Electric Boiler

[JohnW2]

The thermal store seems to be around the 210 litres and I know in brother-in-laws house there were twin units so 420 litres ...

One would need quite a lot of space to accommodate two 210 litre 'stores'.

... so at around 50°C temperature drop around 25 kW of power stored. As to if that is enough to bridge the gap is of course unknown .... not sure when economy 7 turns off, but the fire I would assume is lit when they get home so has to last until around 7 pm when the fire is up to heat, and with solar panels there is input for most of the day.

Yes, perhaps, but in calculating the total heating costs, one would obviously have to add the cost of the fire's fuel from 7pm onwards.

However, I think that one thing which is being overlooked is that, unless one has impractically large radiators, the (space) heating ability of water stored in a thermal store is presumably going to be far from constant. As heat is used up (without being replaced, during the day), the temperature of water in the store will progressively decrease. By the later parts of the day (which is probably when many people most want heating), the water will have become so 'cold' that it would do relatively little heating via 'normal size' radiators. If the thermal store held just enough energy to theoretically keep the house going for the 17 non-E7 hours, the temp of that stored water should be down to around 'ambient' temp towards the end of that period.

I was unaware of the new rates for economy 7 internet says average pay 111% in day and 63% over night ......

Prior to April 2017 (i.e. 2 years ago), I was paying 16.34 p/kWh day and 6.89 p/kWh night, and the corresponding non-E7 rate was 14.39 p/kWh. At that time, my E7 rates were therefore about 114% (of non-E7 tariff) during the day and about 45% at night. With the supplier I switched to last week (which offers almost the lowest figures of any supplier), those figures are now 15.446 p/kWh, 10.836 p/kWh and 16.16 p/kWh respectively - so that my E7 rate are now about 96% (of non-E7 tariff) during the day and about 45% at night.

.... and clearly that 111% is the big thing ... is it worth paying the extra during the day?

That's always been the main consideration for me in the past but, with my day/night split, it has always been financially advantageous to have E7, albeit decreasing so during vthe last couple of years. However, as you can see above, the new E7 tariff I now have is (pretty uniquely) a 'no-brainer' (at least, with this particular supplier), since even the daytime rate is fractionally lower than the non-E7 rate! I've never seen that before, but, as i said, means that electricity bills will always be less with E7 than without it, regardless of my pattern of usage! It will be interesting to see if any other suppliers follow suit!

However the big problem with economy 7 was not the cost, but the silly brick radiators which you can't turn off, it hardly matters what the cost is, if you can't stop the heat escaping on a warm day.

In terms of the physics, the same obviously applies to a thermal store, but I imagine that one can probably insulate the thermal store much more effectively than one can insulate the bricks in a storage heater.

From the power they say they use, it will cost at 13.8p per unit about £900 a year on fuel, so around £600 a year extra on heating to move from oil to electric using their figures of 600 litres per year.

Yep, that's roughly right, and the same as flameport's guesstimate in the first response in this thread ... 600 litres of oil per year is around 6,300 lWh/year, and with your figure of 13.8 p/kWh, that would be about £869 per year. With my (new) E7 tariff, it would be about £683 per year if all the electrical heating was at night.

Kind Regards, John