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Generator and Home Battery Sizing Calculator Ontario (2026): The Well Pump, Sump, Furnace and Fridge in an Outage, the Starting Surge, the kW You Need and How Long a Battery Lasts
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How Much Backup Power Does the House Need? Start With the Well Pump, Not the Brochure.
The loads that matter in an outage, their running and starting watts, the generator size, the battery hours and the installed cost. No email address required.
In town, a winter outage means a cold house and a warm fridge. In rural Simcoe County it means no water, because the well pump stops, and a basement filling because the sump pump stops. Backup power is sized from the few loads you cannot live without, and the one that decides the size is usually not the biggest load but the one with the biggest kick at start-up: the well pump. This calculator adds your loads, finds the surge, and sizes a generator or a battery for them.
Step-by-step
Generator and Battery Sizing (Ontario)
1
The loads you want running in an outage
The one that sizes the generator. Check the pump size on the control box or the well record.
About 800 W running, 2,000 W starting.
About 1,000 W running, 2,500 W starting.
About 650 W running, 1,300 W starting.
About 250 W running, 400 W starting.
About 200 W running, 1,200 W starting.
About 150 W running, 900 W starting.
About 300 W running, 300 W starting.
About 1,200 W running, 1,200 W starting.
About 4,000 W running, 6,000 W starting.
About 4,500 W running, 4,500 W starting.
From the nameplate. Counted as running all the time.
2
The outage you are planning for
Ice storms and derechos in Ontario have meant days, not hours, in rural areas.
A propane tank is bought or rented from the propane supplier, separately.
One Powerwall-class unit is about 13.5 kWh; an Enphase IQ 5P module is 5 kWh.
3
Where is the house?
Sets the installed-price adjustment. Labour rates in the GTA run higher than in the rest of the province.
Used only to tell you whether the house is in our direct service area. Not stored.
Enter a postal code to see service coverage.
BuildersOntario.com
What the House Needs in an Outage
Illustrative sizing
| Line | Value |
|---|
Generator size to look at–
Example only. Running watts add up; only the single biggest starting surge is added on top, because motors rarely start at the same instant. The generator size is that peak plus about 20 percent, rounded up to a common size. Battery hours use the average draw, each load running its typical share of the hour. Real pumps, furnaces and fridges vary; the nameplate is the real number. Prices are October 2026 Ontario ranges before HST. Not a quote and not a design.
Request a Real Quotation
Your loads, the surge and the hours become a written scope a Licensed Electrical Contractor (and a TSSA gas fitter for a standby unit) can quote without guessing. It goes straight to our inbox, and a copy comes back to you; reply to that copy with photos of the panel with the door open, the well pump control box, and the spot outside where a generator would sit.
What backup power costs in Ontario, October 2026
| Option | Installed, before HST |
|---|---|
| Inlet and interlock at the panel for a portable generator (generator extra) | $700 to $1,200 |
| Manual transfer switch for a portable | $800 to $1,500 |
| Automatic standby generator, 7 to 11 kW | $6,000 to $9,500 |
| Automatic standby generator, 14 to 18 kW | $8,500 to $13,000 |
| Automatic standby generator, 20 to 26 kW | $11,000 to $18,000 |
| One Powerwall-class home battery, 13.5 kWh | $15,500 to $24,000 |
| Enphase IQ 5P battery, per 5 kWh module | about $4,400 to $4,500 |
A standby price usually includes the transfer switch, the pad, the electrical tie-in and a 30 to 50 ft gas line by a TSSA gas fitter ($1,200 to $2,200 of it). A propane tank is separate. The calculator uses $7,750 under 14 kW, $10,750 for 14 to 18 kW, $13,000 for 22 kW, $15,500 for 24 to 26 kW and $22,000 above and $19,000 for the first battery, about $12,000 for each one after.
Why Ontario prices are all over the map: the contractors and price guides we checked put a 22 kW standby generator at $11,000 to $16,000 (a Barrie contractor) and $10,500 to $12,500 (a generator dealer’s site), while a GTA company quoted $18,000 to $35,000 for whole-home units of 20 to 48 kW. One Powerwall 3 ran from $15,500 (a Toronto contractor) to $24,000 before HST (an Ontario solar installer). Gas line length, the transfer switch and what is in the price explain most of it. Budget $13,000 for a 22 kW standby, and get two or three quotes on the same written scope.
Three rural houses, worked out
| House | Running + surge | Size |
|---|---|---|
| Well (1/2 hp), sump, propane furnace, fridge, 3 light circuits, microwave | 1,000 + 800 + 650 + 200 + 900 + 1,200 = 4,750 W; well surge +1,500 = 6,250 W; x 1.2 = 7,500 W | 7.5 kW |
| The same plus a freezer and a 1 hp well pump | 5,700 W; well surge +3,200 = 8,900 W; x 1.2 = 10,680 W | 14 kW |
| All-electric ICF house: heat pump, well, sump, fridge, lights | 4,000 + 1,000 + 800 + 200 + 900 = 6,900 W; heat pump surge +2,000 = 8,900 W; x 1.2 = 10,680 W | 14 kW |
Typical wattages; the nameplates on your own equipment are the real numbers. The generator sizes listed are common standby and portable sizes (3.5 to 48 kW).
Where homeowners go wrong
- Sizing from the running watts. The well pump’s start is what trips a small generator or a battery inverter.
- Backing up the whole house. The heat pump and the electric water heater double the size. Back up the loads that matter. Standby generator cost.
- Backfeeding through a dryer outlet. It can kill a lineman down the road. Inlet and interlock, or a transfer switch, installed by an LEC.
- Buying a battery for backup and expecting the rebate. The battery rebate needs new solar and no net metering. Solar grid connection.
- Forgetting the inlet on a new house. $700 to $1,200 at rough-in. In an ICF house the sleeve goes in before the pour. Wiring an ICF house.
Questions people ask
What size generator do I need to run my well pump?
For a 1/2 hp pump plus the basics, about 7.5 kW; for a 1 hp pump with a freezer and the furnace, about 14 kW. The pump’s starting surge decides it. Check the pump size on the control box.
Is a home battery enough in a winter outage?
One 13.5 kWh battery carries a well, a sump, a fridge, a furnace blower and a few light circuits for roughly 12 to 24 hours in a January outage with no sun, depending on how hard the furnace runs; longer in summer. Days-long outages need a generator, more batteries or solar. Generator or battery.
How much does a standby generator cost in Ontario?
$8,500 to $13,000 for 14 to 18 kW and $11,000 to $18,000 for 20 to 26 kW, installed, October 2026, before HST.
Do I need a permit for a generator?
A permanent connection needs an ESA notification and a transfer device; the gas line is TSSA-regulated work. ESA permits.
Can a portable generator run my house?
The essentials, yes, through an inlet and interlock or a transfer switch, with someone home to start and refuel it. Outside, away from windows and doors, never in the garage.
Quotes in hand?
The $99 quote review checks generator and battery quotes against your loads, the well pump surge, the transfer switch and the gas line, and lists what each one left out.
