I have been on fixed contracts for ages. I have known the unit cost and standing charge and know it will be fixed for 2 years.
I know what I use in a year in KWH - and can work out the cost in the house.
However - the company collapsed - so am now on the Price Cap.
21p per unit of electricity and 4p per unit of gas.
Straightforward? But it isn't.
Electricity usage is fairly consistent through the year. But gas obviously varies.
So I am on the current price cap till April, then 6 months of low gas usage to October on the new price cap, and then 6 more months of higher gas usage from then during the winter months on what I think will be a much higher price cap.
I do have a spreadsheet for this 
but basically, the money talked about by Government and the actual increase on the price cap isn't as simple as it sounds, because basicallly:
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The price of gas is going up in April to October, but people use less gas in those months.
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In October, the price of gas will be going up again and that's at a point when we will use more gas.
It's a shame we can't buy our own gas now at this price and store it.
But that would be