Hi OP. First of all, I am very jealous of your night energy charges. We?re not on Economy 7, but something called Total Heating Total Control, where heating is charged at the equivalent of a low night time rate. Our heating rate is nearly 8p a unit!!
£8 for night time energy over 3 months is clearly a nonsense. If it helps I have summised the following for our house:
Electric water heating uses 6 or 7 units a day
A small modern-ish storage heater on minimum will use about the same, but you say your are set to 6 input? So say 10 units a day per heater.
You don?t say how many storage heaters you have, but say 3 ? then your total nightime usage would be in the region of 36 to 40 units per day. For argument?s sake that?s around £1.50 a day. So you?re looking at about £120 for the 3 months for night time charges ? it would have been about £22.50 for just water.
If your storage heaters are old this could be maybe half as much again.
[Woe is me, the same usage would cost us nearly £240 for 3 months just for heating, plus all the standard priced energy Hence we only use one storage heater.]
Then if your average daytime use is, say, a modest 10 units a day you can add another £1.60 per day. 90 days = approx £144. Total approx £264 for 3 months.
If you?ve only been paying £30 per month, when you?ve actually been using closer to £90, and probably more through winter, then it?s easy to see how these charges have arisen.
BTW, when we had Economy 7 our storage heaters automatically came on when the night rate kicked in. Now they come on whenever the electricity company decides they will (satellite switch).
We pay £120 a month ? after the price rises it just covers our usage and we have big arrears from using an old storage heater in our last house.