Space heater vs central heat: when does it actually save money?

Every electric space heater turns one kWh into 3,412 BTU of heat. A cheap one and an expensive one do exactly the same, whether it glows, has oil inside or blows air over ceramic plates. That makes 1 million BTU (MMBtu) of space heater heat cost 293.07 kWh: $53.22 at the national average rate of 18.2¢/kWh.

A 90% gas furnace delivers the same MMBtu with 11.1 therms, $16.44 at the national gas price of $1.48. A heat pump in a climate like the US average (seasonal COP 2.8 in our model) needs 104.7 kWh, $19.01. So per unit of heat, the space heater is the most expensive option in most American homes.

It saves money only one way: by letting you heat one room instead of the whole house. If the central thermostat stays where it was and you add a heater, you're paying for both. If you turn the house down and heat only the room you're in, you may come out ahead, depending on what heats the rest of the house.

What 1 MMBtu of heat delivered into the room costs at current prices:

State	Space heater	Gas furnace, 90%	Heat pump (zone COP)
Florida	$43.93	$27.33	$12.92 (COP 3.4)
Georgia	$46.60	$21.89	$15.03 (COP 3.1)
Washington	$42.88	$18.89	$15.31 (COP 2.8)
US average	$53.22	$16.44	$19.01 (COP 2.8)
New York	$83.96	$18.89	$33.59 (COP 2.5)
Minnesota	$50.41	$11.78	$21.92 (COP 2.3)

In every state the space heater column is the highest. Electric resistance heat everywhere in the house (baseboards, an electric furnace, wall heaters) costs exactly the same per BTU as a space heater, so with that kind of central heat, any space heater that lets you turn the rest of the house down is a pure saving.

Say you work from home in a 150 sq ft office, 8 hours a day, in a 2,000 sq ft house with average insulation. Two options for a winter month:

• Option A: the whole house stays at 68 °F all day.
• Option B: you set the central thermostat to 58 °F for those 8 hours and keep the office at 68 °F with a space heater.

The house loses about 0.35 BTU per hour for every square foot and every degree between inside and outside, the same insulation factor the site's heating calculators use. For 2,000 sq ft that's 700 BTU per hour per °F. Holding the house 10 °F cooler for 8 hours a day cuts its heat need by 700 × 10 × 8 = 56,000 BTU a day, or about 1.68 MMBtu over 30 days.

The heater doesn't run flat out: a 1,500 W heater on its thermostat in a small room cycles, so assume it averages 750 W. That's 6 kWh a day, 180 kWh a month, about 0.61 MMBtu of heat. Here is what the switch is worth, depending on your central heat:

Central heat	Heat it no longer makes	Saving on central heat	Space heater costs	Net
Electric baseboard or furnace	492 kWh	$89.35	$32.69	saves $56.66
Gas furnace, 90%	18.7 therms	$27.63	$32.69	compare the two
Heat pump, COP 2.8	176 kWh	$31.96	$32.69	about even

All figures at US average prices. With electric resistance heat, the office heater saves more than it costs, because the 1.68 MMBtu you stopped heating was paid for at the same price per BTU as the heater's 0.61. With a heat pump, the saving and the heater's cost cancel out: the heat pump was already making each BTU for about a third of the price. With gas, it depends on your state.

In the home office example, the heater beats a gas furnace when your electricity rate in cents per kWh, divided by your gas price in dollars per therm, is below about 10.4. Above that, the gas you save is worth less than the electricity the heater burns.

• Georgia: electricity 15.9¢/kWh, gas $1.97 a therm. The furnace saving is $36.78; the heater costs $28.62. The heater wins.
• Washington: 14.6¢/kWh against $1.70. Saving $31.74, heater $26.33. Cheap hydro power puts the heater ahead.
• Minnesota: 17.2¢/kWh against $1.06. Saving $19.79, heater $30.96. Cheap gas makes the heater a loss.
• New York: 28.6¢/kWh against $1.70. Saving $31.74, heater $51.57. Expensive electricity makes it a clear loss.

At current prices the heater passes this test in about 14 states, mostly in the South and Southeast where gas is expensive relative to electricity, plus Washington and Oregon. In the Northeast, the Midwest and the colder Mountain states, a space heater next to a gas furnace usually costs more than it saves, even when you turn the house down.

If you heat with propane or heating oil, zoning with a space heater does better: at our reference fuel prices, the office example saves money against an 85% propane furnace in every state. Propane heat costs more per BTU than gas heat almost everywhere.

The bigger the setback, the bigger the saving, because the heater's cost stays the same while the central heat saving grows with every degree. In the office example, each degree of setback for 8 hours a day saves 0.17 MMBtu a month.

• Electric resistance central heat: the heater breaks even at a setback of under 4 °F. Anything more is profit.
• Heat pump: the heater wins only if the heat pump's seasonal COP is below about 2.7, which in our model means climate zones 5 to 7, and even there the saving is small. Better to leave a heat pump alone: deep setbacks can trigger its backup resistance strips when the house warms back up, which costs more than the setback saved.
• Gas: use the 10-to-1 test above. A deeper setback raises the break-even in proportion: at 15 °F instead of 10 °F, it's about 15.6.

Two things shrink the real saving. When the setback ends, the furnace works harder for a while to bring the house back up, so you get back somewhat less than the full 1.68 MMBtu. And the office loses heat into the cooler rooms around it, so the heater runs more than it would in a warm house.

• The central thermostat didn't move. A heater in the living room while the furnace still keeps every room at 68 °F is extra heat on top of what you were already buying.
• The room is large or drafty. A 1,500 W heater makes 5,118 BTU an hour. A big, poorly insulated room can need all of it and still be cold, so the heater runs nonstop.
• Several heaters. Two or three heaters in different rooms is whole-house resistance heating at the highest price per BTU, without the efficiency of the central system.
• A bedroom overnight in a house already set back. If you'd set the house back at night anyway, the heater adds cost without saving any central heat.

For safety, plug a space heater straight into a wall outlet, not an extension cord or power strip, and keep it clear of bedding and curtains.

• Space heater vs central heat calculator: a month of space heater use against your furnace's size, AFUE and burner hours, at your state's prices. Your smart thermostat's runtime report gives you the burner hours.
• Space heater cost calculator: what your heater costs by wattage and hours, month by month.
• Thermostat setback calculator: what turning the whole house down saves, for gas, propane, oil, resistance or a heat pump.
• Gas vs electric heat calculator: the whole-house version of this comparison, with the heat pump for reference. See also the heat pump vs gas furnace guide.