matters because a generator can have enough continuous capacity and still fail to start a pump or compressor. The reverse mode reports running-load and startup-load bottlenecks separately instead of pretending one headline wattage answers everything.

120 V vs 240 V can matter more than total watts

Many compact portable generators provide 120 V only. Larger portable generators may provide 120/240 V output. If a selected load requires 240 V, the calculator flags 120/240 V output as mandatory.

This is especially important for rural homes with submersible well pumps and for other hardwired 240 V equipment. An 8,000 W 120 V-only source still cannot operate a 240 V load. Voltage compatibility is a gate, not a footnote.

Franklin Electric's current AIM manual library is one example of why pump sizing deserves exact equipment data: submersible motor and control configurations materially change generator requirements. The calculator's well-pump preset is therefore a planning starting point and is marked for verification.

Why “what size generator for my house?” is the wrong starting question

Generators power electrical loads, not floor area. A large house with natural-gas heat, municipal water, gas cooking, and a gas water heater may have a modest emergency electrical load. A smaller rural all-electric home can have a 240 V well pump, sump equipment, electric water heating, and a heat pump that completely changes the problem.

So there is no responsible “X watts per square foot” rule. Build the outage load list first. Decide what absolutely has to run, what can cycle, and what you are willing to run one at a time.

Load management can save thousands of watts

A kettle or microwave may draw around the same power as several essential loads combined, but only for a few minutes. If you mark those loads Run separately, the calculator excludes them from the must-run baseline and shows a second “everything selected” target.

That lets you compare two very different plans:

1. Managed outage: fridge, freezer, sump, furnace, router, and lights can cycle normally while high-draw conveniences are used one at a time.
2. Everything at once: enough capacity that the selected convenience loads can run without deliberate load rotation.

For many households, the first plan is cheaper, easier to fuel, easier to move, and perfectly adequate during an outage.

What can a 2,000, 3,500, 5,000, or 7,500 watt generator run?

There is no useful universal list because the answer depends on three separate limits: running capacity, startup capacity, and voltage. A 5,000 W generator with weak surge capacity can behave very differently from another 5,000 W model. A 120 V-only model can also fail a 240 V load that a lower-wattage 120/240 V generator could operate.

Use the calculator's “What can my generator run?” mode. Enter the model's published running watts, starting watts, and output voltage, then select the appliances you care about. The result reports the actual bottleneck and tells you whether manually managed loads need to be deferred.

Gasoline, propane, and natural-gas ratings

Multi-fuel generators can publish different output ratings depending on the fuel. Do not apply a made-up universal derating percentage. Use the manufacturer's rating for the fuel you will actually run.

For example, Champion's current 201121 tri-fuel generator is published at 8,000 running watts on gasoline, 7,200 W on propane, and 7,000 W on natural gas. The differences are model-specific. See the manufacturer specifications.

In reverse mode, enter the numbers for your actual fuel. In size-my-generator mode, compare the calculator target with the generator's rating on the fuel you expect to use. If you are buying tri-fuel specifically for natural-gas backup, do not shop from the prettier gasoline wattage printed in the biggest font.

What the preset numbers mean

Presets are there to make the tool usable when the power is out and the manual is hiding somewhere in the junk drawer. They are not substitutes for equipment data. Refrigerators vary. Sump pumps vary. Older PSC furnace blowers and newer ECM motors behave differently. Well-pump motor and control-box configurations can differ dramatically.

Before buying a generator around the result, check these sources in roughly this order:

1. The appliance or motor nameplate.
2. The exact model's manual or manufacturer technical data.
3. The generator's published running and starting ratings for the fuel you will use.
4. A qualified electrician or HVAC/pump professional when the load is hardwired, unusually large, or technically ambiguous.

For AC motor equipment, volts × amps is useful as an apparent-power check, but it is not always identical to true running watts because power factor and motor behaviour matter. If the manufacturer provides running watts, starting watts, locked-rotor amps, or a generator-sizing table, use that information instead of reverse-engineering a guess.

Loads the calculator intentionally refuses to guess

Central air conditioners and ducted heat pumps are the obvious example. Compressor design, locked-rotor current, inverter drives, soft starters, and auxiliary electric heat can change the required generator capacity by an absurd amount. Generic “3-ton AC = X watts” shortcuts are not good enough for a purchase decision.

The calculator therefore pauses the automatic recommendation when an exact-data load is selected. That is not a bug. It is the tool declining to manufacture a confident-looking number where the equipment data actually matters.

Altitude, weather, and other real-world derating

High altitude, ambient temperature, fuel supply, maintenance condition, and exact generator design can reduce real-world available output. Manufacturer derating instructions vary, so this calculator does not apply a universal altitude or cold-weather percentage behind your back.

If your generator manual specifies an altitude or environmental derating, compare the calculator target against the derated available output, not the ideal sea-level box number.

Generator connection and safety

Canada.ca says portable fuel-burning generators should be operated outside at least 6 m / 20 ft from homes or buildings, with exhaust directed away from openings, working carbon-monoxide alarms, and properly rated CSA-approved extension cords when cords are required. It also says household electrical-system connections require approved transfer equipment installed by a qualified electrician.

Read the full generator safety guide before operating one, and the transfer switch / interlock guide before planning household-circuit backup.

Frequently asked questions

What size generator do I need to run a house?

Size the electrical loads you want during the outage, not the house's square footage. A gas-heated home on municipal water may need far less generator capacity than a smaller all-electric rural home with a 240 V well pump. Use the first calculator mode and select only the loads that genuinely matter.

What size generator do I need for a refrigerator and freezer?

The answer depends on each compressor's running and startup demand. Select the refrigerator and freezer presets, then replace the estimates with your actual model data when possible. The calculator will add the larger startup delta to the combined running load instead of summing every peak simultaneously.

What size generator do I need for a sump pump?

Pump horsepower alone does not fully determine the answer. Motor design changes running and starting current. Use the sump-pump preset for planning, then verify the pump label or technical sheet before purchasing a generator around it.

What size generator do I need for a furnace?

A gas or oil furnace still needs electricity for blowers, controls, ignition, and sometimes additional motors. Older PSC blowers can start much harder than modern ECM motors. Use the furnace preset as a starting point and verify the actual furnace electrical data.

What size generator do I need for a well pump?

Well pumps are one of the loads where exact data matter most. Many residential submersible pumps require 240 V, and motor/control configuration can change the starting requirement. Select the well-pump preset to trigger the 240 V check, then verify the exact pump using the nameplate, control-box data, or manufacturer guidance.

Can a 120 V generator run a 240 V appliance if it has enough watts?

No. Total wattage does not create a missing voltage. A selected 240 V load requires a generator with compatible 120/240 V output.

What can a 5,000 watt generator run?

Enter 5,000 W as the running rating in reverse mode, then enter the actual starting rating and voltage from the generator. Select your loads. The tool checks sustained load, startup surge, voltage compatibility, and whether manually managed appliances should be run separately.

Should I add 25% to starting watts?

Not automatically. This calculator uses configurable planning headroom for the sustained running target and reports startup demand separately. Generator surge ratings and motor-start behaviour are not standardized enough for a universal “always add 25% to peak watts” rule.

Can this calculator size my transfer switch or interlock?

No. It estimates generator capacity and voltage compatibility. Transfer equipment, inlet hardware, panel circuit selection, grounding/bonding, and permits belong with a qualified electrician. See the transfer switch guide.