Add up the running watts of everything that must run at the same time, add the extra starting surge of the biggest motor on top, and buy a generator whose running and starting watts both clear those totals. Read the figures off each appliance’s nameplate rather than a generic chart. For the furnace, fridge and sump pump in most detached homes, that points to a mid-size 120/240 V portable such as the Generac GP8000E, not a small inverter or a whole-house standby.
Sizing is the question owners get wrong most often after an outage: a small unit bought in a hurry that can’t start the sump pump, or a standby quoted far bigger than the essentials need. This guide gives you a worksheet to fill in, explains running and starting watts, and shows where a portable, a home standby generator or a battery fits. All of the options are in our backup power section.
Safety first: whatever size you buy, a fuel-burning generator runs outdoors only, at least 6 metres (20 feet) from any building per Health Canada, never in a garage or shed even with the door open. Never plug it into a wall outlet or the panel: backfeeding can kill line crews. Wired-in loads need an approved transfer switch installed by a qualified electrician, as Canada’s Get Prepared guidance puts it; our hookup guide has the provincial and US rules.
Once you know the size, the power outages page shows the four ways to connect backup power and which of them need an electrician.
The short answer
- Size from your own list. The loads you choose to run at once, from their nameplates, decide the size; the house’s square footage doesn’t.
- Starting watts catch people out. Pumps, compressors and blower motors draw a short surge on start-up that can trip a generator sized only on running watts.
- 240 V and the hookup matter as much as watts. A well pump or a transfer switch needs a 120/240 V outlet, and wired-in loads need an electrician.
Step 1: Fill in the load worksheet
List every load you want during an outage, with its running watts, its starting watts if it has a motor, its voltage, and whether it must run at the same time as everything else. Copy the table below onto paper and fill in one row per appliance. We have left the numbers blank on purpose: appliances of the same type vary a lot, so a typical figure from a chart can be well off for yours.
| Load | Running watts (nameplate) | Starting watts (motors only) | 120 V or 240 V | Must run at the same time? |
|---|---|---|---|---|
| Furnace blower and controls | ||||
| Fridge | ||||
| Freezer | ||||
| Sump pump | ||||
| Well pump | ||||
| Lights | none | |||
| Modem, router and chargers | none | |||
| Medical device (CPAP, oxygen) | ||||
| Anything else | ||||
| Total | A = sum of running watts | B = the biggest single surge |
It is worth doing properly, because owners who size from a guess tend to buy twice. Forum threads after outages are full of people whose first generator couldn’t start a pump, and standby buyers who were quoted a unit much bigger than their essential loads needed.
Be honest about what must run together. In a winter outage that is usually the furnace blower, fridge, sump pump and well pump, with lights and chargers. Electric heat, a water heater, an electric range, a dryer or central air each change the answer completely, and are often better left off the list.
Step 2: Find the real figures
Start with the nameplate or rating label that every appliance carries, usually inside the door, on the back or on the motor. It gives watts, or amps and volts; if only amps are listed, multiply the amps by the voltage (120 or 240) for a rough wattage. The appliance’s manual or its maker can confirm the figure.
Wattage charts, like the one Champion publishes, are useful for a first pass and show which loads have a starting surge, but they give ranges. Your own appliances are what the generator has to carry.
To see what a plug-in load actually draws, owners recommend a plug-in watt meter. For wired-in loads such as a furnace or well pump, an electrician can measure the current with a clamp meter, including the start-up surge. Don’t open a panel or a furnace yourself to do it.
Step 3: Running watts vs starting watts
There are two numbers to know. Running watts are what a load uses once it is going. Starting (surge or peak) watts are the brief extra a motor needs to get moving: pumps, compressors in fridges and freezers, and blower motors. Lights and chargers have no meaningful surge. Generators are rated the same two ways; the Generac GP8000E, for example, is rated at 8,000 running and 10,000 starting watts.
The surge is the one that catches people out. If the total running load fits but a pump’s start-up surge doesn’t, the generator stalls or trips its breaker at exactly the wrong moment, and Generac warns that running more than a generator’s rating can damage the generator and whatever is plugged into it.
