I see we have a number of electrically savvy people on the forum, so hope someone can solve this for me.

We recently installed a geyser timer.

Geyser is 150 liters and runs at 55 degrees in Durban area.

Is there some easy way ( software application / spreadsheet, etc ) that could be used to determine the savings vs re-heat power required ?

For example, if I use the time to cut the power to the geyser, then use 75% of the hot water and leave it off for 4 hours, would it use more power to re-heat compared to leaving it on permanently.

Or, if the same as above, but we don't use any water from the geyser, what is the rate of temperature loss for a standing geyser, and how much more energy would be used, compared to just leaving it on all the time.

Naturally, the list of combined factors could be a lot longer, but essentially I am trying to get a feel of weather we would be saving anything if we turned off the geyser for x amount of time each day. There must be some determinable point where it is cheaper to leave it running.