The question is what your furnace burns
This is the largest single variable in household backup power, and a surprising number of people do not know the answer for their own house.
A gas or oil furnace makes heat by burning fuel. Electricity runs the blower that moves air through the ducts, the igniter, and the control board. That is it.
An electric furnace makes heat with resistance elements. Electricity is the heat, and the load is enormous.
The difference is not marginal — it is the difference between a load a battery covers overnight and one no portable equipment touches.
Gas and oil furnaces: just the blower
| Blower motor | Running watts | Starting watts |
|---|
| ⅛ HP | 300 | 500 |
|---|
| ⅙ HP | 500 | 750 |
|---|
| ¼ HP | 600 | 1,000 |
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| ⅓ HP | 700 | 1,400 |
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| ½ HP | 300-800 (Champion) / 875 (Honda) | 800-1,600 |
|---|
The by-horsepower rows come from Honda Power Equipment's wattage estimation guide; the half-horsepower range and the starting figures from Champion Power Equipment's published chart. Where the two publish different figures for the same motor size we give both rather than choosing — that spread is real and it tells you how much variation exists between installations.
So a gas furnace is a 300 to 875 watt load while the blower runs, with a starting surge up to about 1,600W.
Which is the sentence this page exists for: a modest battery can keep a whole gas-heated house above freezing. That is a far better use of stored energy than a space heater, which warms one room and empties the same battery in under ninety minutes.
Electric furnaces and heat pumps: a different category
An electric furnace's resistance elements draw thousands of watts — the same order as an electric water heater or an oven, and continuously rather than in bursts.
Heat pumps are more efficient but complicate the picture:
- In moderate cold, a heat pump moves heat rather than making it, so its draw is closer to an air conditioner's than to a furnace's.
- In severe cold, most systems fall back to resistance strip heat — and then the load is an electric furnace's.
- Which is exactly when you need it. The auxiliary heat comes on in the conditions that also cause outages.
For backup purposes, treat a heat pump with strip heat as an electric furnace and plan accordingly. The whole-house tiers put it in context.
Duty cycle matters as much as the figure
A furnace blower does not run continuously. It runs while the thermostat calls for heat and then stops, and how often depends on outside temperature, insulation and the thermostat setting.
A reasonable planning assumption for a cold night is roughly a third of the time, which at 500W average gives about 4,000Wh across 24 hours, or around 1,500-2,500Wh for the overnight portion that actually matters.
On a very cold night with a marginally sized system it will run much more than a third. Treat the figure as a planning basis rather than a prediction — your own thermostat behavior is the real answer.
The other electrical parts of a heating system
Small, but worth knowing they exist because they can be on separate circuits.
- The igniter. A hot-surface igniter draws a few hundred watts briefly at the start of each cycle.
- The control board and thermostat. A few watts, but nothing works without them.
- A condensate pump on a high-efficiency furnace. Small, and if it stops the furnace may lock out.
- A zone valve or circulator pump on a hydronic system.
- An oil burner motor and pump on an oil-fired system, which is a larger load than a gas igniter.
The practical instruction: put the whole furnace circuit on your transfer switch rather than trying to power components individually.
Everything is hardwired
The constraint that makes winter backup a different problem from summer backup.
A furnace has no plug. Neither does a well pump or a sump pump. So a battery or generator that covers the load in watts still cannot reach it, and the missing component is a transfer switch or an interlock kit.
That is licensed-electrician work with a permit in most jurisdictions, and it is the thing to arrange in October rather than during a storm warning. The connection guide walks the whole chain.
What this means for equipment
- Gas or oil heat: a 2kWh battery with 2,400W or more of inverter covers the blower overnight. A 5,300W generator covers it plus everything else.
- Electric heat: a generator, and a large one. No portable battery is viable.
- Heat pump: size for the auxiliary strip heat, because that is what runs in the conditions that cause outages.
- Either way, a transfer switch. Without it the arithmetic is academic.
And the substitution worth knowing: a 1,500W space heater warms one room and empties a 2kWh battery in under ninety minutes. The same battery running a 500W blower heats the entire house for four hours or more. The winter kit covers the rest of the season.
