Producer Surplus and Total Economic Surplus

A seller who would have taken 3 dollars but gets 8 pockets 5 dollars of pure gain: that's producer surplus. It's the mirror image of consumer surplus. The supply curve traces out each seller's minimum acceptable price, their marginal cost, ranked from cheapest to priciest. Every unit sold at the market price earns the seller the difference between that price and their marginal cost. Stack up every unit and you get a triangle: the area above the supply curve and below the price line, running from zero up to the equilibrium quantity.

Consumer surplus is the triangle above price and below demand. Put the two triangles together and you get total economic surplus, the whole pie of value created by the market. This is maximized exactly at the competitive equilibrium quantity. Push quantity below equilibrium and you strand units where buyers would gladly pay more than it costs sellers to produce them, leaving gains on the table. Push quantity above equilibrium and you force trades where the cost of producing exceeds what any buyer values the unit at, destroying value. Only at equilibrium does every unit traded have a buyer value at or above seller cost, and no unit is left untraded despite value being available.

This sets up everything that follows in the unit: taxes, price floors, and price ceilings all work by pushing quantity away from equilibrium, which is exactly why they create deadweight loss.