Gas furnace blowers are modest but need a start surge. Learn the watts to plan for and how to size a power station or generator safely.
- Gas and oil furnaces need only hundreds of watts; electric furnaces need far more than a portable battery can supply.
- Size for start surge as well as running watts, with about 20 per cent margin.
- Use a qualified electrician for the hookup and never run a generator indoors.
People are often surprised that a gas or oil furnace can run on a fairly small backup source. The burner uses fuel, and only the blower motor, the control board and the ignition need electricity. An electric furnace is a different story and is far beyond what a portable battery can handle.
Here is how to figure out the watts, why start-up surge matters, and how to match a power station or generator to your furnace.
Gas, oil and electric furnaces are not alike
- Gas or oil furnace: electricity runs the blower fan, the inducer fan, the igniter and the control board. Typical draw is in the hundreds of watts, but it varies by size and by whether the blower is a basic motor or a variable-speed one.
- Electric furnace: heating elements use thousands of watts. This is not a realistic load for a portable power station and needs a large generator or whole-home system. Use other heat instead. See how to stay warm without power.
- Heat pump: draw depends on size and mode, and backup resistance heat can be very large. Check your model.
Find your furnace's numbers
Open the furnace door or look on the nameplate for amps and volts. Multiply amps by volts to get watts. In North America most homes use 120 V for the furnace circuit. For example, a nameplate showing 8 amps at 120 V means about 960 W maximum. Typical running draw is often below the nameplate figure, but use the nameplate when sizing so you have margin.
If the nameplate is not clear, ask an HVAC technician. They can read the numbers in minutes.
Running watts versus start watts
Motors draw more for a moment when they start. The blower can briefly pull several times its running watts. A power station or generator has two ratings: continuous output and surge output. Both need to cover your furnace.
- Add the nameplate watts for the blower, inducer, and controls.
- Allow extra for the start surge, often as much as two to three times the running load for a moment.
- Make sure the power station's continuous rating covers the running total and its surge rating covers the start.
- Leave margin of about 20 per cent so the unit is not working at its limit.
Variable-speed and electronically commutated motors ease in more gently, which helps. Some furnaces have sensitive electronics that dislike rough power, so look for a pure sine wave output. For sizing in general, see what size power station do I need and what size generator do I need.
How long will it run?
A furnace cycles on and off. It does not run flat out all day. A rough way to plan: battery watt-hours x 0.85, divided by the average watts while running, gives hours of fan time. The furnace may run a share of each hour depending on the cold and your house. In extreme cold the burner runs longer, so be conservative. A mid-size battery may handle one night, while a bigger one may stretch to a couple of days. Cold also reduces battery capacity, so keep the station indoors. See power stations in cold weather.
How to connect it safely
Furnaces are normally hard-wired. Plugging one into a portable source needs the right method.
- A qualified electrician can install a dedicated outlet or an inlet or a transfer switch. Do not improvise by opening panels or modifying wiring yourself.
- Do not back-feed your house by plugging a generator into a wall outlet. This can kill utility workers and neighbours.
- Check the furnace manual and local code. Some homes have a cord-connected furnace, but many do not.
Read more in transfer switch vs extension cords and how to keep a furnace running in an outage.
Plan ahead
Look up your nameplate, choose a source that covers surge and runtime, and have an electrician set up the connection before winter. Keep a backup heat plan, because a furnace can fail for reasons other than power. For gear options visit our power station hub.
Quick answers
Can a power station run an electric furnace?
In nearly all cases no. Electric heating elements draw thousands of watts, far beyond typical portable units.
Why does my furnace need a surge rating?
Motors briefly draw more when starting. If the source cannot cover that, it trips or shuts off.
Can I plug my furnace into an extension cord?
Only if the furnace is designed for a cord connection. Most are hard-wired, so ask an electrician about a safe inlet or transfer switch.