As the price rises, I use the good for higher valued uses to share with my sister or husband. As the price falls, I use the good for lower valued uses to share with my dog or mother in law. It is the 4 movie rentals that are given up. Consumer Surplus Consumer surplus is another name for net total utility. It measures the difference between total utility and total expenditure.
Economists assume that firms maximize profit, while consumers maximize consumer surplus.
The last pint offers no surplus because the optimal purchase rule sets marginal utility equal to price. An increase in income reduces quantity demanded. If the price of a good falls, it falls relative to other goods. This should encourage the substitution of night calls for day calls. Thus, a normal good must have a downward sloping demand curve.