Today's portable electric heaters are relatively safe compared to liquid fuel or open flame heaters, but utility companies appear to disagree about the cost of that comfort.

"The cost of using an electric space heater can add up over the course of a month," says the Eugene, OR-based Eugene Water and Electric Board.

"You can stay warm and save money without turning up the thermostat on your furnace," says the Lincoln, NE-based Lincoln Electric System.

What makes an electric portable heater useful is its ability to quickly warm an area or object. The heaters work by running electric current through high resistance wires -- the heating elements. Generally the heat is distributed by convection -- naturally or fan forced -- or in radiant form.

Convection heaters warm the air and are best at heating enclosed, relatively draft-free spaces. Radiant heaters warm people and objects directly and are better suited for garages, drafty spaces and larger or more open areas.

In either case, what makes portable electric heaters viable is that you can turn them on until the area warms and then turn it off. Newer models with thermostats work the controls for you.

Crank up the furnace to take the chill off you in one room and you'll spend much more because you'll unnecessarily heat your entire home.

The key to using portable electric heaters economically is knowing how much it costs to operate. That knowledge will clue you to how much you should or shouldn't use it, cost-wise.

The cost of portable heat

Cedar Rapids, Iowa-based Central Iowa Power Cooperative offers the following steps to determine a portable electric heater's cost of use.

First, says the cooperative, you must determine the heater's wattage.

Most portable heaters are adjustable and operate from about 750 watts to 1,500 watts.

Next calculate the kilowatt/hour (kWh) use of the heater. The formula is watts x hours/1000 = kWh use.

If your heater uses 750 watts and runs for 1 hour to heat up the room you multiply 750 by 1 and divide the answer by 1,000. That equals 0.75 kWh. At the higher 1,500 watts adjustment, it's 1.5 kWh.

Finally, you need to know your per kWh cost. It's likely included on your utility bill or your utility company will supply you with the figure. The per kWh cost can vary from season to season and based on the time of day, even how much electricity you use, depending upon how your utility bills you.

You can also figure your cost per kWh by getting out your last utility bill and dividing the electricity cost by the number of kWh used. For example, if your electric tab was $120 and you used 1200 kWh, your per kWh cost is $0.10 or 10 cents.

Multiply your per kWh cost by your kWh use.

In the first example, multiply 0.75 by $0.10 and it costs $0.075 (7.5 cents) to operate a heater at 750 watts for one hour. It'll cost 15 cents to operate it for an hour at 1500 watts.

One hour of operation is relatively cheap, but run that heater for a couple of hours a day during cold snap and the cost could mount. Still, it's likely a pittance compared to running your furnace for the same amount of time.

Your utility company can help you further compare what it costs to run your portable heater with what you are saving by not running your furnace as much.

Safety issues

Safety concerns also matter when it comes to portable electric heaters, though less so today than in the past.

With automatic shut-off switches, non-glowing elements, casing that's cool to the touch and other innovations, portable electric heaters are inherently less risky than using a heater that requires you to store, handle and burn flammable fuel.

The U.S. Consumer Product Safety Commission nevertheless offers a list of more than two dozen portable electric heater recalls, and some older models can be risky, recalled or not.

Northbrook, IL-based Underwriters Laboratories, Inc. (UL) offers these safety tips to avoid mishaps with portable electric heaters.

Purchase only UL approved heaters. The independent, not-for-profit product safety testing and certification organization has tested products for public safety for more than 100 years.