Producer Surplus

— Economics · Supply, Price & Surplus —

Key idea: Producer Surplus (PS) = the gain a seller gets for selling at the market price instead of the lowest price he would have accepted. On a graph it is the area above the supply curve and below the price line.

1. What is Producer Surplus?

Definition: Producer surplus is the difference between the minimum price a producer is ready to accept and the actual price he receives.

PS = Actual Price − Minimum Price

Where: Actual price = market price P (what he really gets) | Minimum price = lowest price he would accept (point A on the supply curve)

① Reading PS on a supply graph

PS = triangle between the price line and S

Draw Price (P) on the y-axis and Quantity supplied QS (Q) on the x-axis.
Supply curve S slopes upward (+ve). It starts at A = minimum price on the price axis.
Market price = P (actual price); it cuts S at point a, quantity Q.
The vertical gap between P and A (green double arrow) is the surplus per unit; the whole triangle P A a = PS.

Note: Producer surplus is the area above the supply curve and below the price level.

2. Law of Supply (why S slopes up)

Law of SupplyPrice ↑ ⇒ QS ↑ ⇒ TR ↑
Price ↓ ⇒ QS ↓ ⇒ TR ↓

Price and QS move in the same direction = positive (+ve) relationship.
That is why the supply curve is an upward-sloping supply curve.
TR = total revenue = P × QS (both rise together, so TR rises).

Extra (not on board): for a straight-line S, PS = ½ × Q × (P − A) — area of the triangle (base = Q, height = P − A). Handy when numbers are given.

3. PS with Elastic Supply

Why elastic?

More resources available → readily available, so producers can expand fast.
Eg: Mobile phones.
Price ↑ 10% ⇒ QS ↑ 50% ⇒ TR ↑ (QS responds much more than price).
⇒ Producer surplus increases.

flat S: small P rise, big QS rise, PS grows

Surplus change (elastic)PS: triangle P A a → PS₁: triangle P₁ A b ↑ (bigger)

Flatter supply curve (flatter curve S) ⇒ elastic supply.

4. PS with Inelastic Supply

Why inelastic?

Less resources available; production is time taking, so supply cannot expand quickly.
Eg: Coffee beans (they take time to grow).
Price ↑ 50% ⇒ QS ↑ only 10% ⇒ TR ↑.
⇒ Producer surplus increases.

steep S: big P rise, small QS rise, PS grows a lot

Surplus change (inelastic)PS: triangle P A a → PS₁: triangle P₁ A b ↑ (bigger)

Steep supply curve (steep curve S) ⇒ inelastic supply.

5. Elastic vs Inelastic — side by side

	Elastic supply	Inelastic supply
Resources	more, readily available	less, time taking
Example	mobile phones	coffee beans
Price change	↑ 10%	↑ 50%
QS change	↑ 50%	↑ 10%
TR	↑	↑
Supply curve	flatter	steep
Producer surplus	P A a → P₁ A b ↑	P A a → P₁ A b ↑

Remember: in both cases price rises ⇒ PS rises. The difference is how: elastic ⇒ producers sell much more; inelastic ⇒ they sell about the same amount at a higher price.

6. How to write it in an answer

Start with: Producer surplus is the area above the supply curve and below the market price.

Agricultural products (quick to grow)elastic supply

Supply is elastic because production time is quicker.

A rise in price (due to an increase in demand) gives a larger % change in quantity supplied.

Hence total revenue increases (O P a Q → O P₁ b Q₁)
and producer surplus also increases (A P a → A P₁ b).

Coffee beans (slow to grow)inelastic supply

Coffee beans take time to produce, so supply is inelastic.

A rise in price by a larger % makes quantity supplied extend by only a smaller %.

The producer is selling about the same amount at higher prices ⇒ PS rises from A P a to A P₁ b.

Note on letters: the board's written answers use letters from an exam diagram that isn't on these pages (R, C, P₂, Q₂, B), and the two paragraphs name the starting triangle differently (APR vs APB). Both mean the original surplus triangle A P a, so here the letters are matched to this chapter's labels (a, b, P₁, Q₁).

7. The two extreme cases

① Perfectly elastic supply

Perfect resources available; spare capacity available.
Infinite quantity can be sold; same product, same price.
⇒ PS = zero, because Minimum price = Actual price (m = P).
Supply curve is a horizontal line (H.C. on the board).

② Perfectly inelastic supply

No resources and no spare capacity; only a fixed quantity can be sold.
⇒ PS = infinite; minimum price cannot be determined.
Price ↑ 10% ⇒ QS does not change ⇒ TR ↑ 10%.
Supply curve is a vertical line (V.C. on the board) at quantity Q.

perfectly elastic: no gap ⇒ no surplus

perfectly inelastic: area reaches down without limit

8. Quick Recap — Cheat-Sheet

✓ Producer Surplus — everything on one page

PS = Actual price − Minimum price (the minimum price a seller will accept).
On a graph: area above S and below the price line (triangle P A a).
Law of supply: Price ↑ ⇒ QS ↑ ⇒ TR ↑ (positive relationship, upward-sloping S).
Elastic (flatter S; mobile phones): price ↑ 10%, QS ↑ 50% ⇒ PS ↑.
Inelastic (steep S; coffee beans): price ↑ 50%, QS ↑ 10% ⇒ PS ↑.
Perfectly elastic (horizontal S): min price = actual price ⇒ PS = 0.
Perfectly inelastic (vertical S): fixed quantity, min price undetermined ⇒ PS = ∞.