Hot tub
In Norway it's quite popular to have a hot tub in the garden, my daughter has recently bought one. I was a bit surprised to learn that it is kept on all the time. They say that it costs more to heat up the water than it costs to keep it hot. To keep it hot you only need to supply an amount of energy equal to the heat lost, but to warm it up you need to supply an amount of energy equal to the mass of water x the specific heat capacity of water x the temperature change. I was wondering it this is true so I have asked my daughter to measure the temperature drop in 1 hour. Then I can find the heat lost. It is possible to buy extra insulation but first let's see how much energy we are talking about. I will post my conclusion when I get the measurements from my daughter.