The price cap is given as 2 numbers per region.
1 – the “nil” consumption cap. This is the maximum amount a supplier can charge for a year as standing charge
2 – the average consumption cap. This is the maximum amount a supplier can charge for a year of average consumption (3100kWh for electricity, 12000kWh for gas), this amount includes the standing charge
To work out the max for the standing charge you divide the nil consumption cap by 365.
To work out the max for the unit rate you subtract the nill consumption charge from the average consumption cap and then divide by the average consumption.
e.g For the October cap (before it was just superseded today). In the North West region, the cap for electricity (flat rate metering), when paid by direct debit is:
nil consumption: £150.38
average consumption: £1665.67
So the max standing charge in that region is 150.38/365 = 0.412
And the max unit rate is (1665.67 – 150.38)/3100 = 0.4888032258
and then you need to add VAT.