Advanced options
What The Hot Water Bottle Cost Calculator Works Out
- Per fill: the energy to heat the bottle's volume of water from about 10C to boiling (litres x 4.186 kJ per litre per degree x 90 degrees, converted to kilowatt-hours) multiplied by your unit rate, which defaults to the current Ofgem price cap rate (26.32p per kWh from October 2026). A standard 1.7-litre bottle is about 0.18 kWh, roughly 5p.
- Per month: the per-fill cost x fills a day x 30. One bottle a night is about GBP 1.40 a month, which embarrasses everything plugged in near it.
- Vs alternatives: the quick comparison with a 100W electric blanket at about 3p an hour and a 2kW fan heater at about 53p an hour at the current price cap rate. The bottle wins for one warm person, the heater for a warm room.
- Temperature rise (advanced): you should never pour boiling water straight in, so set a lower rise if you let the kettle cool or boil less: heating to 70C instead of 100C cuts the energy by a third.
- Kettle efficiency (advanced): the simple result assumes all the electricity ends up in the water. Real kettles lose a little to the body and the steam, and overfilling the kettle wastes the rest, so 85% to 95% is realistic.
- Bottles filled each time and days per winter (advanced): scale up for a household of bottles and work out the whole season. 120 days is November to March; 150 to 180 covers a long winter.
- Boiled on a gas hob instead (advanced): the same heat bought as gas at your gas unit rate, allowing for a hob kettle wasting around 60% of the flame's heat. Gas is roughly a third of the price per kWh, so it usually comes out slightly cheaper despite the waste.
- Electric blanket for 2 hours (advanced): a typical evening's warm-up at the wattage you enter, so you can see whether the bottle or the blanket is the cheaper warm bed.
- Microwave wheat bag (advanced): two minutes in a 1,000W microwave, about 0.03 kWh, which plays the same role at the same pennies. Retire rubber bottles every two to three years, use a cover against the skin, and for infants, the elderly and diabetic feet warm the bed then remove the bottle rather than cuddle it all night.