On electricity meter readings - and bills

I'm with SSE Airtricity, and they send reminders to submit your meter reading, before the next bill. They give you three days from the reminder, to submit the reading, or else they'll guesstimate your reading, and bill you for that - if it's too much, it'll be reduced from your next bill.

Fair enough, but I'm just wondering how they estimate your reading, if you haven't submitted your actual reading..

For example, I usually DO submit my reading, so I'm billed for exactly the electricity I've used.

However, before my previous bill, I forgot to submit the reading, and as my previous bills had been about the same, I expected the estimated bill (in late December) to be about the same as well.

But no. As you can see, below, the estimated bill was WAY more than my usual bill!

I was not impressed, especially as that time of the year is pretty heavy on your wallet, anyway, with Christmas presents, and other Christmas and New Year related spending..

Yesterday, I got the reminder for my February bill. I've just submitted it now, and I'm STILL not where they estimated my electricity reading to be, three months ago!

Well, the positive side is, that I won't have to pay anything now as I've already paid it in December, and there will still be a little bit money left, towards my May bill..

But how the fuck did they decide to bill me for that much, in December?!? What's the estimated billing based on? Obviously, not on your billing history as in that case, my December bill would have been much smaller..