Energy suppliers cannot just blame "old and draughty" houses for rising fuel bills, a leading comparison site has said.
The chief executive of npower earlier claimed gas and electricity prices were rising because British houses waste so much energy.
"The reason why UK energy bills are high is both that the housing stock is not energy efficient but also that suppliers have kept whacking up prices year after year," Mark Todd, co-founder of switching website energyhelpline.com, said.
"Suppliers have raised prices by 140% in the last nine years while users have cut usage. Typical gas usage is down 34% and electricity consumption by 3% and this is in part because many customers can no longer afford to heat their homes.
"The biggest reason for customers cutting usage is unaffordability so without doubt UK homes are now colder than they were in the past. It's crunch time for the energy market and suppliers can't just blame the houses."
Labour's planned freeze on energy prices would hamper investment, the chief executive of npower has said.
"I think it is already having a dramatic effect already. What it's doing is freezing investment," Paul Massard told BBC Radio 4's Today programme.
Mr Massara, who earlier claimed 'Old and draughty houses' were to blame for the UK's rising energy bills, also criticised the Government's plans to reform green levies.
"They made some technical changes on what carbon we actually have to deliver. That reduced the bills by about £53," he said.
"We don't want them to scrap it because actually we think that insulating old people's homes and insulating homes which are leaky is the right thing to do.
"What they should have done is taken all of it into taxation because actually it is unfair that everybody gets hit by the same way in the terms of insulating people's homes."
The chief executive of npower has said he wants to dispel myths about the energy industry in a bid to restore trust in the under-fire sector.
Mr Massara said the actual unit price of energy in the UK was one of the lowest in Europe, but claimed bills are high because British houses waste so much energy.
In the supplier's second "Energy Explained" report, the chief executive said: "At times during the debate on energy, facts have been in short supply, but we urgently need to dispel some myths to restore trust in the energy industry.
"Suppliers control less than 20% of a bill and I want to shine a light on all the different aspects of energy, particularly to reassure my customers that there is no hidden profit. We made a 3.2% margin in our retail business in the first nine months of 2013.
"Over the same period our power stations were struggling to recoup the hundreds of millions of pounds in investment required to build them, and made a loss of £59 million."
Earlier, industry regulator Ofgem accused npower of "misleading" consumers with "incorrect" data on network costs.
Paul Massara said the actual unit prices of gas and electricity in the UK are among the lowest in Europe, but bills were high "because British houses waste so much energy".
In the supplier's second "Energy Explained" report, Mr Massara said: "If we can increase the efficiency of the UK's old and draughty housing, we can ensure that annual energy bills are some of the lowest too."
The npower chief executive warned consumers that the cost of bills was set to increase unless households introduce efficiency measures.
An official Ofgem response to npower's company costs announcement read:
We welcome npower's effort to inform the energy debate, however their data on network costs is incorrect and misleading.
We offered to help npower improve the accuracy of their numbers for network charges and it is disappointing that they did not engage fully with us until after the document had been circulated.
Ofgem directly regulates the money that network companies can earn through charges.
Given this level of certainty we can see that after 2014 network costs per household are expected to remain broadly flat in real terms.
It is unclear how npower can state with any authority otherwise.
Energy regulator Ofgem has accused npower of producing a "misleading" report about the cost of producing gas and electricity.
The firm had argued that rising costs of transporting energy meant they could do little to control the prices charged to consumers.
Npower had released documents claiming those transportation costs would increase by around 20% by 2020, but Ofgem said the data was "incorrect and misleading".
Energy firm npower has issued an apology after it was found to have breached energy sales rules, saying, "We made some mistakes and we've put things right".
The firm's website states: "After a thorough investigation, it is fair to say there were some requirements of our licence conditions we didn't meet at that time. We're sorry.
"We've identified the relevant issues and addressed them completely, putting right what was wrong and learning valuable lessons in the process.
"We'll be giving at least an extra £25 to around 125,000 customers who already receive the Warm Home discount".
Energy regulator Ofgem said npower has "done the right thing" after the firm agreed to pay £3.5 million for breaching energy sales rules.
Sarah Harrison, Ofgem's senior partner in charge of enforcement said:
npower has done the right thing by stepping forward and recognising that, whilst it was making changes to improve its sales processes, weaknesses remained which affected consumers’ ability to compare supplier offers fairly.
These issues have been fully addressed by npower and Ofgem welcomes the company’s actions and its agreement to pay £3.5m to directly benefit vulnerable consumers.
Energy firm npower "fell short of the standards set by Ofgem’s tougher 2009 marketing rules", the regulator said.
Ofgem's investigation centred on the sales processes and information used by the firm when customers were making decisions about whether to switch supplies to npower.
The regulator said npower has "remedied all the issues raised" by the investigation, adding, "all breaches ceased by September 2012".
npower has agreed to pay £3.5 million to help vulnerable customers after it was found to have breached energy sales rules on the doorstep and over the phone, Ofgem announced: