Advice groups warn some people could be paying hundreds of pounds more than they should be.
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Gary Day, a retired engineer from Abergavenny, has saved himself and three neighbours more than £2,300 after discovering their meter clocks were wrong by several hours. Tariffs such as Economy 7 and Economy 10 offer customers a lower rate for electricity during some hours.
Nearly four million households across the UK are reliant on tariffs, according to the consumer watchdog Which? Mr. Day told Which? "I have only checked four meters and every single one of them was wrong. I am horrified that there might be others that have these clock errors and don't realise it."
Mr Day and his neighbours were all on time-of-use tariffs with Swalec. A spokesperson from the company offered to recalculate any charges for customers whose meter clock is incorrect.
"We will always make sure our customers are not left out of pocket. If anyone suspects their meter clock may be incorrect, or would like us to check, please give us a call on 0800 975 8317."
Energy company SSE Power - which has around 1 million SWALEC customers in Wales - says fuel bills will be cut by around £50 a year on average.
Prices are due to drop from March 24th following a government decision to change green levies.
SSE says it will cost less to supply people with energy in 2014/15 and those on variable tariffs will see their rates cuts by 3.5%.
That means the annual bill for a typical dual fuel customer, averaged across all regions, will come down from £1,224 to £1,174 says SSE.
Unlike some other energy providers SWALEC customers with fixed and capped price tariffs will also have their rates cut by the same amount.
Around a million households in Wales will see energy costs rise from today.
The owners of SWALEC, SSE, announced last month an average hike of 8.2 per cent.
It says that equates to two pounds a week for a typical dual fuel customer.
Wholesale prices and environmental, social and delivery costs are said to be to blame.
Shadow Welsh Secretary, Owen Smith, has accused Swalec owner SSE of 'spin' by claiming to have reported a £150m loss. He criticised the energy group for 'selling its own product to itself,' a reference to its wholesale and retail divisions.
No matter how hard SSE spin by highlighting a £150m loss in its retail arm, the reality is the company’s overall profit came in at around £350m. The attempt to plead poverty will fall on deaf ears in Wales, where we already have the highest energy bills of anywhere in the UK.
SSE’s profits of £350m have allowed it to give shareholders inflation-busting pay-outs while customers face inflation-busting price hikes. It can only do this because it operates in a flawed market. The group sells its own product to itself and hasn’t got enough serious competition.
– Owen Smith MP, Shadow Welsh Secretary
A long-term solution is to reform the energy market, which a Labour Government will tackle head on. But we also need a short-term answer to sky-high prices. That’s why Labour is calling on David Cameron to freeze energy prices and address the cost of living crisis facing families across the country.
The group managing director of SSE's retail business, which owns Swalec, has defended the level of the company's dividend payments to shareholders, while raising prices for consumers.
It comes as the energy giant announces a £115.4m loss in its retail supply business during the six months to 30 September, compared with a £48.3 million operating profit a year earlier.
Will Morris, group managing director of SSE's retail business, said: "Some politicians and media commentators have claimed recently that we value our shareholders more than our customers.
"Or to put it another way, we're focussed on paying them a dividend on their shares, regardless of what that means for our customers.
"Nothing could be further from the truth."
He added: "Without the investment made by shareholders, we couldn't afford to build the infrastructure or buy the equipment needed to deliver what customers need."
Energy giant SSE, which owns Swalec, has revealed it made a loss of £115.4 million in its retail supply business, just weeks after announcing a hike in household bills.
The company insisted it was battling "difficult" energy market conditions.
The group, which has more than nine million customers and is the UK's second largest generator of electricity, blamed the performance on rising costs of wholesale energy, environmental and social policies and distribution.
Its overall underlying group profits fell 11.7% to £354 million in the six months to September 30.
The first half loss in its retail supply arm compares with a £48.3 million operating profit a year earlier.
SSE was the first of the major suppliers to announce a tariff rise, saying last month that it would lift prices by an average of 8.2% from Friday.
Shadow Welsh Secretary Owen Smith has sharply criticised the decision by SSE to increase energy prices.
Families in Wales are being particularly hard hit by this increase.
Wales – where there are more than one million SSE customers - already suffers from the highest energy bills and lowest levels of disposable income of anywhere in Britain.
This is nothing more than an act of gross profiteering. People in Wales are clearly being ripped off by this energy company and it needs to stop.
People are already suffering under David Cameron's cost of living crisis and now, thanks to his failure to stand up to the big energy companies, consumers will be hit by ever high bills, with SSE increasing prices by nearly 10%. This is a sign that David Cameron only stands up for the privileged few.
If ever there was proof that Ed Miliband was right and we need a freeze on energy prices, this is it.
Labour Leader, Ed Milliband, has taken to Twitter to criticise the announcement that SSE, which owns energy supplier SWALEC, will increase its prices by 8.2% from next month.
Today’s energy price rise hitting especially hard in Wales where SSE has more than one million customers http://t.co/XMMU9Om6Be