The worksheet turns this into a simple rule. Total A is the running watts of everything on at once. B is the largest single starting surge (that appliance’s starting watts minus its running watts). The generator’s running watts should be above A, and its starting watts above A plus B. Then stagger motor loads: start the biggest one first and switch the others on one at a time, so two surges never land together. Don’t plan to run at the very limit of the rating.
Step 4: Check voltage and the hookup
Watts are only half the check. Small inverter generators give 120 V only; larger portables add a 120/240 V outlet, usually an L14-30R twist-lock, sometimes a 14-50R. Many well pumps run on 240 V, and a home transfer switch or inlet is normally fed from that 240 V outlet. So a generator with enough watts but no 240 V outlet can’t run a 240 V well pump or feed a whole-panel transfer switch. The Westinghouse iGen2200c, for instance, has no 30 A or 240 V outlet at all; the Champion 201319’s largest outlet is a 120 V TT-30R.
If any row in your worksheet says 240 V, or the load is wired in rather than plugged in, look for an L14-30R or 14-50R and budget for an electrician. Plug-in loads can run on heavy-duty, outdoor-rated extension cords with a current rating to match, plugged straight into the generator. Interlock or transfer switch explains the two ways to connect a panel, and the transfer switches we track are listed separately.
Step 5: Your furnace and clean power
Modern gas furnaces have electronic control boards, and an inverter generator produces power with less waveform distortion (owners call it low THD, or clean power) than a typical open-frame one. Several owners report that a high-efficiency furnace ran erratically or not at all on an open-frame generator, and fine on an inverter. Not every furnace is affected, and a hardwired furnace still needs a proper transfer switch whatever the generator.
Before you choose, check your furnace manual or ask its maker what generator power it accepts. If you want inverter power with propane, the Champion 201319 is our inverter pick; if you need 240 V too, an open-frame model like the Generac GP8000E gives it, so confirm your furnace’s requirements first.
Step 6: Essentials or the whole house
Your total can be covered three ways. A portable generator covers essentials, and you start it and connect it yourself. A standby generator is permanently installed, runs on natural gas or propane and starts on its own. A battery power station is silent, makes no exhaust and stores a fixed amount of energy.
Standby sizes are much bigger. Our essentials pick is Generac’s 14 kW standby; its 22 kW model is the one we suggest for whole-house backup. Generac’s spec sheet also says output drops about 3.5% per 1,000 ft of altitude and about 1% per 10 °F above 60 °F, and recommends a cold weather kit below freezing. A battery’s continuous output is only part of the answer, because its capacity runs down: the EcoFlow DELTA Pro 3 gives 4,000 W at 120/240 V from 4,096 Wh, and expands with extra batteries. Dividing the battery’s watt-hours by your total A gives a best-case figure for hours of runtime.
A portable is the simplest answer if your total A fits one and you are happy to run it. If you need automatic backup, are away often, or have medical equipment, look at standby or a panel-connected battery. All three need a licensed electrician to connect to the panel. Power station vs generator compares the two portable options in depth.
Step 7: Fuel and runtime
Makers quote runtime at a stated load, usually 25% or 50%. Generac gives the GP8000E about 10 hours at half load on its 7.9-gallon tank; DuroMax gives the XP13000HXT about 8.5 hours at half load and 17 at a quarter.
Plan the fuel before the storm. In an ice storm, gas stations lose power and roads close, and owners in long outages describe running out of fuel before the power came back. From the maker’s runtime figures, work out how much fuel a day your load needs, then how much you can store safely outdoors. Propane keeps better than gasoline; Generac advises adding stabiliser to gasoline and draining it before storing a generator for more than 30 days. A standby on natural gas doesn’t need refuelling.
What each budget buys
Rough bands, in US dollars; Canadian prices usually run higher. Installation is extra wherever a transfer switch is involved.
