As winter weather bites, so have the electricity bills, with heat pumps being blamed for skyrocketing accounts.
Some South Island consumers who complained after seeing their power bills more than double from last year have been have been told there is nothing faulty with their metering or billing.
Contact Energy told the Southland Times it had investigated a number of complaints this week, including several from customers who had power bills leap by more than $500.
Retail general manager Jason Delamore said the company had looked into each case and established a change in user patterns, such as the installation of heat pumps and the use of oil column heaters, was behind the increases.
Contact communications manager Jonathan Hill said estimated accounts were based on usage at the corresponding time the previous year, so if the company was unaware of change in energy use, the next actual reading would factor in a significant catch-up.
The installation of a heat pump in Christchurch woman Selena Cannell's home was "almost certainly" the key reason her bill had more than doubled from $160 in May to $400 in July, he said.
Heat pumps were efficient but still impacted heavily on power bills, Mr Hill said.
"There is a perception out there that heat pumps are almost free to use - that is not the case at all.
"We don't think it would be uncommon for people who are running them really hard to incur costs of up to $5 a day," he said.
Trustpower staff told disgruntled Timaru customer Moira Melhopt that a heat pump could cost up to $7 a day to run.
"More and more New Zealanders are getting heat pumps, so all retailers will be facing similar issues," Mr Delamore said.
However, Ms Cannell disputed Contact's claim a heat pump had caused her power bill to double.
The company that installed the heat pump had told her it should be costing only $60 to $70 maximum a month, she said.
Ms Melhopt said she felt disappointed about what she had been told.
"We have been promised efficient heating and therefore lower costs with a heatpump."
Daikin New Zealand, which specialises in heat pumps, said they cost between 10c and 20c an hour to run.