Three-quarters of British households are wasting £68 million a year by overfilling the kettle, according to a report.
Homes use nine billion litres of water every day, with showers using a quarter of the total and toilets taking 22%, the Energy Saving Trust (EST) Foundation found.
The study found 95% of people boil the kettle every day, and 40% boil water five times a day or more.
However, three quarters of households boil more water than they need, with overfilling costing them £68 million a year.
EST water strategy manager Andrew Tucker said: "When people think of energy use they think of heating and lighting, running electrical appliances or filling the car with petrol. It's all too easy to turn on the tap and not think about the consequences.
"But there is an environmental and energy cost attached to water which many people do not consider. On average, hot water use contributes £228 to the average annual combined energy bill.
"It's clear that we are all using more water-consuming appliances regularly, especially showers, but that doesn't mean we're powerless to control our water use."
The study of 86,000 British homes found the average shower lasts seven and a half minutes, while cutting 60 seconds off that time could save households £215 million on their energy bills each year.
Britons use more than two billion litres of water showering every day, with each person showering on average 4.4 times a week and taking 1.3 baths.
Just over a fifth (22%) of household water is used by kitchen appliances such as dishwashers, kettles, taps and washing machines.
The average household washes dishes by hand 10 times a week and uses a dishwasher three times a week, the study found.
But the EST said larger households could make greater energy and water savings by using an efficient, modern dishwasher rather than washing by hand.
The EST said consumers could save money and water by installing an "eco" shower head, washing clothes at 30C and only filling the kettle to the required amount.
