Npower has increased off-peak night tariffs meaning some households have seen bills rise by hundreds of pounds.
One customer was told her prices are changing from 1 October and from that date, her off-peak electricity will be charged at the 'standard' rate, meaning a potential rise of £304.87 a year.
Calling the increase 'scandalous', Jan said that when she contacted Npower about this, she was simply advised to change to another supplier.
She asked for a meter exchange instead only to be told that wasn't possible due to Covid, despite her meters being external.
Rise: Npower customers have complained that their tariff will be increasing from October
Another customer, Ken, said he was informed his deal would be increasing from 9 pence per KW to 16 pence per KW on the first of October, making his bill £167 more expensive a year.
He said when he contacted Npower, which merged with fellow Big Six supplier Eon last year, it was unable to offer an alternative, such as the Economy 7 plan.
The Economy 7 energy offer is popular with those looking to save money as it allows customers to get cheaper electricity for seven hours each night.
The cheaper rate period typically falls for seven hours between 10pm and 8.30am.
Both Jan and Ken are currently on the off-peak night tariff with Npower in order to save money by using overnight storage heaters.
This means their bills should be reduced as the heaters work overnight, storing heat, which it then gives out during the day.
This is Money contacted Npower and was told that when Ken contacted the energy firm a couple of weeks ago, he wasn't put through to the correct team who would have been able to discuss changes on the customer's account, adding it sincerely apologises for this.
Since making contact a few weeks ago, it said all of its customer service colleagues had been re-briefed to ensure any customers who call through about changes to their restricted meters, go through to the dedicated team to discuss their options.
Customers are encouraged to change supplier if they are being charged over the odds
A spokesperson for Npower said regarding the payment changes: 'Like many energy suppliers, we have simplified our restricted meter rates, which are multiple meters that have multiple rates, to align with the one rate SVT price cap, meaning that these customers pay the same unit rate no matter what time of day energy is used.
'The new rates which come into effect from 1 October means that some customers will see decreases in their annual estimated costs whilst others will see increases.
'We have written to customers impacted by this change with details of a dedicated team who are on hand to offer a package of support which include alternative tariff options, energy efficiency advice, affordability reviews and the removal of exit fees if a customer chooses to switch tariff or supplier.
'Our specialists are also proactively calling customers to discuss their options and to understand whether we could offer any additional support.'
The energy price cap, introduced by Ofgem, was introduced in January 2019 as a way of keeping down the cost for households across the UK.
It is currently set at £1,126 for standard variable tariff customers but is due to drop to £1,042 per year from 1 October, saving homes £84.
Suppliers are not allowed to charge above the cap but they can offer cheaper deals for savvy customers.
The savings are bigger for prepayment meter customers who will see their bills drop by £95 from £1,164 to £1,069.
The cap will remain in place until April 2021 when the level will be reviewed.
Customers stuck on pricey tariffs are encouraged to use price comparison services to see if they could save money by switching to a different supplier or move to a fixed tariff, which are typically much cheaper than default options.
Moving to an Economy 7 tariff can be good for those who use more energy during the night than during the day.
However, with many households still working from home, energy usage is likely to increase during the daytime hours, with energy bills set to soar for some.
Could you cut your energy bills... or help the planet and go green?
Millions of people could be needlessly overpaying for their energy as they fail to switch to providers who offer cheaper deal.
They may also be missing out on the opportunity to help the planet and fight climate change, by switching to green deals that offer electricity from renewable sources and more environmentally-friendly gas.
With our partner, Compare the Market, you can compare energy tariffs and exclusive deals.
Why not find out if you could save hundreds of pounds a year on your energy or go green?
>> Check to see if you can start saving money now