Space Heater Cost Calculator

The formula behind the bill

Electricity is sold by the kilowatt hour, so the first step is to turn watts into kilowatts. A 1,500 watt heater is 1,500 / 1,000 = 1.5 kW. Run it for 8 hours and it uses 1.5 x 8 = 12 kWh. At 17 cents per kWh that is 12 x 0.17 = 2.04, or $2.04 for the day.

Nothing else in the arithmetic is hard. The rate comes from your bill, the hours come from how you use the heater, and the wattage is printed on the label on the back of the unit. Most portable heaters sold in the United States draw 1,500 watts on high and 750 watts on low, and that 1,500 watt figure is a ceiling set by the circuit, not a coincidence: a standard 15 amp household circuit at 120 volts carries 15 x 120 = 1,800 watts, so heaters are capped below that.

The hours are where estimates go wrong. A heater that runs 8 hours a night for a month is 240 hours, and a heater on a thermostat that cycles on and off half the time draws power for about 120 of those hours. Duty cycle matters as much as the setting, which is why the number at the end of this page is a ceiling, not a promise.

What 1,500 watts costs by the hour, day and month

The table uses a rate of 17 cents per kWh, close to the U.S. average for residential customers in recent years. Your own rate is printed on your bill, and the calculator lets you type it in.

A 1,500 watt heater at 8 hours a day is 12 kWh a day and 360 kWh a month, which is $61.20. The same schedule on the low setting is half of that, because the resistance element draws half the power. There is no trick to a low setting on a resistive heater: half the watts means half the heat and half the bill.

For scale, the EIA household electricity data puts an average U.S. home at about 899 kWh a month. One heater on that schedule is 360 / 899 = 40 percent of the whole house's monthly electricity, spent heating one room.

Hours run matter more than the wattage setting

Doubling the hours doubles the bill, and most people underestimate how long a heater runs. A bedroom heater that goes on at 9 pm and off at 7 am is 10 hours, not the 6 hours people remember.

Four hours a day is $30.60 a month and 12 hours a day is $91.80, a difference of $61.20 for the same heater in the same room. A five month heating season at 8 hours a day comes to $306.00, which is more than most people expect to spend on one appliance.

That is the argument for a timer or a thermostat plug. Cutting two hours a night off an 8 hour schedule takes 30 kWh and $5.10 off the month, and nobody notices the difference in a room that was already warm when they went to bed.

Use the all-in rate, not the headline rate

The rate printed in large type on the front of a bill is usually the supply charge only. Delivery charges, a customer fee, and state or local taxes land in the same bill, and the all-in number is often 20 to 30 percent higher than the headline.

Take the same 360 kWh month. At 17 cents it is $61.20. At an all-in rate of 21.5 cents it is 360 x 0.215 = 77.40, or $77.40, which is $16.20 more for the same heater and the same hours.

Two other things change the real rate. Many utilities bill in tiers, so a big winter month pushes you into a higher price for every kWh above the threshold. Some also charge more in winter than in summer. If your bill has a tier table, use the top tier you expect to reach, because the heater is what pushes you there.

The EIA explanation of how electricity is measured is a plain-language refresher on watts, kilowatts and the difference between the two if the bill is unclear.

Space heater, heat pump, propane: the same heat for the money

A resistive heater turns every kWh into about 3,412 Btu of heat. A heat pump moves heat instead of making it, so the same kWh delivers two to four times as much heat, depending on the outdoor temperature. That difference is the whole story in one table.

The arithmetic for each row is short. A kWh is 3,412 Btu, so 3,412 / 0.17 = 20,071 Btu per dollar for resistance heat, and a heat pump at COP 3 gives 10,236 / 0.17 = 60,212. Propane carries 91,500 Btu per gallon, so 91,500 x 0.8 = 73,200 Btu of delivered heat, and 73,200 / 2.50 = 29,280 Btu per dollar. Gas is 100,000 x 0.8 = 80,000 Btu per therm, and 80,000 / 1.20 = 66,667.

Fuel prices move every winter, so treat the last two rows as illustrations and redo them with your own delivered price. The ranking is what matters: in most of the country a heat pump or a gas furnace beats a plug-in heater by a wide margin, and a plug-in heater beats propane only when propane is expensive.

The safety numbers behind the cost

Cost is the second question with a space heater. The first is whether it is safe where you plan to put it. The U.S. Fire Administration heating fire data reported 32,200 home heating fires in 2021, with 190 deaths, 625 injuries and $442 million in property loss. Heating was the second leading cause of home fires that year.

Portable heaters are a small share of those fires and a large share of the deaths: they caused 3 percent of home heating fires in 2017 to 2019 but 41 percent of the fatal ones. The pattern is almost always the same, something flammable within a few feet of the element, or a heater left running in an empty room.

Three rules cover most of it. Keep anything that burns at least 3 feet from the heater, including bedding and curtains. Turn it off when you leave the room or go to bed. Plug it directly into a wall outlet, because an extension cord or power strip rated for 15 amps is still a poor match for a load that runs for hours.

Cutting the bill without freezing

The cheapest kilowatt hour is the one you do not use, and there are four moves that cut a space heater bill without making a room uncomfortable.

First, heat the room, not the house. Closing the door to the room you are in and running the heater there for an hour costs less than heating the whole floor. Second, seal the obvious leaks: a door draft stopper and plastic over a single pane window can hold a room two or three degrees warmer, which is an hour of heater time you no longer need.

Third, use a plug-in thermostat or a timer so the heater runs in bursts instead of all night. A heater on a 50 percent duty cycle uses half the electricity of one that runs flat out, and the room stays within a degree or two. Fourth, drop the house thermostat a few degrees and use the heater only in the room you occupy. The common guideline is that each degree you lower a thermostat for eight hours saves about 1 percent of the heating bill, which adds up across a winter.

What does not work is running a 1,500 watt heater as primary heat for a house. It delivers 5,118 Btu an hour, and a typical gas furnace delivers 60,000 to 100,000 Btu an hour, so you would need about 12 of them to match one furnace. As a supplement for one room, a space heater is a reasonable tool. As a furnace replacement, the bill and the fire risk both climb fast.