Re: How much!!
It is true that energy suppliers often over-estimate energy use, but at the end of the day they are obliged to repay any over-payment.
I recently left British Gas in favour of EDF (yes, I know, bloody Frogs!) because the latter were offering a more competitive tariff and British Gas were dragging their feet in offering something competitive. Incidentally, I made use of the Cheap Energy Club who deal with it all for you.
When the two parties had finally sorted themselves out, and both had accepted my final meter readings (for some reason it can take up to 28 days, and I've never understood why), I received a refund of about £360 from British Gas.
With interest rates being so low, I was not too bothered whether they or I had my money for a few months. Fortunately, we are not too short of money at the moment although, to be fair, I know that some people are and live almost hand to mouth. In such cases, people should keep a close eye on their accounts - suppliers are legally obliged to supply this information - and demand a refund if they find they are too much in credit.