Energy websites have been crashing as customers rush to submit meter readings ahead of bills rising on Friday.
Suppliers including Shell Energy and EDF said they were working to resolve issues with their websites and apps.
Experts have encouraged people to submit readings so their usage is covered by the current cheaper rates.
On Friday, the energy price cap – the maximum price suppliers in England, Wales and Scotland can charge households – is being raised.
Recording a meter reading prevents suppliers estimating usage and potentially charging for energy used before 1 April at the higher rate.
A spokesman for Shell Energy said: “Our website is experiencing disruption as we deal with a surge in the number of customers trying to access their account.
“Our teams are working hard to keep it online and we thank customers for their patience.”
A spokesman for EDF said it was also aware of technical issues affecting its website and app when customers tried to submit meter readings.
“We are working hard to try and resolve this and would encourage customers to try again later and apologise for the inconvenience this is causing,” he said.
A message on the British Gas website said it was also facing “some technical issues we’re trying to resolve as quickly as possible”.
It said customers could still submit readings but warned it could take “a little time to update your account with the meter reading you provided today”.
E.On, Scottish Power, British Gas and SSE are also experiencing problems with their websites, according to Downdetector, which monitors outages.
What should I do if I’m struggling to submit a meter reading?
- Take a photo of the reading alongside something that shows the time and date
- Try submitting it online later when websites may be less busy – some websites allow you to say when you took the reading
- Many suppliers also offer alternative ways of submitting readings, such as automated phone lines and apps
- Customers with smart meters don’t need to worry as readings are automatically sent to their supplier
One British Gas customer, Izmir Smajlaj from Streatham, said he had tried for almost two hours to submit a meter reading online.
“Every time it pops up as an error. The system is probably overloaded with people trying to do the same,” he told the BBC.
“I tried a laptop, phone, my partner’s phone. Tried to reach the British Gas helpline for half an hour then gave up [because] I’m at work.”
Graham Ruddock, a retired civil engineer from Bracknell in Berkshire, said he had been trying to log into his account with E.On Next all morning to enter his meter readings.
He told the BBC he had been unable to access his account, instead getting a message advising him to try again later because of “higher than usual levels of traffic”.
Graham said his gas and electricity bill was rising from £80 a month to £200 a month from April, adding that energy price rises would be “the biggest single change in my finances”.
In response to one customer who complained they were unable to submit meter reading online, E.On Next, in a now deleted tweet, blamed consumer expert Martin Lewis for “creating unprecedented demand bringing down Britain”. Mr Lewis had urged his social media followers to submit meter readings ahead of 1 April when prices will rise.
An E.On spokeswoman said it was seeing “unprecedented volumes” of traffic to its 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.”
Meanwhile, a message on Octopus Energy’s website warned customers of long waits for its phone lines, saying they were “super busy”.
It advised people to make a note or take a photo of readings and submit them on Saturday instead, adding that prices for its standard variable tariffs do not change until 00:00 on 2 April.
“You will have a week to submit [readings] to us, so please don’t worry if you can’t get it to us that day,” the message added.
