How to Heat a House When the Power Is Out

The most common heating failure during a winter outage is not that the furnace broke. It is that the furnace was fine and could not run.

A modern gas or oil furnace needs electricity for its blower motor, its ignition system, and its control board. Gas in the line is not heat if the appliance cannot operate. During the February 2021 Texas event, households with uninterrupted gas service sat in cold houses because the furnace needed a few hundred watts it did not have. That is the single most preventable failure in residential preparedness, and it is also one of the cheapest to fix.

This page covers what actually works, what the numbers are, and what will kill you if you get it wrong.

Start by finding out what your furnace draws

Before buying anything, establish the actual electrical load of your existing heating system. Most people assume it is large. Usually it is not.

A residential gas furnace with a standard blower typically draws somewhere between 300 and 800 watts while running, with a brief surge two to three times that at startup as the motor spins up. Furnaces with electronically commutated motors, common on newer high efficiency units, often run considerably lower, sometimes under 300 watts at low speed.

Oil furnaces add a burner motor and an igniter, which pushes the running draw somewhat higher, often in the 500 to 1,000 watt range.

Boilers with hydronic distribution are the best case in the whole category. A circulator pump on a hot water system may draw as little as 50 to 100 watts, because it is moving water rather than air. If you have a boiler, keeping heat running through an outage is a genuinely small electrical problem.

Heat pumps and electric resistance heat are the worst case by a wide margin. Electric baseboard or a strip heater draws thousands of watts continuously, and no realistic battery or portable generator setup will carry it for long. Households on electric heat need a fundamentally different plan, covered further down.

The nameplate on the furnace, usually inside the access panel, gives you the figure. If it lists amps rather than watts, multiply by 120. A clamp meter on the supply line while the unit runs gives you the real number, which is often lower than the plate rating.

Once you have that number, the off grid power sizing calculator will tell you what battery capacity or generator size actually covers it, including the startup surge that determines inverter selection rather than battery size.

Option one, and usually the right one: power the furnace you already have

For most households with gas or oil heat, the cheapest and safest solution is not a new heat source. It is a way to run the existing one.

This works because the load is small and intermittent. A furnace does not run continuously; it cycles. In a well insulated house at moderate outdoor temperatures a furnace might run twenty to forty percent of the time, so the average draw over a day is a fraction of the running draw.

Three ways to do it, roughly in order of cost.

A portable power station, meaning a lithium battery with a built in inverter, sized at 1,500 watt hours or more with a 1,800 watt or greater pure sine inverter, will typically run a gas furnace through a night or longer depending on cycling and outdoor temperature. This is the simplest option because there is nothing to install and nothing to fuel. It is also silent, which matters more than people expect during a multi day event.

A portable generator with a proper transfer switch is the traditional approach and the one most likely to be done unsafely. The important part is the transfer switch, not the generator. Never connect a generator to household wiring through an outlet, which is called backfeeding. It energises the utility line outside your house and can kill line workers, and it bypasses every protective device in your panel. An interlock kit on the main panel is the inexpensive legitimate version, typically a few hundred dollars installed, and a manual transfer switch is the more conventional one.

A hardwired inverter with a battery bank is the permanent version of the first option and makes sense if you are already building solar. The furnace circuit lives on a critical loads subpanel and simply does not notice the outage.

Whatever you choose, buy pure sine wave. Modified sine damages motors and confuses the control boards on modern furnaces, and many furnace boards will refuse to run on it entirely.

Option two: a heat source that needs no electricity at all

If your heat is electric, or if you want redundancy that does not depend on any powered system, you need combustion or stored heat.

Wood stoves and fireplaces

A properly installed wood stove is the most reliable residential heat source that exists. It needs no electricity, no fuel delivery during an event, and it produces heat for as long as you have wood.

Two caveats matter. An open masonry fireplace is a poor heater and frequently a net loss, because it pulls more heated air up the chimney than it delivers to the room. A fireplace insert or a freestanding stove is a different appliance entirely and delivers real output.

Installation is not optional and not a place to improvise. Clearance to combustibles, proper chimney or class A flue, hearth protection, and local code compliance all matter, and the failure mode is a house fire. The National Fire Protection Association's NFPA 211 standard governs chimneys and solid fuel appliances, and most jurisdictions adopt it directly.

