Millions of British households are warned they must take urgent action before New Year's Day or risk overpaying their bills.
The warning comes as energy bills are set to rise from £1,717 to £1,738 per year for a typical household using both electricity and gas.
Energy customers on all major providers including British Gas, Octopus, Eon, EDF, and OVO risk having their December usage estimated at January's higher rates.
Customers on standard variable tariffs who don't have smart meters need to submit their meter readings before January 1, when the Ofgem price cap increases.
The energy price cap controls what most households in England, Scotland and Wales pay for energyGETTYUswitch warns that suppliers will bill customers based on estimates, which are often higher, if accurate readings aren't submitted.
EON has advised its customers on standard variable tariffs to submit readings before January 5 to prevent estimated usage charges.
According to Uswitch research, households with typical energy consumption will spend £165 on energy in January, compared to £135 in December, due to higher usage and rates.
The difference for a week's worth of energy at January rates compared with December is £6.67 per home on average.
This could lead to a collective overpayment of £66million across Britain if households fail to submit readings.
Research shows 14 per cent of Britons needing to submit readings are unsure how to do so, while 12 per cent don't know where their meter is located.
For standard electricity meters, customers need to record the first five numbers shown from left to right, ignoring any red numbers. If their meter has two rows of numbers, both readings must be noted down.
For dial electricity meters, Britons need to check which way the dials turn, as each rotates in the opposite direction to the one before.
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For digital metric meters, energy customers should note the first five numbers from left to right, ignoring numbers after the decimal point.
Digital imperial meters require reading the four black numbers while disregarding the two red ones. Smart meter users don't need to submit readings as their usage data is sent automatically.
Elsie Melville, energy expert at Uswitch, said: "Customers who don't have a smart meter should aim to submit their readings before or on Wednesday 1 January, so their supplier has an updated - and accurate - view of their account.
"If you leave it any later than this, then some of your December energy usage could end up being estimated and therefore charged under the higher January rates."
Cornwall Insight predicts the price cap will rise by one per cent again in April to £1,762.
This would mark a third consecutive increase for households on default tariffs, following the one per cent rise in January and 10 per cent increase in October.
Fixed tariffs could help households avoid price rises and lock in rates for at least a year, with some deals currently cheaper than the January price cap.
