This is worrying. You're an adult paying bills. Learn how they work.
The cap is not a cap on how much they can charge in total. It is a cap on price per unit.
They are capping how much each KW of electricity can be charged at. Same for gas.
If you use 5KW of electricy a day, then you pay for 5 KW a day. If you use 15KW a day then you pay for 15Kw a day. Each KW is charged at a rate which cannot be more than the cap.
A fixed rate of £166 a month does not mean you will only pay £166 a month no matter how much power you use. They have fixed the price you pay per unit, then they have looked at how many units you normal use annually. They then offer you a monthly price which should cover your annual usage. If you stopped using all power, you'd have a huge credit with them. If you triple your energy use by having the heating and tumble dryer on all day then you would get a huge bill because you need to pay for what you use.
A fixed rate means they fix the price per unit, not the price you pay overall.
The energy cap price used in the press is just the maximum an average house would pay on the proposed maximum price per unit. Millions will pay less than the suggested figure and millions will pay more. That's how you get an average.
Everyone pays for what they use, some pay tens of thousands have some pay less than two thousand. But no one can be charged more per each unit than the cap.