Solar water heating

I have been experimenting with a birthday present which is a portable water heater – well in fact a portable shower.  It holds about 20 litres of water in a bag arrangement which can be laid out in the sun and when it gets hot enough hung up and the water drained through a sprinkler on the end of a tube.  Primitive enough but it does work, and I like the low-tech nature of it.  I confess I have not actually showered using it but I have made general use of the water in the kitchen.