Introduction:

Excess capacity refers to the situation where a firm is not operating at its maximum production level given its available resources. In other words, it is the difference between a firm's actual production level and its maximum production capacity. This excess capacity can occur in different market structures, and in this case, we are discussing its presence in various types of market structures.

Explanation:

1. Monopoly:
In a monopoly market structure, there is a single seller with significant market power, and there are significant barriers to entry for potential competitors. In this scenario, a monopolist often operates at a point where it maximizes its profits, which means there is no excess capacity. The monopolist sets its output level where marginal cost equals marginal revenue, ensuring that it maximizes its profits.

2. Monopolistic Competition:
In monopolistic competition, there are many firms that produce differentiated products, and entry barriers are relatively low. Each firm has some degree of market power due to product differentiation. In this market structure, firms often operate with excess capacity. This is because firms differentiate their products to attract customers, and this product differentiation requires investing in excess capacity to produce different variations of the product.

3. Perfect Competition:
Perfect competition is a market structure where there are many buyers and sellers, homogeneous products, perfect information, and ease of entry and exit. In perfect competition, firms are price takers and have no market power. As a result, they operate at their efficient scale of production, where there is no excess capacity. Any firm operating below its efficient scale would be outcompeted by other firms in the industry.

4. Oligopoly:
Oligopoly is a market structure characterized by a few large firms dominating the market. These firms have significant market power and often engage in strategic interactions. The presence of excess capacity in oligopolistic markets depends on various factors, such as the degree of competition, barriers to entry, and the strategic behavior of firms. In some cases, firms may operate with excess capacity to maintain market stability or deter potential competition. However, in other cases, firms may operate at their efficient scale to maximize their profits.

Conclusion:
Excess capacity is not found under perfect competition because firms in perfect competition operate at their efficient scale of production, where there is no excess capacity. In monopolistic competition and oligopoly, firms may operate with excess capacity depending on market conditions and strategic considerations. Monopoly, on the other hand, does not have excess capacity as the monopolist operates at a profit-maximizing level of output.