There is a physical infrastructure, owned by company A. You pay a fixed fee for access as part of your energy bill. You have no choice in that. Your actual subs

There is a physical infrastructure, owned by company A. You pay a fixed fee for access as part of your energy bill. You have no choice in that. Your actual subscription is with energy company B, Who you can choose freely. Of course company B doesn’t hook up directly to your home, instead they look at the total amount of power used by customers of company B and they are obligated to supply that amount of power to the national grid.