Model Essay
(b) Using real-world examples, evaluate how a change in supply affects the size and distribution of consumer surplus and producer surplus. Include a calculation of the changes where possible.
A change in supply alters both the size of total welfare and the distribution of welfare between consumers and producers. Consumer surplus (CS) is the difference between what consumers are willing to pay and what they actually pay, while producer surplus (PS) is the difference between the market price and the minimum price producers are willing to accept. When supply shifts, equilibrium price and quantity change, so the areas representing CS and PS change as well. In some cases there is mainly a redistribution of surplus between the two groups; in others, total surplus also falls because production has become more costly.
On a standard demand and supply diagram, with price on the vertical axis and quantity on the horizontal axis, the initial equilibrium is where demand, , intersects supply, , at . If supply decreases, the supply curve shifts left to . The new equilibrium, , occurs at a higher price and lower quantity. This means consumers lose surplus because they pay more and buy less. Producers may gain or lose surplus depending on how large the price rise is relative to the fall in quantity and any increase in costs. Total surplus may fall because higher production costs reduce the gains from trade; the new competitive equilibrium remains allocatively efficient relative to the new supply curve.
This can be shown numerically. Suppose demand is given by and initial supply by . Setting them equal gives the original equilibrium: , so and .
Initially, consumer surplus is the triangle below demand and above price: CS = .
Initial producer surplus is the triangle above supply and below price: PS = .
So initial total surplus is .
Now suppose supply decreases to , where . Equating this new supply curve with demand: , so and .
New consumer surplus becomes: CS = .
New producer surplus becomes: PS = .
New total surplus is therefore .
The changes are:
So in this example, a fall in supply reduces both consumer and producer surplus by $5.50 each, and total surplus falls by $11. This illustrates that a negative supply shock does not necessarily help producers overall. Although the market price rises from $6 to $7, producers sell fewer units and the supply shift itself reflects higher costs, so their surplus also falls. The $11 reduction is a fall in total surplus caused by the higher-cost supply curve; it is not a deadweight loss relative to the new market conditions, because the new equilibrium remains competitive.
However, the real-world effect of a supply decrease is often more complex than this linear example suggests, especially because elasticities matter. A useful example is OPEC oil production cuts. When OPEC restricts output, supply shifts left, causing a higher world oil price and lower quantity traded. For consumers, especially households and firms using petrol, diesel or jet fuel, consumer surplus clearly falls because they face higher prices. But for oil producers, producer surplus may actually rise, particularly in the short run, because demand for oil is relatively price inelastic. If the percentage increase in price is greater than the percentage fall in quantity sold, producer revenue rises, and producer surplus can also rise. This contrasts with the numerical example above, where PS fell. Therefore, the impact of a supply reduction on the distribution of surplus depends heavily on the price elasticity of demand.
The 2022 European energy crisis provides a strong real-world illustration. Gas supply to Europe fell sharply, shifting supply left. Equilibrium energy prices rose dramatically, and households and businesses experienced a severe loss of consumer surplus as heating and electricity bills increased. At the same time, major gas exporters such as Norway and Qatar benefited from much higher prices, so producer surplus rose substantially for these suppliers. This shows that a supply decrease can redistribute surplus away from consumers and towards producers, rather than reducing both equally. It also highlights that the effects are unevenly distributed across countries: importing nations lost welfare, while exporting nations gained.
A similar pattern can occur in agricultural markets. If a drought reduces coffee supply, the supply curve shifts left. Consumers pay more for coffee and lose consumer surplus. Yet if demand for coffee is relatively inelastic, farmers’ total revenue may rise despite lower output, which can increase producer surplus for some producers. Nevertheless, this depends on the scale of cost increases and on whether the drought affects all producers equally. Some farmers may actually produce so little that they lose out, even if market prices rise. This shows that “producers” are not a single homogeneous group; the distribution of PS within the producer group may also change.
Evaluation therefore requires distinguishing between a simple textbook welfare diagram and real markets. First, the exact size of CS and PS changes can only be calculated precisely when the demand and supply functions are known, as in the numerical example. In reality, demand and supply curves may be non-linear, and economists often estimate welfare changes using elasticities rather than neat triangles. Secondly, time matters. In the short run, supply and demand are often more inelastic, so a leftward shift in supply may create very large price increases and large transfers from consumers to producers. In the long run, consumers may switch to substitutes and firms may invest in alternatives, making demand more elastic and reducing producer gains.
