What a furnace needs
A typical forced-air gas furnace uses electricity for the blower fan, the draft inducer, the igniter and the control board. Blower motors commonly draw somewhere between 300 and 800 W while running, with a higher surge at start-up. Check the nameplate inside the furnace door for your own figure.
The furnace does not run constantly. In cold weather it might run for 8 hours out of 24. At 500 W, that is 4,000 Wh a day, which is more than a fridge and freezer combined.
| Blower draw | Hours run per day | Daily energy | Battery at 85% usable |
|---|---|---|---|
| 300 W | 6 | 1,800 Wh | 2,150 Wh |
| 500 W | 8 | 4,000 Wh | 4,750 Wh |
| 800 W | 10 | 8,000 Wh | 9,450 Wh |
This is why a furnace is usually a job for a generator, or for a large battery with a plan to recharge it.
Connecting it safely
Most furnaces are hard-wired, so you cannot simply plug them into a power station. The safe options are:
- A transfer switch or panel interlock with a generator inlet, installed by a licensed electrician. This is the standard answer.
- A single-circuit furnace transfer switch, also fitted by an electrician, which lets the furnace circuit take power from a cord.
Do not rewire the furnace yourself with a plug and cord unless your local electrical code allows it and a qualified person does the work. Never backfeed through an outlet.
Some furnace control boards are sensitive to the rough power from a conventional generator. An inverter generator or a pure sine wave power station avoids that problem.
Holding the heat you have
- Close doors to rooms you are not using and gather in one room, ideally on the sunny side of the house.
- Close curtains and blinds at night; open them on sunny windows by day.
- Block drafts under doors with towels.
- Dress in layers and use blankets and sleeping bags.
- Eat regular meals and drink warm fluids if you can heat them safely.
Heating you must not use
- No barbecues, camp stoves, patio heaters or generators indoors or in a garage. They produce carbon monoxide.
- Do not heat the house with a gas oven or stovetop.
- Only use an indoor-rated heater exactly as its manual says, with ventilation and a working CO alarm.
An electric space heater draws about 1,500 W. Eight hours of that is 12,000 Wh, far beyond any ordinary power station, and a heavy load for a small generator.
Protecting pipes
- Open cabinet doors under sinks on outside walls so room air can reach the pipes.
- Let cold taps drip slightly. Moving water is slower to freeze.
- Know where your main water shut-off is. If the house will go below freezing and you must leave, shut the water off and open the taps to drain the lines.
- When heat returns, check for leaks as pipes thaw.
When to leave
If the indoor temperature keeps falling and you have no safe heat, go to a relative, a friend or a public warming centre before conditions become dangerous. Infants, older adults and people with health conditions are affected by cold first. Take medicines, chargers and pets with you.
Sources
- Government of Canada: how to prepare for power outages
- Ready.gov: Power Outages
- Health Canada: preventing carbon monoxide exposure
Related
Reviewed October 2026 by Lee Palmer. This guide is general information, not professional advice. Suggest a correction.