@martyyn: yes, that's pretty much it. We also have monthly Simple Saver specials which should buy as they have a good discount.

@BraaiGuy is correct in the way he describes how prices are calculated, and that it takes a few month's meter reads for your estimated usage to settle down. It sounds like in his case usage went up from original estimates, but it is also possible it could go down. If you think you're a high-ish user then his advice to wait is correct. The other thing I would note is that we expect prices to increase in Apri next year (because network costs will be increasing) - current powerpack prices do not yet factor in any change, so buying Autmn and Winter powerpack before 1 April will allow you to beat the price increase. Squirrelling a bit of winter power away during summer also allows you to take the sting out of your winter power bills :)