Energy websites crashed on the last day of March as customers rushed to submit meter readings before April's price jump.
The issue has become an industry-wide problem, with energy giants such as Shell Energy, E.ON, British Gas, EDF, SEE and Scottish Power reporting faltering websites.
It comes after Money Saving Expert Martin Lewis urged householders to submit meter readings for gas and electricity to their supplier on Thursday ahead of Ofgem's price cap increase.
The early submission would prevent firms from estimating usage and charging energy used before 1 April at the new higher tariff.
How big is the price jump?
Households will see the most significant rise in the cost of energy in living memory from Friday with bills increasing by 54%, or almost £700, to just under £2,000 a year.
Gillian Cooper, head of energy policy at Citizens Advice, said: "We'd recommend sending meter readings to your supplier ahead of the price cap rise on 1 April. This means your energy company will have an accurate picture of your usage before higher rates come in."
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Advice that industry bosses have claimed has led to the website crashes.
She added: "If you're struggling to pay your bill, speak to your energy provider as they have to help you. Citizens Advice can also provide you with free, independent support."
Those on default tariffs who pay by direct debit will experience a rise of £693 from £1,277 to £1,971 from 1 April.
While prepayment customers will see an even larger jump, with their price cap going up by £708, from £1,309 to £2,017.
The regulator was forced to hike the energy price cap to a record £1,971 for a typical household as gas prices soared to unprecedented highs.
'Biggest energy shock in living memory'
Fuel poverty charity National Energy Action (NEA) warned the cost of heating an average home has now doubled in 18 months.
Which will leave 6.5 million households unable to live in a warm, safe home across the UK.
NEA chief executive Adam Scorer said: "This is the biggest energy price shock in living memory.
"Millions of people will be priced out of adequate levels of heating and power. For all the anticipation of these price rises, many people on the lowest incomes will be crushed by the reality.
"Quality of life for millions of people will plummet."
"Warm homes, cooked food, hot water, clean clothes – all cut back or cut out. Debt will spiral. Physical and mental health will suffer.
"Last week, the UK Government chose not to prioritise support for those on the lowest incomes. It has crossed its fingers that the market will right itself. This 'wait and see' policy could cost lives next winter."
What should you do if you are unable to submit your meter reading?
Energy firms have assured customers that their websites will be up and running properly as soon as possible.
Scottish Power, for example, told customers it had increased its website capacity by 500% but still could not cope with the "unprecedented demand."
But Energy UK, the trade body for the industry, urged people not to worry if they were unable to submit a meter reading ahead of April.
The firm said: “Most suppliers are offering alternative options such as submitting at a later date, and different methods to send meter readings such as text, social media and email.
“This demonstrates the scale of the problem and how worried people are about high prices, which is why we have been asking government to intervene to provide further support to consumers."
Among those experiencing difficulties are E.On, who seem to be facing issues with letting customers log in to their accounts.
An E.On spokeswoman said: "We are seeing unprecedented volumes of customer traffic to our website and app.
"While we work to resolve this, we can confirm to our customers that any meter readings they take today can be updated to their account online in the coming days.
"We apologise for the inconvenience caused. Smart meter customers do not need to do anything as their readings will automatically be shared with us."
Several other firms have experienced the same or comparable issues but until issues are completely resolved finance specialist Martin Lewis took to social media to offer further advice.
He suggested a number of steps people can take if their providers site has crashed, including:
Taking a photo of the reading accompanied by something that showcased the time and date in case of a future dispute.
Attempt to submit readings online later when websites are functioning as normal.
Check to see if suppliers offer alternative ways of sending off reading.
And, if you have a smart meter, your readings will be sent off automatically.