The concept of consumer surplus

Consumer surplus represents the extra benefit that buyers gain when they purchase goods or services at a price lower than what they would have been willing to pay.

Consumer surplus

Consumer surplus is the difference between the price that a consumer is willing to pay for a good or service and the price that they actually pay. For example, if a customer is willing to pay £40 for a pair of trainers but buys them for £22, the consumer surplus is £18.

Key features:

• It varies due to differences in individual preferences, income levels, and perceptions of value.
• On a demand and supply diagram, consumer surplus appears as the area below the demand curve and above the equilibrium price line.

The concept of producer surplus

Producer surplus captures the additional profit that sellers earn when they receive a price higher than the minimum they would accept for supplying a good or service.

Producer surplus

Producer surplus is the difference between the price that a producer is willing to supply a good or service at and the price that they actually receive for it. For example, if a manufacturer would accept £30 to sell a gadget but the market price is £50, the producer surplus is £20.

Key features:

• It depends on variations in production costs among different suppliers.
• On a demand and supply diagram, producer surplus is shown as the area above the supply curve and below the equilibrium price line.

How changes in supply and demand affect surpluses

Shifts in supply or demand curves alter the equilibrium price and quantity, which in turn impact the sizes of consumer and producer surpluses.

General effects on surpluses:

• Any factor causing a demand or supply curve to move leads to a new equilibrium, adjusting the surpluses.
• A change in price will bring a good closer to or further away from the amount the buyer was willing to pay or the supplier was willing to sell for, which changes the consumer and producer surpluses.
• Changes in equilibrium quantity also affect the total surplus areas on diagrams.

Effects of shifts in supply and demand curves

Specific shifts in the supply or demand curves have predictable impacts on consumer and producer surpluses, which can be visualised on diagrams.

Effects of a rightward shift in the supply curve

A rightward shift (increase) in supply lowers the equilibrium price and raises quantity. Price falls from the original level (P₀) to a new level (P₁), while quantity rises from Q₀ to Q₁.

Impact on surpluses:

• Consumer surplus increases - It expands from the original triangular area below demand and above P₀ to a larger area below demand and above P₁.
• Producer surplus decreases - It shrinks from the original triangular area above supply and below P₀ to a smaller area above the new supply and below P₁.

Effects of a leftward shift in the demand curve

A leftward shift (decrease) in demand reduces both equilibrium price and quantity. Price drops from P₀ to P₁, and quantity falls from Q₀ to Q₁.

Impact on surpluses:

• Consumer surplus decreases - It reduces from the original area below demand and above P₀ to a smaller area below the new demand and above P₁.
• Producer surplus also decreases - It contracts from the area above supply and below P₀ to a reduced area above supply and below P₁.