Permanent Automatic Generator
This is the most efficient-and the most costly-option. You can easily spend $10,000 to $15,000, plus installation, but it gives you a seamless power transition. In the event of an outage, selected circuits such as the one for your furnace are on a subpanel and automatic transfer switch, which is powered by the generator.
Portable generators aren’t designed to take over any portion of your home’s circuitry via a subpanel and transfer switch. Their outlets are for plugging in appliances.
But there’s a safe method for powering just your furnace with a portable generator. Connect a heavy-duty double-pole, double-throw switch (such as Leviton No. 1286; $40 at home centers) to the furnace, along with a power inlet receptacle (such as Leviton No. 5278-CWP; $40 at home centers). If the utility power goes out, flip the double-pole switch to disconnect the permanent furnace circuit, and the run a cord from the portable generator to the inlet. The double-pole, double-throw switch prevents the portable generator from back-feeding the entire house or, worse, the utility’s system. To be extra safe, turn off the main circuit breaker at the main service, disconnecting the house, to eliminate any chance of back-feeding the utility.