Excel formula to calculate electricity bill

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However even though (per the graph) it is (under thermostatic control) 'on' for only a couple or so of those seven hours per day, that still represents a substantial proportion of my total consumption - "a third" might have been a bit of an over-statement, but it is certainly usually more than a quarter.

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Rather than having it come on at 1:45am to do the bulk of the heating, then cycling on and off several times to replace lost heat - why not if you can, have it start the bulk heating cycle at 6am?
 
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Rather than having it come on at 1:45am to do the bulk of the heating, then cycling on and off several times to replace lost heat - why not if you can, have it start the bulk heating cycle at 6am?
We used to do that, but it actually makes very little difference, presumably because the total amount of heat loss during the 7-hour E7 cheap period is much the same, regardless of when one heats the water during that period. In other words, when we delayed the immersion start time by a few hours, until around 6am, the initial period of continuous heating became longer (than if it came on at about 1:30am), by not much less than the total of the 'top-ups' (each only a very small number of minutes long) with the earlier start - so the total amount of energy used was almost the same.

Whilst that approach was fine for us, the reason we changed it was because we quite frequently have family members staying here who (at least, pre-Covid) had to get up between 05:00 and 06:00 to get to work (and wanted hot showers!). That was a double whammy - having more people in the house inevitably meant that the temp of water in the cylinder was lower at the start of the night, but, in any event, starting heating around 06:00 was no good for people wanting to have showers before or soon after that time!

We could, maybe should, have changed back to the original system, or some intermediate one - but since my experiments back then indicated that (presumably for the above reason) it made very little difference, I haven't bothered. I would add that the above comments are in the context of an extremely-well insulated cylinder (probably 500-600mm all around - including above and below)!

Kind Regards, John
 

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