When watching television, particularly during the winter months, it is easy to get chilly. So many people may cuddle a hot water bottle as they sit down for an hour and a half of strictly or His Dark Materials, my mother, on the other hand, has a different device, a miniature electric blanket meant for relieving muscular pain that we often use in place of a cumbersome, time consuming hot water bottle. Over the weekend however, my sister raised the question, of a hot water bottle and the small heat pad, which is the most environmentally friendly?
Having a degree in theoretical physics and, more importantly, an internet connection, I decided to research it properly.
First we must define our terms. Neither the hot water bottle nor the electric blanket directly emit carbon pollution, so it is straight forward enough to compare electrical input.
The hot water bottle uses electricity through the kettle. A kettle requires about 1200 Watts [W] while the miniature blanket we have needs up to 110 W depending on which of the five settings you choose.
So that would seem to be an easy enough comparison, 110 is much less than 1200 so the blanket must be the better winter warmer for the environment. Well, no. actually it’s not that simple.
The unit Watt, named after James Watt who invented the steam engine, isn’t a unit of energy but a unit of power which is energy measured in Joules divided by time in seconds.
W=J/s
The hot water bottle only uses energy while you are boiling the kettle. The kettle will take about 2 minutes to boil, or 120 seconds. So the total energy would be:
1200 joules per second times 120 seconds equals 144000joules.
The blanket on the other hand continues using energy as long as you are using it. However most blankets default to a setting that turns itself off after an hour. Assuming the blanket is set to switch off after an hour or 3600 seconds:
110 joules per second times 3600 seconds equals 396000 joules.
So actually the blanket will use more than double the energy a hot water bottle would use over all if using the highest setting. This means that if you need to be warmed for more than half an hour then the bottle would be more energy efficient.
There are further complications of course. A lower setting on the blanket would change the numbers. So would using one kettle to boil water for two bottles. You may also feel that the bottle is too cool to be useful before an hour is passed.
An entirely separate problem is the possibility of micro plastics. Although there appears to be little creditable evidence that hot water bottles leak plastic particles into the water that is then thrown away, there is credible research suggesting that water stored in plastic bottles for a long period of time does contain microscopic pieces of plastic. So if you forget to empty out your hot water bottles after using them, then you may later be pouring micro plastics in to the water system which is its own blight on the environment.
So if you are feeling the cold as you sit down to watch an ITV drama or David Attenborough documentary this weekend, you may care to take a second and think scientifically about how best to warm yourself without warming the planet.