Energy Quiz/washing up with a kettle

I was on a stall in a shopping centre today with my Transition Cambridge hat on, raising awareness of how much energy we use for different things. I wrote a little quiz for this event  - people were asked to pair up activities, such as driving a car 3 miles, or vacuum cleaning the house for 30 minutes - with how much energy they used. We only put out 7 activity cards but even so no-one got all the answers right. For example,  most people vastly over estimated how much energy it takes to dry your hair with a hair dryer and considerably underestimated how much energy it takes to drive a car. You can see more details of the quiz here.

Also at this event I was asked if it saves energy to use a kettle to warm water for washing up rather than using hot water from the tap. As usual, the answer depends on your behaviour:


An electric kettle is around 90% efficient whereas the average gas boiler, allowing for losses in heating and in pipes and from the tank, is more like 75%. So all other things being equal, the kettle will use less energy but because the carbon emissions from electricity is about 3 times that from gas heating, the kettle solution definitely has more carbon emissions.

However, in my book I estimate that the average household uses 30 litres of water for washing up a day (this is half the waterwise estimate for how much water is used but I can't believe that is all hot). Assuming the washing up water is at 40C, heated from 10C, and your kettle takes 1.7 litres which seems to be typical, then  you would need about 6 kettles full to heat 30 litres of water. (1 kettle full can be diluted with twice the amount of water to get you just over 5 litres of hot water).  You aren't going to wait for 6 kettles to boil are you? You're going to use about 2 or 3 aren't you? Well if you use only 2 kettles full you win on carbon emissions too.

Of course, if your hot water comes from a solar panel, you are definitely better off with water from the tap.