People should not buy low-wattage heaters expecting to save money on heating, experts say. Low-wattage electric panel heaters, such as the heavily-advertised Econo-Heat or Ecosave panel heater, are marketed as cheap options. Many are only 400W, compared to 2kW for a small fan heater.
But Energy Efficiency and Conservation Authority technical adviser Christian Hoerning said a 400W heater would give the heat equivalent of only two to four lightbulbs.
"Most rooms need six or 10 times that," he said.
People often bought low-wattage heaters such as panel heaters thinking they would be cheap to run.
They cost about 9c an hour, against more than 50c for a standard electric heater. But people would generally have them on much longer.
"They use a lot of electricity because they are on even when you're not using the room. People are better to buy a much gruntier electric heater, turn it on to heat up the room and then when they leave the room, turn it off. That's more economical.
"The bigger heater uses more electricity when you are running it but much less overall."
Consumer recommends convection heaters, such as oil column heaters or fan heaters, for insulated homes because the heat generated is retained long enough to heat the air in the room.
Heat can pool above oil column heaters but Consumer NZ adviser Maggie Edwards suggested using a small fan to distribute the warm air more evenly around the room.
In a recent test, Consumer rated the $140 Kent Mica, available from The Warehouse, best of the radiant convection heaters tested. It was easy to use and heated a room quickly.
Edwards said that in poorly insulated older houses with high ceilings, radiant heaters, which have elements that shine heat on an area, were likely to be more effective as the heat was felt more directly.
Consumer conducted a test comparing a 400W flat-panel heater, a dehumidifier and a 2,300W thermostatically controlled oil-column heater in a well-insulated bedroom. The World Health Organisation says bedrooms should not be colder than 16°C.
The oil heater took less than an hour to heat the room from 8°C to 16°C, and maintained it at that temperature for another hour. It cost 45c to run for two hours.
The dehumidifier raised the temperature quickly, but tailed off, giving a final temperature of 11.6°C after two hours. The dehumidifier used 11c of electricity.
The flat-panel heater slowly raised the temperature to 12.2°C after two hours and cost 19c.
Econo-Heat director Mark Shepherd said most of his customers understood panel heaters should be used to take the chill off a room. The firm sold about 40,000 heaters a year.
He said a 400W panel heater would keep an insulated shed 4°C or 5°C warmer than the outside air. A bedroom could be up to 10°C warmer with a panel heater than it would have been. But people who tried to heat one bedroom would have problems with internal heat loss.
Which heater where?
First thing in the morning, while the kids are having breakfast: You need to heat the air quickly. A fan heater is probably best.
At home during the day, for background heat: A panel or oil column heater with thermostat will keep the temperature at a comfortable level
When you've been out all day and are coming home to a cold house: A radiant heater will offer the option of a lower setting and no fan noise.
At night in a baby's room: Choose an oil column heater with a thermostat.