Age UK London said there was "a significant and concerning lack of awareness among some respondents" about different ways to save energy.
The report said: "Many people told us how they either do not turn on the heating or find many ways to cut back."
Polling carried out by Opinium for the charity focused on how older people were managing the cost of living in a time of rising prices.
It found that one in four older Londoners said their home was colder than they would like it to be "all of the time" or "most of the time".
More than one in three (36%) older Londoners said they felt less financially secure heading into 2025 than at the start of 2024.
The government's decision in July make Winter Fuel Payments means-tested, with only households receiving Pension Credit or certain other means-tested benefits eligible to receive the benefit, has also had an impact.
One respondent told Age UK London: "Withdrawing the Winter Fuel Payments is a blow.
"A lot of struggling pensioners, who have small occupational pensions and a modest amount of savings, are not considered poor enough to qualify for Pension Credit and will lose out and may have to choose between heating and eating."