It all depends on the devices and how they're setup. If you're in an area that gets really cold AND has intermittent power (which is not uncommon, snow and ice on the lines can take down power) you can either design your setup to work without external power (think: emergency generator, etc) or you can design parts of it to be usable in an emergency (think: stove you can light with a match, fireplace; kerosene heaters, etc).

My gas furnace will not ignite without power and the power necessary to run the blower is 15 amps or so, wiring in a portable generator would be simple.

The water heater works without any power whatsoever.

The stove has electric ignition but you can light it without power. The oven needs power (I think? never tried it).