There are also important policy implications. If a fall in supply gives rise to windfall producer gains while consumer surplus collapses, governments may intervene to redistribute surplus. For example, the UK imposed a windfall tax on energy company profits in 2022. This did not reverse the original supply shift, but it sought to transfer some surplus from producers to consumers or to public spending. Equally, if governments use production subsidies to offset higher costs, supply may shift right again, potentially increasing both CS and PS. However, the subsidy is paid for by taxpayers, so the net welfare effect depends on the opportunity cost of government spending and whether the intervention corrects a temporary shock or merely distorts the market.
Overall, a change in supply affects both the size and distribution of consumer and producer surplus by changing equilibrium price and quantity. A decrease in supply usually reduces consumer surplus and often reduces total surplus through deadweight loss. In the simple numerical case, both CS and PS fall by $5.50 and total surplus falls by $11. However, real-world examples such as OPEC oil cuts and the 2022 energy crisis show that producer surplus may rise when demand is inelastic, even as consumer surplus falls sharply. Therefore, the most convincing judgment is that a supply change almost always redistributes welfare, but whether producers gain or lose, and by how much total welfare changes, depends mainly on elasticity, costs and the time period considered.
Tip:
A change in supply alters both the size of total welfare and the distribution of welfare between consumers and producers.
This opening sentence answers both parts of the task immediately: how much surplus changes and how it is shared. It gives the essay a clear controlling line that the later examples and calculations keep returning to.
On a standard demand and supply diagram, with price on the vertical axis and quantity on the horizontal axis,
This orients the reader to the diagram before any shifts are discussed, which is a smart explanatory move in economics writing. It shows the response is not just naming the diagram but using it as the framework for the argument.
If supply decreases, the supply curve shifts left to .
This is the precise diagrammatic detail the explanation rests on. The answer identifies the exact directional change rather than speaking vaguely about 'less supply'.
Total surplus may fall because higher production costs reduce the gains from trade; the new competitive equilibrium remains allocatively efficient relative to the new supply curve.
This is strong causal analysis: it explains why total surplus can shrink after production costs rise while correctly distinguishing the new competitive equilibrium from a market-failure deadweight loss.
This means consumers lose surplus because they pay more and buy less.
The sentence links price and quantity changes directly to welfare effects in one clean step. That concise chain of reasoning is a model of disciplined economic explanation.
This can be shown numerically.
A brief transition like this helps the essay move from theory and diagram to calculation in a controlled way. It signals a new stage of the argument without breaking the flow.
Suppose demand is given by and initial supply by .
This supplies the exact functional evidence needed for the later welfare calculations. Because the functions are stated explicitly, the numerical results feel derived rather than asserted.
Setting them equal gives the original equilibrium:
This shows correct method: equilibrium is found where demand equals supply, and the answer makes that procedure explicit. Stating the method before the arithmetic helps the reader follow the economic logic, not just the numbers.
So initial total surplus is .
This is a neat argumentative checkpoint: the answer pauses to total the surpluses before introducing the shock, creating a clear baseline for comparison. That makes the later evaluation much more persuasive.
Now suppose supply decreases to , where .
This is the key changed condition in the numerical example, giving the reader the exact form of the new supply curve. It provides the concrete evidence needed to track how welfare changes after the shift.
New producer surplus becomes:
This signals that the writer is systematically recalculating both sides of the market after the shock. That symmetry matters because the question asks about the distribution of surplus, not just one group.
The change in total surplus is calculated explicitly rather than left implied, which is exactly what allows the deadweight loss to be demonstrated clearly. The negative result makes the welfare loss visible and measurable.
However, the real-world effect of a supply decrease is often more complex than this linear example suggests, especially because elasticities matter.
This is a strong evaluative pivot from textbook model to applied economics. It prevents the numerical example from becoming over-generalised and prepares the essay for balanced judgment.
A useful example is OPEC oil production cuts.
The essay introduces a specific, relevant case study rather than a generic reference to oil markets. That concreteness strengthens the link between theory and actual market behaviour.
If the percentage increase in price is greater than the percentage fall in quantity sold, producer revenue rises, and producer surplus can also rise.
This is effective analytical use of elasticity: the answer explains the condition under which producers benefit from a leftward supply shift. It moves beyond description to conditional reasoning about outcomes.
This contrasts with the numerical example above, where PS fell.
