How to Size a Generator for Your Home: Complete Wattage Guide

A generator is only as useful as the load it can actually support. Many Ontario homeowners buy a portable unit on sale, take it home, and then trip breakers when the furnace, well pump, and fridge start together. Sizing starts with a list of what must run during an outage, not a guess. This guide walks through the math in plain language. If you are still comparing whole-home options, see our best-home-backup-power-2026 roundup.

The key concept is the difference between running watts and starting watts. A fridge may need 700 watts once it is running, but its compressor can briefly draw 2,200 watts when it starts. Motors in furnaces, sump pumps, and well pumps have the same behaviour. Ignoring that surge is the most common sizing mistake. For a full week-long outage plan, read how-to-survive-week-long-power-outage.

Safety shapes the electrical side of sizing. In Ontario, you cannot simply backfeed a portable generator into a wall outlet. Doing so can kill a utility worker and is illegal. A manual transfer switch or interlock kit is required for any connection to your home wiring. Our generator-safety-tips guide covers placement, carbon monoxide, and fuel handling.

This article uses the 2026 Ontario landscape, including current Electrical Safety Authority permit rules and CSA certification expectations. By the end, you will know whether a 5,000-watt portable is enough, when you need a 10,000-watt surge rating, and how to plan fuel for a 72-hour outage. Accurate sizing saves money, reduces fuel burn, and keeps critical loads running.

What You’ll Need

• Notebook and pen for load inventory
• Appliance watt meter or manufacturer spec sheets
• Calculator
• CSA-certified generator with rated running and surge watts
• Manual transfer switch or interlock kit installed by a licensed electrician
• Battery-powered carbon monoxide detector
• Heavy-duty outdoor extension cords, 12-gauge or thicker
• Approved fuel containers and fuel stabilizer

How Do You Size a Generator for Your Home?

1. List the critical loads you need in an outage

Start with survival and health, not comfort. Write down the fridge, freezer, furnace blower, sump pump, well pump, a few lights, internet router, and any medical equipment. If someone in the house uses a CPAP, oxygen concentrator, or home dialysis, that load goes first. See our emergency-power-medical-equipment guide for specific wattage values.

The reason this step matters is simple: a generator is sized for the actual connected load. Too many appliances on a small unit will trip the breaker and leave you in the dark. Milk, meat, and medication spoil faster than most people expect. A frozen pipe from a dead well pump can cost far more than a larger generator.

A common mistake is trying to power the whole house from a portable. That often requires 15,000 to 20,000 watts. Instead, rotate non-critical loads. Run the freezer for four hours, then switch to the fridge. To understand how long a power station can carry a fridge, see how-long-power-station-run-refrigerator. This rotation method keeps the generator smaller and lowers fuel burn.

2. Find running watts and starting watts for each load

The next step is to put a number beside each item on your list. Look for the specification plate on the back, side, or bottom of the appliance. It may list watts directly, or list volts and amps. If it shows amps, multiply amps by volts: 120 volts times 12 amps equals 1,440 watts.

For motor-driven appliances, you need two numbers. Running watts is the continuous draw after the motor reaches speed. Starting watts, sometimes called surge watts, is the brief spike when the motor starts. A typical refrigerator uses 700 running watts but can need 2,200 starting watts. A well pump may use 1,500 running watts and 4,100 starting watts. These are real Ontario field values, not worst-case guesses.

If the label is worn or missing, use a plug-in watt meter. Many are available at hardware stores and let you measure running watts safely. For starting watts, check the manufacturer’s spec sheet or search the model number online. Do not guess a surge value. Underestimating a furnace blower surge is why many generators stall on the first cold morning.

3. Add running watts and the single largest surge

Now do the math. Add the running watts of everything you plan to run at the same time. Then add the largest single starting watt value from your list. That total is the minimum surge watts the generator must deliver for a few seconds. The generator’s running watt rating must cover the first number, not the surge total. For well pump details, see power-well-pump-outage.

