The following is a cut and paste from the SimplySwitch website:
How cheap is it?
Prices vary from supplier to supplier, but an example could be 8 pence per unit for standard rate, and 2.5 pence per unit for the cheaper rate. The 'standard' rate will usually be a bit more expensive than the standard rate for non-Economy 7 accounts. If you pay a standing charge on your account, this too will be higher if you have Economy 7 (possibly around 50% more). If your account does not have a standing charge, then instead the first 'day' units you pay each quarter will be charged at a much higher rate (the same way as ordinary accounts without a standing charge work).