This comparative sentence is a sophisticated move because it actively tests one example against another. It shows the argument developing rather than listing examples separately.
The 2022 European energy crisis provides a strong real-world illustration.
This is another well-chosen real case that is directly relevant to supply contraction and welfare effects. The phrase also signals that the example will be used analytically, not just dropped in.
This shows that a supply decrease can redistribute surplus away from consumers and towards producers, rather than reducing both equally.
Here the writer draws the exact evaluative lesson from the example. The wording focuses sharply on redistribution, which is the central economic issue in the question.
It also highlights that the effects are unevenly distributed across countries: importing nations lost welfare, while exporting nations gained.
This is a strong analytical extension because it widens 'distribution' beyond consumers versus producers to international winners and losers. That added layer shows confident control of the concept.
A similar pattern can occur in agricultural markets.
This transition keeps the structure coherent while broadening the range of industries considered. It helps the essay build comparison across cases rather than repeating the same market.
If a drought reduces coffee supply, the supply curve shifts left.
This is a precise real-world application of the theory to agriculture. The cause of the supply shift is clearly identified, so the example feels economically grounded.
Some farmers may actually produce so little that they lose out, even if market prices rise.
This is insightful analysis because it distinguishes market price effects from individual producer outcomes. It adds nuance by showing that producer surplus may not move uniformly even within the producer group.
This shows that “producers” are not a single homogeneous group; the distribution of PS within the producer group may also change.
This is an excellent argumentative move: it refines the category of producers and deepens the meaning of distribution. That kind of qualification makes the evaluation much more convincing.
Evaluation therefore requires distinguishing between a simple textbook welfare diagram and real markets.
This is a clear evaluative topic sentence that tells the reader exactly what lens the next discussion will use. It marks a shift from examples to broader judgment and keeps the essay purposeful.
In reality, demand and supply curves may be non-linear, and economists often estimate welfare changes using elasticities rather than neat triangles.
This is strong analytical qualification of the model’s limits. It shows awareness that the earlier calculations are useful abstractions, not exact descriptions of complex markets.
Secondly, time matters.
This compact phrase is an effective organising device. It signals the second strand of evaluation and gives the paragraph a controlled, easy-to-follow shape.
There are also important policy implications.
This transition broadens the essay logically from explanation and evaluation to consequences for decision-making. It helps the response feel complete and well-sequenced.
For example, the UK imposed a windfall tax on energy company profits in 2022.
This is a sharply relevant policy example tied to the earlier energy discussion. It shows the student can connect welfare analysis to an actual government response.
However, the subsidy is paid for by taxpayers, so the net welfare effect depends on the opportunity cost of government spending and whether the intervention corrects a temporary shock or merely distorts the market.
This is balanced analysis because it does not treat intervention as automatically beneficial. The sentence weighs gains against wider costs and introduces opportunity cost with precision.
CS = .
This is a precise numerical detail showing the initial consumer surplus derived from the demand intercept, equilibrium price, and quantity.
the triangle below demand and above price
This identifies consumer surplus geometrically, linking the formula to the area under the demand curve and showing why the calculation captures consumer welfare.
PS = .
This gives the exact initial producer surplus using the equilibrium quantity and the gap between price and the supply intercept.
the triangle above supply and below price
This phrase explains producer surplus as a geometric area, showing why the triangular formula is appropriate in this linear market setup.
so and .
These new equilibrium values show the expected effect of a leftward supply shift: lower quantity traded and higher market price.
Equating this new supply curve with demand:
The response explicitly applies equilibrium methodology again after the shock, making the comparative-static change transparent and logically structured.
CS = .
This is the exact post-shock consumer surplus, calculated from the new quantity and the reduced gap between willingness to pay and market price.
New consumer surplus becomes:
The wording signals a comparison with the initial position, highlighting that the welfare measure is being recalculated under changed market conditions.
PS = .
This exact figure shows producer surplus after the supply decrease, using the new equilibrium and the upward-shifted supply intercept.
This numerical total gives the new level of combined market welfare after the supply shock.
New total surplus is therefore
The phrase shows that total welfare is being inferred from the recalculated consumer and producer surpluses, preparing the ground for identifying welfare loss.
Therefore
This connective move clearly links the new total surplus to the preceding component calculations, showing coherent stepwise reasoning.
This exact change quantifies the loss in consumer surplus between the initial and new equilibria.