Here is a practical example. Fridge 700 watts, freezer 600 watts, furnace blower 800 watts, three LED lights 30 watts total, and a sump pump 1,000 watts. Running total is 3,130 watts. The largest starting surge is the sump pump at 2,900 watts. Minimum generator sizing: 3,130 running watts and 6,030 surge watts.

Do not add all starting surges together. Each motor starts at a different moment if you stagger them. The exception is automatic equipment that can start together. In a wet Ontario basement, two sump pumps can cycle at the same time. If that is possible, add both surge values to the running total instead of only the largest. This matters for spring thaw and heavy storm backup.

4. Apply Ontario electrical code and transfer switch rules

A portable generator cannot be connected to your home wiring through a dryer outlet or any wall receptacle. This is backfeeding and it is illegal in Ontario. It can energize the utility line and kill a line worker. The Electrical Safety Authority requires a permit and a licensed electrician for a transfer switch or generator panel installation.

The correct way is a manual transfer switch or an interlock kit. A manual transfer switch has breakers for selected circuits, so you choose which loads get generator power. An interlock kit physically prevents the main breaker and generator breaker from being on at the same time. Both must be certified to Canadian standards and installed by a licensed electrical contractor.

The CSA Group publishes safety standards for generators and transfer equipment. Look for a CSA mark on any generator, cord, or transfer switch you buy. In Ontario, unlisted equipment from online marketplaces may not pass inspection. This is not a paper detail. It is a fire and shock hazard.

If you plan to use extension cords only, you can skip the transfer switch. That keeps the generator separate from the home electrical system. But you must still use outdoor-rated cords and follow load limits. Many smaller setups use this method for a fridge and a few lights.

5. Choose the right generator type and fuel

Now match your numbers to a generator category. Inverter portables are quiet and fuel-efficient for 1,800 to 4,500 watts. Conventional portables cover 5,000 to 12,000 watts but are louder. Portable power stations are silent and good for small medical or electronics loads. See best-portable-generators-2026 and best-portable-power-station-2026 for current Ontario picks.

For a critical-load panel of 5,000 to 7,500 running watts, a conventional portable like the Generac GP6500 works. It is rated at 6,500 running watts and 8,125 starting watts. Check the Generac GP6500 on Amazon. For a smaller setup with a fridge and CPAP, the Honda EU2200i offers 1,800 running watts and 2,200 surge watts. Check the Honda EU2200i on Amazon. These are examples, not blanket recommendations for every home.

Fuel matters as much as wattage. Gasoline is widely available but degrades and is hard to store. Propane stores for years and burns cleaner, and it works better in cold temperatures. Natural gas makes sense for standby units where the line is reliable. Compare the trade-offs in propane-vs-natural-gas-generator. Choose a fuel you can actually source during a winter outage.

6. Add a buffer and check the 80 percent rule

Never run a generator at full rated output for long. Most manufacturers and electricians recommend keeping continuous load at or below 80 percent of rated running watts. If your total running load is 4,000 watts, choose a generator rated at least 5,000 running watts. This buffer reduces heat, voltage drop, and breaker trips.

In the earlier example, running load was 3,130 watts. At the 80 percent limit, you need a generator rated at least 3,913 running watts. That means a 4,000-watt unit is barely enough, while a 5,000-watt unit is a safer choice. The extra capacity also handles small lights or chargers you forgot to list.

Cold weather and altitude reduce output. In Ontario, winter cold thickens oil and makes starting harder. At elevations above 1,000 feet, derate output about 3 percent for every additional 1,000 feet. Not a big issue in Toronto, but it matters near Collingwood or Algonquin. For continuous run limits, see how-long-can-a-generator-run-continuously.

