This post overlaps with a fuller one: Electric or Gas Furnace: Which Costs Less to Run Here.
A gas furnace burns natural gas in a heat exchanger and vents the exhaust outside. An electric furnace heats air with resistance elements and has no flue. The electric one converts almost all of its energy into heat, and the gas one loses some up the vent, yet in Southern California the gas furnace usually costs less to run, because electricity costs more per unit of heat than gas. That is the whole comparison in one paragraph. The details follow.
Efficiency versus cost to run
Department of Energy guides rate all-electric furnaces at 95% to 100% AFUE, and put gas furnaces at roughly 56% to 70% for old standing-pilot units up to 90% to 98.5% for condensing models. AFUE measures how much of the fuel became heat, not what the fuel cost. To compare on your own bills, take the Pasadena Water & Power or LADWP price per kilowatt-hour and the SoCalGas price per therm, remembering that one therm is roughly 29.3 kilowatt-hours of heat. Our short heating season, November to March, keeps the gap smaller than it would be in a cold climate, which is why an electric furnace can still fit a small ADU with no gas service.
The house decides more than the fuel
A gas furnace needs a gas line and a proper vent; condensing models vent through plastic pipe and need a drain. An electric furnace needs a heavy dedicated circuit, and many older ranch homes in the Valley are on 100-amp service. Panel work belongs to an electrician, a separate licensed trade. Either replacement usually triggers a duct leakage test under California's Title 24, and when more than 25 feet of new duct goes into the attic, it must be R-8.
If you are going electric, look at a heat pump
A heat pump moves heat instead of making it, and ENERGY STAR puts it at up to three times more heat energy than the electrical energy it consumes. On a Pasadena, CA winter night in the 40s it runs comfortably. That is the electric system that competes with a gas furnace; the resistance furnace rarely does. Get a furnace installation quote and a heat pump installation quote from the same visit and compare them in writing.
Our diagnostic visit is $69, told to you before booking and credited toward the work. Boost Services Heating and Cooling answers at 818-277-5056 or contact us here.
The full guide: Electric or Gas Furnace: Which Costs Less to Run Here
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.