The figures come as the Met Office warned of plummeting temperatures which could cause blasts of snow, frost and ice in parts of the UK, following an unseasonably warm December.
The survey of 2,000 people aged 65 and over found nearly half (46%) feared the cold snap would increase their bills, 54% worried their income or pension would not be enough to cover the cost and a fifth (22%) admitted they would have to use their savings or credit.
Some 61% admitted they cut down energy use during the winter.
Campaigners called for companies to pass on reductions in wholesale energy prices to consumers, warning many elderly people faced a greater risk of dying from the cold.
Caroline Abrahams, charity director at Age UK, said: "Many older people will be feeling anxious about the current cold snap, not least because high heating costs are prohibitive for many, resulting in large numbers of older people finding it virtually impossible to stay adequately warm.
"The UK has an appalling record on cold-related deaths, with one older person dying every seven minutes from the winter cold. Even 'normal cold' temperatures of around six degrees significantly raise the risk of life-changing health problems such as heart attacks and strokes.
"There's no doubt that with wholesale costs representing about half the total cost of energy, passing on some of the reduction to customers could make a big difference to the affordability of their bills.
"But we also urge the Government to be far more ambitious about eradicating fuel poverty by introducing a comprehensive energy efficiency programme to help those who are most at risk from the cold."
The research suggested the majority of elderly people relied on other measures than central heating in trying to keep warm, including wearing more clothes, using draught excluders and hanging thick curtains.
James Padmore, head of energy insurance at comparethemarket.com, said: "When it comes to staying warm in winter, nobody should be left out in the cold. It is deeply concerning that, when the temperature drops, so many of the elderly generation are rationing their fuel usage to save money, at the expense of keeping warm.
"There are other ways to reduce energy bills apart from turning off the central heating. As the wholesale cost of energy has decreased, many fixed tariffs have also fallen. However the cheapest tariffs available often require people to change energy supplier, something that the elderly in particular still seem reluctant to do."
He added the firm's research showed 65 to 79 year olds who changed energy provider saved on average £530, but that age group made up only 9% of those who switched.