7. Plan fuel supply and runtime for a multi-day outage

Fuel planning is part of sizing. A 5,000-watt generator at roughly 50 percent load burns about 0.75 gallons of gasoline per hour. Over a 24-hour day, that is 18 gallons. For a 72-hour outage, you need 54 gallons, not counting warm-up and cool-down cycles. That is a lot of fuel to store and rotate safely.

Propane is easier to store. A 20-pound propane tank on a 5,000-watt generator may run 8 to 10 hours at half load. A 100-pound tank can give you roughly 40 to 50 hours. You need a safe outdoor spot for tanks, and you need to know which local supplier can refill during a storm.

Never store gasoline inside your house or attached garage in large quantities. Use an approved container, add fuel stabilizer, and rotate fuel within 30 days if untreated. For generator runtime limits, see how-long-can-a-generator-run-continuously. For fuel trade-offs, revisit propane-vs-natural-gas-generator. A generator that runs out of fuel at 3 a.m. is not a backup plan.

8. Install, test, and run the system safely

Hire a licensed electrician for the transfer switch or interlock installation. In Ontario, the electrician pulls the permit through the Electrical Safety Authority. The work must meet the Ontario Electrical Safety Code. Never attempt to wire a generator inlet or transfer switch yourself if you are not licensed. The risk is line voltage and it can be lethal.

Position the generator at least 20 feet from the house, away from windows, doors, and vents. Carbon monoxide from a small generator can exceed Health Canada’s 10 ppm 24-hour indoor guideline very quickly if it enters a living space. Never run a generator in a garage, even with the door open. Use a battery-powered CO detector inside the home.

Extension cords must be outdoor-rated and thick enough. For a 15-amp load up to 50 feet, use at least 12-gauge cord. For 20 amps or distances over 75 feet, use 10-gauge. Feel the cord after 30 minutes. If it is hot to the touch, reduce the load or use a heavier cord.

Test the system monthly. Start the generator, transfer the critical circuits, and run for 15 minutes under load. Check for fuel leaks, stale fuel, and CO detector battery. Annual service by a qualified technician keeps a standby unit ready. For the full safety checklist, see generator-safety-tips.

Red Flags & Warnings

• 🚨 Never backfeed a generator into a wall outlet or dryer plug. It is illegal in Ontario and can kill utility workers. Install a transfer switch or interlock kit with a licensed electrician.
• 🚨 Never run a generator inside a home, garage, basement, or enclosed porch. Carbon monoxide can build to lethal levels in minutes. Keep the generator at least 20 feet from any door, window, or vent.
• 🚨 Do not overload your generator. If continuous load exceeds 80 percent of rated running watts, the breaker may trip and appliances can be damaged. Use the buffer method in Step 6.
• 🚨 Let the generator cool before refueling. Spilled gasoline on a hot engine can ignite. Store fuel in an approved container away from the house.
• 🚨 Do not use undersized extension cords. A 16-gauge cord on a 15-amp load can overheat and melt. Match cord gauge to amperage and distance.
• 🚨 Do not rely on the manufacturer’s maximum wattage sticker. Use actual starting surge numbers. Undersizing can stall the fridge or furnace blower exactly when you need it.

Frequently Asked Questions

What Should You Remember?

• Running watts: Add the continuous watts of each appliance you must run at the same time.
• Starting surge: Add the single largest starting watt value, not all surges together, unless two motors auto-start together.
• 80 percent rule: Keep continuous load at or below 80 percent of the generator’s rated running watts.
• Transfer switch: Any connection to home wiring needs a transfer switch or interlock kit and a licensed Ontario electrician.
• Safety margin: A typical critical-load panel needs a 5,000 to 7,500-watt portable generator.
• Fuel plan: A 5,000-watt generator can burn about 0.75 gallons of gasoline per hour at half load, so store or source fuel accordingly.

This article is for general information only. Always follow manufacturer instructions, local electrical codes, and hire a licensed electrician for transfer switches and standby generator installation. Carbon monoxide is deadly , never run a generator indoors.