Economy 10 hot water

You are not comparing like with like.

To start with you are comparing different time periods.

In 30 mins you would be lucky if the top half of the tank had been heated. (As you keep heating it it will gradually heat right to the bottom and then the thermostat will cut the power to the heater).

An accurate comparison would be an hour of Peak water heating compared to a hour of Off-Peak water heating.

You are also trying to compare.the cost of heating a water tank to the cost of general appliance use.

Your above “3-hour” omparision is like comparing the cost of boiling a 1,000 Watt kettle for 10 minutes with the cost of having an 8 Watt LED lightbulb on for 30 minutes. Of course the lightbulb will use less - even though it is on for longer it's kW rating is much, much, lower than heating water.

Thanks ​@Nukecad , I do get what you're meaning, I don't need an entire tank of hot water so 30 mins twice a day is enough for my needs without wasting unnecessary energy and I do know how much that costs outwith off peak hours, my trial was to see roughly how much it would cost to implement it during off peak hours and see if there was a saving and after an hour I had to turn it off because of cost. I know exactly how much energy I use, I have been monitoring it for weeks and am averaging out the same energy and credit costs every day so today's experiment didn't work out in my favour at all. I probably worded it wrongly but I know how much credit I use in every 24 hour period running appliances, heating and immersion as needed and I used excessive credit in 1 hour of off peak hot water than I had ever used previously with all my appliances, heating , kettles etc. Ideally I would have liked to have done it over the entire 10 hours off peak to get a real daily credit difference but I wasn't willing to waste credit and energy, sorry

TBH when doing such testing/experimenting you are best using the kWh figures rather than £’s and pence.

If you have a smart meter set to half hourly then that is the best way to monitor your usage throughout the day.

You can see the half hour usages in your online portal and in the app.

(I've recently moved to a supplier who doesn't do that, so I'm going to have to get the now going to  get the Bright app or similar to monitor my half hourly usage.

I'm sure others will be along with thoughts about your E10 questions.

Thanks ​@Nukecad I am a traditional meter pay as you go so basically in my mind it's all down to £££s, I'm currently £9 per 24 hours for heating, immersion hot water and everything else thrown in so I was trying to improve on that even if it was just a little less. I'll give it a couple of days and try again with the off peak hot water over 24 hours and see how much I use in £££s but between you and me I think it will be a lot more than £9 and at the end of the day that's probably what counts more than an unused tank of hot water 

Do you have two immersion heater elements in your hot water tank ? Do you mean you’ve usually run the upper one for 30 mins at a time, and now you’re trying the lower element which is on a different circuit. I may have misunderstood so sorry if I’ve got that wrong!

If I’m right the lower element will be heating a much larger volume of water so will cost much more. Heat rises but having it on for 30 mins may not give you the same amount of hot water as just having the upper element. Also the elements may not be the same Wattage of course. 
Another setup may be one long element that is inserted into the top of the tank and runs most of the way to the bottom. 

Hi ​@juliamc  it's just 1 long element on a standard hot water tank, it's well insulated and 30 mins morning and afternoon is enough to heat the water for our requirements, we don't need a whole tank of hot water, I'm just trying to be a bit more economical and trying to utilise the off peak times. I'm pay as you go so am counting in £££s not units, currently at £9 per 24 hours and trying to improve on that as the further into the season we go that will obviously increase