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The price cap applies to the domestic energy providers (e.g. Bulb) and it's based on the wholesale price of energy. The domestic energy suppliers are buying energy at high prices, they don't produce energy themselves. Because the prices are rising so fast, there are periods where the price cap is lower than wholesale prices, which is why so many of them went bust over the last year (ie. they lose money for every kWh of electricity their customers use).
The windfall tax applies to energy producers who are selling energy at insane prices (e.g. Shell). The prices are high because of Russia/Ukraine and COVID.
There is a little bit of overlap of course - Shell does household energy for example - but by and large they're different companies
Why doesn't the government just tell/legislate OFGEM that it can't raise the energy cap a) at all, or b) more than inflation?
It seems the windfall tax just takes some ££ from the energy companies and they just put the prices up so it all flows back to them. Same with the 5p petrol thing - prices went down a bit (though not 5p round us) - and then carried on going on up again with the fuel companies pretty much pocketing the cash as far as I can see.