Generator Size Calculator for Canadian Power Outages

Quick answer

To estimate generator size, add up the running watts of your essential loads, identify the largest motor-starting surge, and add headroom. Do not guess on hardwired equipment, medical devices, or safety-critical loads. This calculator is an estimate for planning — not electrical advice.

How the calculator works

The calculator follows a straightforward approach that avoids false precision. Here is the logic:

• Total running watts: Sum of the running watts for every load you toggle on.
• Startup surge (delta): For each motor or inductive load, starting watts are higher than running watts. The calculator takes the difference for each load.
• Largest startup delta: Rather than summing every startup surge (which overestimates — motors rarely all start at once), the calculator adds the single largest startup delta to the total running watts.
• Conservative mode: Adds the two largest startup deltas instead of one. Use this if you expect multiple motor loads could start close together.
• Headroom: A percentage buffer (default 25%) added to both running and starting recommendations. Adjust down if you know your loads well, or up for extra safety.

The result is an estimate, not a guarantee. Always compare against the actual nameplate ratings on your equipment. Check the label and manual before buying gear around this number.

What to check on real appliances

The default watt values in the calculator are representative estimates. Your actual appliances may differ. Before buying a generator, check:

• Nameplate labels on the appliance — look for watts (W) or amps (A) and volts (V).
• Owner manuals — many list both running and starting watts.
• Manufacturer specs online for your exact model.
• Running watts vs starting watts — motors, compressors, and pumps can draw 2–5 times their running watts for a split second during startup.
• Watts = Volts × Amps — if the label lists amps, multiply by the circuit voltage (typically 120V or 240V in Canada) to estimate watts.

What not to run on a portable generator

Some loads are a bad match for portable generators. Do not plan around these unless you are working with a licensed electrician on a properly sized system:

• Electric space heaters — a 1500W heater draws near the limit of many small portables by itself.
• Electric ranges and ovens — these can pull 3000–8000W.
• Clothes dryers — typically 3000–5000W.
• Central air conditioning — high starting surge and continuous draw. Not a portable generator load.
• EV chargers — Level 1 chargers draw 1200–1800W continuously. Level 2 chargers need dedicated circuits and professional install.
• Whole-home panels without professional transfer equipment — never backfeed.

Extension cords and connection safety

If you are plugging appliances directly into the generator, follow these guidelines:

• Use outdoor-rated extension cords rated for the total load.
• Keep connections dry and off the ground.
• Do not daisy-chain power bars or extension cords.
• Do not overload cords — match cord gauge to load and length.
• Do not backfeed a house through a dryer outlet, range outlet, or any improvised wiring.

Transfer switch and interlock safety

If you want to power household circuits rather than running extension cords, a transfer switch or approved interlock is required. This is electrical work that needs a licensed electrician — not a DIY wiring project.

A licensed electrician can assess your panel, determine whether an interlock is permitted for your panel model and local jurisdiction, and install the appropriate inlet, cabling, and safety hardware.

Read the full transfer switch guide for questions to ask an electrician and what to expect. Also see the generator safety page for placement, CO, and fuel safety.

Frequently asked questions

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

A fridge typically runs at 150–200W and surges to 600–1200W. A freezer is similar. Together, a small portable generator rated around 2000W starting is usually enough — but confirm your specific models.

What size generator do I need for a sump pump?

A 1/2 HP sump pump can run at 800–1000W and surge to 2000W or more. Add fridge/freezer loads on top, and you are often looking at a mid-size portable with 3000W+ starting rating. Confirm your pump specs before buying.

Can a portable generator run a whole house?

Not in the sense of running every circuit the way grid power does. A portable generator can power selected essential loads through extension cords or a professionally installed transfer switch. Whole-home standby generators with automatic transfer switches are a different category — higher cost, professional install, and often running on natural gas or propane.

How much starting watts do I need?

Add the running watts of everything you plan to run, then add the largest single starting surge. That gives you a minimum starting-watt target. Then add 25% headroom. The calculator does this automatically.

Can I plug a generator into my house panel?

No. Backfeeding a panel through a suicide cord or improvised wiring is dangerous, illegal in most jurisdictions, and can kill utility workers, damage your equipment, and start fires. Use a licensed electrician to install a transfer switch or approved interlock.

Can I run a generator in a garage if the door is open?

No. Carbon monoxide from a generator can build up to lethal levels in a garage within minutes, even with the door open and a fan running. Never run a fuel-burning generator indoors, in a garage, shed, balcony, or any attached structure. Always place it outside, well away from doors, windows, and vents.

Is a portable power station safer indoors?

Battery-based portable power stations do not produce carbon monoxide, so they can be used indoors. However, they have limited capacity compared to fuel generators. Check running watts and watt-hour capacity to see if the unit can handle your loads. See the portable power station guide for details.