Wood also needs to be seasoned. Green wood contains a great deal of water, burns cool, produces little usable heat, and deposits creosote in the flue at a rate that causes chimney fires. Wood cut and split this spring is not ready this winter. Twelve months minimum for softwoods, closer to two years for dense hardwoods, stacked off the ground and covered on top but open on the sides.

Propane and kerosene heaters rated for indoor use

Portable propane heaters designed and certified for indoor use, meaning they carry an oxygen depletion sensor and are rated for the space, work well as supplemental heat for one or two rooms. They are inexpensive, need no installation, and run on fuel that stores indefinitely.

The constraints are real. They consume oxygen and produce water vapour and combustion byproducts, so they require ventilation, which means cracking a window somewhat, which costs you some of the heat you are generating. They should never run while everyone is asleep. And the indoor rating is not a formality: a construction heater or a propane appliance rated for outdoor use only is not made safe by being small.

Kerosene heaters occupy similar ground with more odour and more maintenance, and are prohibited outright in some jurisdictions.

The thing that must never happen

Never burn charcoal indoors. Never run a generator in a garage, even with the door open. Never use a gas range or oven as a heat source.

Carbon monoxide is the leading cause of death in winter power outages, and it is entirely preventable. It has no smell, no colour, and the early symptoms are headache and drowsiness, which people in a cold dark house attribute to being cold, dark, and tired. The Centers for Disease Control publishes guidance on this every winter for the reason that people keep doing it.

Buy a battery powered carbon monoxide alarm and test it before you need it. If you are using any combustion heat indoors, that alarm is not optional equipment.

Option three: stop needing as much heat

This is the option nobody sells, and in a genuine emergency it is often the most effective one.

Close off the house. Pick one room, preferably small, on the south side if there is any sun, away from the prevailing wind, and ideally with a heat source. Close doors to everywhere else. Hang blankets over doorways that do not close. Heating 150 square feet is a completely different problem from heating 2,000.

Insulate the windows. Window film or even bubble wrap against the glass, blankets over curtains at night, and cardboard in the worst offenders. Glass is the largest heat loss surface in most rooms.

Sleep together and sleep low. Body heat in a small enclosed space is substantial. A tent pitched indoors, which sounds absurd, works extremely well because it traps a small volume of air around the people in it.

Insulate yourself before the room. Layers, hats indoors, and something under you when sitting or sleeping. Conduction into a cold floor or couch costs more than most people realise.

These measures cost nothing, need no fuel, and buy you a great deal of time. They also stack with everything above, extending whatever fuel or battery capacity you have.

The pipe problem

Heat is not only about comfort. Below freezing, an unheated house destroys its own plumbing, and that damage frequently exceeds the cost of everything discussed on this page.

If you cannot keep the house above freezing, drain the system. Shut the main valve, open the lowest faucet in the house and the highest one to break the vacuum, and drain the water heater if the outage will be long. Add non toxic plumbing antifreeze to traps if you know how.

If you can keep it marginally above freezing, let faucets drip on exterior walls, open cabinet doors under sinks so warmer room air reaches the pipes, and concentrate what heat you have on the side of the house where the plumbing runs.

The Texas event produced enormous burst pipe losses, a large share of which were preventable by shutting a valve. This is the cheapest insurance in the entire subject.

What actually to do, in order

Find your furnace's running watts. That single number determines whether this is a small problem or a large one, and for most gas and oil households it is a small one.

Get a battery powered carbon monoxide alarm before anything else, particularly if there is any chance you will use combustion heat.

Solve the electrical problem for the furnace you have, with a power station, a generator and interlock, or an inverter and battery bank. Size it against the real load rather than the nameplate.

Add one non electric heat source if you can. A wood stove if the house supports it, an indoor rated propane heater if not.

Know how to shut off and drain your water, and practice it once while the weather is fine.

And plan for the cascade rather than the outage. The Texas 2021 breakdown covers how power, water, and heat failed in sequence rather than independently, which is the shape most winter events take.

Related: How to read a grid reliability report for your region covers whether your area is flagged for winter risk. The water storage planner covers the supply side of the same event.

This page is general information, not a substitute for a licensed installer or your local building code. Solid fuel and combustion appliance installation is regulated in most jurisdictions for good reason.

Last reviewed: July 2026