 Millions of energy customers face a 20pc increase in their utility bills this winter, just months after gas and electricity providers raised prices by more than 10pc. Scottish Power is the first of the big six to make the move and others are expected to follow. The shock increases come at a time when turning up the thermostat will be furthest from people's minds.  From august, Scottish Power will increase the cost of its gas and electricity bills by 19pc and 10pc respectively - adding an additional ?175 a year to the average bill.  audrey Gallacher, head of energy at Consumer Focus, the statutory consumer champion, warned: "Suppliers like the comfort of the pack and, because of that, price rises often come in waves."  Energy providers have been accused of profiteering and many consumers will also be questioning the steep rises they are about to face.  Last winter, all the big six energy suppliers announced price rises, with Scottish Power raising electricity by 8.9pc and gas bills by 2pc last November. E.ON added 9pc to electricity prices and 3pc to gas prices, while British Gas raised both its gas and electricity prices by 7pc. Scottish and Southern Energy increased gas prices by 9.4pc and npower added 5.1pc to gas and electricity prices. EDF raised electricity bills by 7.5pc and gas by 6.5pc.  It all means that the average dual fuel bill will have increased by 50pc since 2007 to around ?1,450, once households factor in another 20pc rise this year.  Providers vehemently deny accusations of profiteering and argue that they have had little choice but to inflict steep increases on customers because of the wholesale price which has risen 30pc since November.  However, industry experts argue that wholesale prices are still lower than they were three years ago during the financial crisis, yet household bills are reaching record levels.  andrew Horstead, risk analyst at energy and carbon management specialists Utilyx, said that the wholesale price of gas is around ?410 per year, compared with March last year when the cost was ?485 per year.  according to Bloomberg, the wholesale price for gas in autumn 2008 hit prices above 70p a therm. This compares to 59p per therm today, showing that wholesale gas prices have actually dropped 15pc since then.  Similarly, prices in the wholesale electricity market reached ?120 per Megawatt hour in autumn 2008. Today, they are ?51.20 per Megawatt hour - less than half the price back then.  When the prices peaked more than two years ago, the average household needed to find ?1,100 a year to pay their gas and electricity bills. However, despite lower wholesale prices now, households will have to pay more like ?1,400 per year, once Scottish Power's increases come into effect in august.  "The latest tariff hike should set alarm bells ringing within the Government," said Mr Horstead. "It comes at a time when its forthcoming energy White Paper detailing its measures to stimulate investment in low carbon generation will also feed through to higher prices."  Raymond Jack, UK retail director at Scottish Power, said: "Wholesale prices for gas and electricity have increased significantly since the end of last year and continuing unrest in global energy markets means future prices are volatile.  "The rising burden of non-energy costs faced by Britain's energy suppliers, including the cost of meeting government environmental and social programmes, has also placed further upward pressure on energy bills."  On the environmental programmes, Mr Jack has a point. Whether you have an ethical conscience or not, every household is footing the bill for the Government's target to meet 30pc of the country's energy needs from renewable sources by 2020.  For instance, gas and electricity customers pay ?48 a year towards the Carbon Emissions Reduction Target and ?13 towards the EU Emissions Trading Scheme. all the extras add around ?80 to the typical household bill.  Tim Yeo, Conservative MP and the chair of the Commons Energy and Climate Change select committee, said that energy companies should be forced to publicise the prices paid for wholesale energy to prove their reasoning for price increases.  He said: "There is a very strong public suspicion, probably justified, that prices tend to go up faster than they come down."  In March, Ofgem said that when it conducted a retail market review of the sector it had found evidence that energy providers raised prices faster than they cut them. It also said tariffs were complicated and pricing was unfair.  Ms Gallacher of Consumer Focus said: "It is ironic that the [Scottish Power] announcement comes exactly when the regulator is deciding whether energy firms are serious about treating consumers properly and if energy prices are fair.  "Ofgem has put the big six in the dock, saying suppliers have been quicker to raise prices than to cut them and are bamboozling consumers with complex tariffs. Scottish Power itself is under investigation by the regulator for unfair pricing and miss-selling."  again, energy providers deny any wrongdoing. In its official response to the review, npower said: "Ofgem claims to find evidence that prices rise faster than they fall in response to changing wholesale costs, but concedes there are limitations to the work and found no clear evidence of consumer detriment. The review has not found any evidence of excess profits."  Consumer group Which? said that all energy tariffs should be structured in the same way so that customers can easily compare different deals.  Richard Lloyd, executive director of Which?, said: "This latest price rise is yet another example of the big six blaming the wholesale energy market, but energy companies have a lot of work to do to convince consumers that energy prices are fair."  WHaT CaN CUSTOMERS DO?  There is little doubt that Scottish Power's announcement is a fresh blow to households whose budgets have been squeezed after repeated increases in inflation.  Research by the website 