The survey of 1,000 adults looked into how energy efficient people are and discovered that just 14 per cent of respondents knew that the optimal temperature for a home is 21 degrees, with a quarter (24 per cent) believing it to be higher than 21 degrees. This factor alone affects energy bills, since, according to the Energy Saving Trust, £65 is added to the average annual heating bill for each degree Celsius that the thermostat is turned up.