Households across the UK could end up collectively spending £2.2 billion more than they need to on energy over the next four months, according to analysis.
Energy provider First Utility, which made the calculation, said the potential overspend is due to large numbers of customers sitting on the standard variable tariff of a major provider, rather then switching to a cheaper deal.
It reviewed gas and electricity usage and spending data since January 2014, to identify the months of the year in which the average household consumed the most energy.
First Utility said more than half (52%) of household energy is typically consumed during the colder months between November and February - making this time of year particularly expensive for households.
Ed Kamm, UK MD of First Utility, said: "Households who have never switched are likely to be overpaying to the sum of £300 annually on a standard variable tariff."