To better understand the difference between the fixed price and the indexed price, we must take into account a concept, Risk.
With the indexed rate, the price of energy is the one set by the electricity market. In this type of rate, the risk is assumed by the user since the fluctuation of the electricity market will be reflected in his bill.
On the other hand, in the fixed rates, the marketer marks a fixed price with the client that applies for a certain time (normally one year). The trading companies are the ones that assume the market risk and apply premiums to their rates. In this type of contract, the fixed price will largely depend on when the fixed price of electricity is negotiated.