The below has been taken from the Current Cost website faq
My tariff alters after a number of units, what figure should I enter into the display?
The Current Cost monitor is designed to highlight wastage and is not intended to be a revenue meter. The reason a cost is on there is to offer some relevancy to the wattage used. The best currency to highlight wastage is energy (expressed as watts) although money ultimately saved is an obvious benefit. A good solution, offered by a user is: Add the two tariffs together, then add five per cent VAT, finally divide by two, and enter this value in as a single tariff.
My rate is
First 225kWh used each quarter = 21.338p
All/Day remaining kWh = 10.555p
Maths was never my strong point!