Budget
Under about $1,000
A small inverter for a fridge, lights and chargers on cords, or a basic 8,000 W gasoline portable with a 240 V outlet. You give up quiet running with the bigger one, and 240 V with the small one.
Mid-range
About $1,000 to $3,000
Large dual-fuel or tri-fuel portables with 50 A outlets for more of the house, or a large battery power station for silent backup of a few circuits.
Premium
Over $3,000, plus installation
A home standby generator or an expanded battery system that starts by itself. Owners report installed costs well above the price of the unit, so get quotes based on a load calculation.
Our top picks
One pick for each common answer to the worksheet, from a fridge-and-lights inverter to whole-home backup. For the full ranking of portables, read the best generators for a power outage in Canada.
-
Best overall
Generac GP8000E Portable Generator with COsense
8,000 W with COsense, L14-30R and a CSA version for Canada
- 8,000 W running
- Gasoline
- CO shutoff
-
Best budget
Westinghouse iGen2200c Inverter Generator with CO Sensor
Light 1,800 W inverter with CO shutdown for the basics
- 1,800 W running
- Gasoline
- CO shutoff
-
Best inverter
Champion 4500W Electric Start Dual Fuel Inverter with CO Shield
Quiet dual-fuel inverter, CO Shield, stated PGMA G300-2018
- 3,500 W running
- Gasoline or propane
- CO shutoff
-
Best for essentials
Generac 14kW Air-Cooled Home Standby Generator
14 kW on either fuel, US and Canada certified, smaller fuel draw
- 14 kW on propane
- 14 kW on natural gas
- 5-yr warranty
-
Best whole-home
EcoFlow DELTA Pro 3 Power Station
4–48 kWh LFP with 120/240 V for home-panel backup
- 4,096 Wh battery
- 4,000 W AC output
- expandable to 48 kWh
CA$2,999–CA$4,499at EcoFlow Canada · prices may have changedPrice checked . Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on EcoFlow Canada at the time of purchase will apply to the purchase of this product.Check price at EcoFlow Canada (opens in a new tab)More retailers for the EcoFlow DELTA Pro 3 Power Station$2,799–$3,299at EcoFlow US · prices may have changedPrice checked . Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on EcoFlow US at the time of purchase will apply to the purchase of this product.Check price at EcoFlow US (opens in a new tab)More retailers for the EcoFlow DELTA Pro 3 Power Station
How we choose
- Started from the sizing questions owners ask after outages, not from the spec sheet.
- Used only the makers’ own published figures for the products named; we give no typical appliance wattages.
- Checked safety advice against Health Canada, Get Prepared and the US CPSC, and prices at the retailers that let us read their pages, with the date we checked.
- Revisit the picks when prices move, a recall is announced or a model is replaced.
What we don’t do: lab-test generators or measure appliance loads ourselves. Runtime and noise figures are the makers’.
Sizing questions
What size generator do I need for a fridge, furnace and sump pump?
It depends on your appliances, so add their nameplate running watts and the biggest starting surge using the worksheet above. Forum threads put this trio in mid-size portable territory rather than small-inverter territory, and a hardwired furnace also needs a transfer switch fitted by an electrician.
Is a 2,000-watt inverter enough for a fridge?
Often, but check. Add the fridge’s running watts to its compressor’s starting surge from its nameplate, and compare that with the generator’s starting watts; the Westinghouse iGen2200c, for example, is rated at 1,800 running and 2,200 starting watts. Defrost heaters and anything else on the same generator add to the load.
Will my furnace run on an inverter generator?
Owners report that furnaces with electronic controls are happier on inverter power than on open-frame generators. A hardwired furnace still needs a transfer switch or interlock fitted by an electrician, and the generator must cover the blower’s starting surge. Check the furnace maker’s guidance before you buy.
Is a 22 kW standby generator too big?
For essentials alone, it can be. Owners on forums describe being quoted 22 kW when a load calculation pointed to less. Generac’s 14 kW model, our essentials pick, is the smaller option. Ask the installer for a written load calculation, and check how much fuel the bigger unit burns at full load.
