You use that method fair enough but don’t go around telling people it’s a more convenient way of heating water what can be more convenient than hitting a switch and having hot water,the method you use gives you warm water but even if you have a jacket round the tank from the second you disconnect you’re plug it’s cooling down and it won’t stop the hose reel freezing on cold nights in the depths of winter.
An interesting point here.
While that is technically true, of course the water in the tank will cool during the day (and more so when the ambient temperature is colder) the temperature of the water actually in use during the day doesn't actually change much.
How so?
When you heat a tank of water, using an immersion heater or whatever, you do not get the same temperature of water throughout the tank. I've done some quite extensive testing on this over the last 2 years, and what I have found is that the water is usually about 10-15 degrees hotter at the top of the tank than the bottom.
As an example, I have heated the water in my 700 litre tank up to around 45 degrees, as measured at the bottom. But the temperature at the top is around 55-60 degrees. I have measured this differential during the day, and it seems to stay almost the same no matter how much driving and mixing is done. (It's a baffled Ionic tank)
So, what this means is that, as you use the water, you use the coldest water first, and gradually as the level goes down you start to use the hotter water. If all were equal, the water you use during the day would gradually get hotter and hotter.
Now, the cooling of the water has the opposite effect. By amazing co-incidence, my tank loses about 10 - 15 degrees of heat during the day due to cooling (depending on how cold the day is). This cooling effect is actually very welcome as the heat the tank loses is radiated into the van, which keeps the whole van warm throughout the day, even in sub-zero conditions.
So, these two opposing effects pretty much cancel each other out, to give a pretty consistent 45 degrees of working water.
TL;DR - The water does cool during the day, but as you use the water, you start to use the hotter water at the top of the tank which cancels out the effect.