Can refer to either an energy supplier who is automatically assigned to serve an existing customer immediately following deregulation, or an energy supplier who must supply a given classification or subclassification of customers who may not be able to acquire energy from any other provider. In most cases, the supplier of last resort is the utility company that the customer used before deregulation.

When the first competitors emerge in a deregulated market, a significant percentage of all customers will stick with their existing supplier and not even bother to shop around. This gives an existing utility company a built-in advantage over new competitors, so this advantage is often offset by legislation that requires these utilities to act as suppliers of last resort. They may be required to sell energy only at a rate determined by regulators, a rate referred to as standard offer. In some locales, suppliers of last resort may not even be allowed to sell energy that they produce to their former customers.

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