Consumer Surplus

1) What is consumer surplus?

Consumer Surplus (CS) is a welfare measure representing the difference between:

• what consumers are willing to pay for a given quantity of a good (their valuation), and
• what they actually pay in the market.

In standard partial equilibrium analysis (Marshallian):

Graphically, CS is the area under the demand curve and above the market price line, up to the quantity purchased.

CUET PG relevance:
Consumer surplus is frequently tested in:

• welfare questions (tax/subsidy/price control effects),
• market efficiency questions,
• comparing consumer welfare across different prices/market structures.

2) The demand curve as marginal willingness to pay

In many interpretations, the demand curve can be read as a curve of marginal willingness to pay (MWTP):

• At each quantity $q$, the demand curve shows the maximum price a consumer would pay for the $q^{th}$ unit (marginal valuation).

Then total willingness to pay for $Q$ units is:

Total expenditure is:

So:

3) Graphical measurement (core method)

Consider a market price $P^*$ and quantity demanded $Q^*$.

• Draw the demand curve.
• Draw a horizontal line at $P^*$.
• Consumer surplus = area between demand curve and price line from 0 to $Q^*$.

3.1 Linear demand (common for numericals)

If demand is linear and intersects price axis at $P_{max}$ (the choke price), then:

This is the area of a triangle.

CUET tip: Many quick CS numericals are built on this triangle formula.

4) Marshallian consumer surplus: assumptions and interpretation

Marshall’s CS is based on cardinal utility ideas and often implicitly assumes:

1. Utility is cardinally measurable (or at least comparable in money terms).
2. Marginal utility of money is constant (so money is a stable yardstick).
3. Demand curve reflects MWTP accurately.

These assumptions make CS intuitive and usable, but also imperfect.

5) Hicksian welfare measures vs Marshallian CS (conceptual upgrade)

In modern ordinal utility framework, welfare changes due to price change are ideally measured by:

• Compensating Variation (CV)
• Equivalent Variation (EV)

These use indifference curves and expenditure functions, and do not require constant MU of money.