Steve S wrote:
Now that my heating controller is nearly done I've been using it to collect
data. Some of it is quite surprising, at least to me.

We have a ~25 year old 120L HW cylinder with foam insulation. Over a period
of 6 hours with no heat input (and just a couple of handwashes) the
temperature dropped from 61.5 C to 56.5 C. Measurements have a resolution of
0.1C. According to my calculations that represents a loss of ~2.5
megajoules, which over 6 hours is an average of ~115W.

I daresay modern cylinders could do a lot better than this, but somehow I
was expecting the losses to be higher.

If you've got the space, an extra wrap of loft-insulation can be a good
plan.